Scott L. Delp, Ph.D.

All Publications

• Hamstrings are Stretched More and Faster during Accelerative Running Compared to Speed-Matched Constant-Speed Running. Medicine and science in sports and exercise Gurchiek, R. D., Teplin, Z., Falisse, A., Hicks, J. L., Delp, S. L. 2024

  Abstract

  Hamstring injuries are common in field-based sports and reinjury rates are high. Recent evidence suggests hamstring injuries often occur during accelerative running, but investigations of hamstring mechanics have primarily considered constant-speed running. Thus, our objective was to compare hamstring lengths and velocities between accelerative running and constant-speed running.We recorded videos of 10 participants during 6 accelerative running trials and 6 constant-speed running trials. We used OpenCap to estimate body segment kinematics and a 3-dimensional musculoskeletal model to compute peak length and step-average lengthening velocity of the biceps femoris (long head) muscle-tendon unit. We compared running conditions using linear mixed models with running speed as the independent variable.At running speeds below 75% of top speed, accelerative running resulted in greater peak lengths than constant-speed running. For example, the peak hamstring muscle-tendon length when a person accelerated from running at only 50% of top speed was equivalent to running at a constant 88% of top speed. Lengthening velocities were greater during accelerative running at all running speeds. Differences in hip flexion kinematics drove the greater peak lengths and lengthening velocities observed in accelerative running.Hamstrings are subjected to longer lengths and faster lengthening velocities in accelerative running than in constant-speed running. This provides a potential biomechanical perspective towards understanding the occurrence of hamstring injuries during acceleration. Our results suggest coaches and sports medicine staff should consider the accelerative nature of running in addition to running speed to quantify exposure to high-risk circumstances with long lengths and fast lengthening velocities of the hamstrings.

  View details for DOI 10.1249/MSS.0000000000003577

  View details for PubMedID 39446022

• A randomized clinical trial testing digital mindset intervention for knee osteoarthritis pain and activity improvement. NPJ digital medicine Boswell, M. A., Evans, K. M., Ghandwani, D., Hastie, T., Zion, S. R., Moya, P. L., Giori, N. J., Hicks, J. L., Crum, A. J., Delp, S. L. 2024; 7 (1): 285

  Abstract

  This randomized clinical trial evaluated the effectiveness of short, digital interventions in improving physical activity and pain for individuals with knee osteoarthritis. We compared a digital mindset intervention, focusing on adaptive mindsets (e.g., osteoarthritis is manageable), to a digital education intervention and a no-intervention group. 408 participants with knee osteoarthritis completed the study online in the US. The mindset intervention significantly improved mindsets compared to both other groups (P < 0.001) and increased physical activity levels more than the no-intervention group (mean = 28.6 points, P = 0.001), but pain reduction was not significant. The mindset group also showed significantly greater improvements in the perceived need for surgery, self-imposed physical limitations, fear of movement, and self-efficacy than the no-intervention and education groups. This trial demonstrates the effectiveness of brief digital interventions in educating about osteoarthritis and further highlights the additional benefits of improving mindsets to transform patients' approach to disease management. The study was prospectively registered (ClinicalTrials.gov: NCT05698368, 2023-01-26).

  View details for DOI 10.1038/s41746-024-01281-8

  View details for PubMedID 39414999

  View details for PubMedCentralID 7000096

• Multiscale hamstring muscle adaptations following 9 weeks of eccentric training. Journal of sport and health science Andrews, M. H., S, A. P., Gurchiek, R. D., Pincheira, P. A., Chaudhari, A. S., Hodges, P. W., Lichtwark, G. A., Delp, S. L. 2024: 100996

  Abstract

  Eccentric training, such as Nordic hamstring exercise (NHE) training, is commonly used as a preventive measure for hamstring strains. Eccentric training is believed to induce lengthening of muscle fascicles and to be associated with the addition of sarcomeres in series within muscle fibers. However, the difficulty in measuring sarcomere adaptation in human muscles has severely limited information about the precise mechanisms of adaptation. This study addressed this limitation by measuring the multiscale hamstring muscle adaptations in response to 9 weeks of NHE training and 3 weeks of detraining.Twelve participants completed 9 weeks of supervised NHE training, followed by a 3-week detraining period. We assessed biceps femoris long-head (BFlh) muscle fascicle length, sarcomere length, and serial sarcomere number in the central and distal regions of the muscle. Additionally, we measured muscle volume and eccentric strength at baseline, post-training, and post-detraining.NHE training over 9 weeks induced significant architectural and strength adaptations in the BFlh muscle. Fascicle length increased by 19% in the central muscle region and 33% in the distal muscle region. NHE also induced increases in serial sarcomere number (25% in the central region and 49% in the distal region). BFlh muscle volume increased by 8%, and knee flexion strength increased by 40% with training. Following 3 weeks of detraining, fascicle length decreased by 12% in the central region and 16% in the distal region along with reductions in serial sarcomere number.Nine weeks of NHE training produced substantial, region-specific increases in BFlh muscle fascicle length, muscle volume, and force generation. The direct measurement of sarcomere lengths revealed that the increased fascicle length was accompanied by the addition of sarcomeres in series within the muscle fascicles.

  View details for DOI 10.1016/j.jshs.2024.100996

  View details for PubMedID 39461588

• Personalizing the shoulder rhythm in a computational upper body model improves kinematic tracking in high range-of-motion arm movements. Journal of biomechanics Maier, J. N., Bianco, N. A., Ong, C. F., Muccini, J., Kuhl, E., Delp, S. L. 2024; 176: 112365

  Abstract

  Musculoskeletal models of the shoulder are needed to understand the mechanics of overhead motions. Existing models implementing the shoulder rhythm are generic and might not accurately represent an individual's scapular kinematics. We introduce a method to personalize the shoulder rhythm of a computational model of the upper body that defines the orientations of the clavicle and scapula based on glenohumeral joint angles. During five static calibration poses, we palpate and measure the orientation of the scapula. We explore the importance of representing shoulder elevation by introducing clavicle elevation as a degree of freedom that is independent of the glenohumeral angles. For ten subjects, we record the five calibration poses, ten additional static poses, and dynamic arm raises covering the participants' full range of motion in each body plane using optical motion capture. We examine the data using a dynamically-constrained inverse kinematics analysis. Shoulder rhythm personalization, independent clavicle elevation, and both in combination reduce the average upper body marker tracking error compared to the generic model in the static poses (26 mm to 17-20 mm) and in the dynamic trials (22 mm to 14-17 mm). Only personalization reduces the average scapula marker error (51 mm to 36-38 mm) and scapula axis-angle error (15° to 10°) compared with the palpated ground truth measurements in the static poses, and in the dynamic trials at instances that best match the static poses (53 mm to 37-40 mm, 15° to 9°). Our results show that personalizing upper body models improves kinematic tracking. We provide our experimental data, model, and methods to allow researchers to reproduce and build upon our results.

  View details for DOI 10.1016/j.jbiomech.2024.112365

  View details for PubMedID 39426356

• Impact Of Time Of Day On Neuromuscular Performance Of The Star Excursion Balance Test In Young Women Weed, L., Thompson, S., Kraus, E., Delp, S., Zeitzer, J. LIPPINCOTT WILLIAMS & WILKINS. 2024: 257-258

  View details for Web of Science ID 001315123201146

• Quantification Of 3D Knee Morphology In Patients With Patellar Instability Lee, M. R., Gatti, A. A., Wright, C. E., Bartsch, A., Veerkamp, M. W., Parikh, S. N., Chaudhari, A. S., Shea, K. G., Sherman, S. L., Delp, S. L. LIPPINCOTT WILLIAMS & WILKINS. 2024: 61-62

  View details for Web of Science ID 001315123200134

• Marker Data Enhancement For Markerless Motion Capture. bioRxiv : the preprint server for biology Falisse, A., Uhlrich, S. D., Chaudhari, A. S., Hicks, J. L., Delp, S. L. 2024

  Abstract

  Human pose estimation models can measure movement from videos at a large scale and low cost; however, open-source pose estimation models typically detect only sparse keypoints, which leads to inaccurate joint kinematics. OpenCap, a freely available service for researchers to measure movement from videos, addresses this issue using a deep learning model-the marker enhancer-that transforms sparse keypoints into dense anatomical markers. However, OpenCap performs poorly on movements not included in the training data. Here, we create a much larger and more diverse training dataset and develop a more accurate and generalizable marker enhancer.We compiled marker-based motion capture data from 1176 subjects and synthesized 1433 hours of keypoints and anatomical markers to train the marker enhancer. We evaluated its accuracy in computing kinematics using both benchmark movement videos and synthetic data representing unseen, diverse movements.The marker enhancer improved kinematic accuracy on benchmark movements (mean error: 4.1°, max: 8.7°) compared to using video keypoints (mean: 9.6°, max: 43.1°) and OpenCap's original enhancer (mean: 5.3°, max: 11.5°). It also better generalized to unseen, diverse movements (mean: 4.1°, max: 6.7°) than OpenCap's original enhancer (mean: 40.4°, max: 252.0°).Our marker enhancer demonstrates both accuracy and generalizability across diverse movements.We integrated the marker enhancer into OpenCap, thereby offering its thousands of users more accurate measurements across a broader range of movements.

  View details for DOI 10.1101/2024.07.13.603382

  View details for PubMedID 39071421

  View details for PubMedCentralID PMC11275905

• ISB recommendations on the definition, estimation, and reporting of joint kinematics in human motion analysis applications using wearable inertial measurement technology. Journal of biomechanics Cereatti, A., Gurchiek, R., Mündermann, A., Fantozzi, S., Horak, F., Delp, S., Aminian, K. 2024; 173: 112225

  Abstract

  There is widespread and growing use of inertial measurement technology for human motion analysis in biomechanics and clinical research. Due to advancements in sensor miniaturization, inertial measurement units can be used to obtain a description of human body and joint kinematics both inside and outside the laboratory. While algorithms for data processing continue to improve, a lack of standard reporting guidelines compromises the interpretation and reproducibility of results, which hinders advances in research and development of measurement and intervention tools. To address this need, the International Society of Biomechanics approved our proposal to develop recommendations on the use of inertial measurement units for joint kinematics analysis. A collaborative effort that incorporated feedback from the biomechanics community has produced recommendations in five categories: sensor characteristics and calibration, experimental protocol, definition of a kinematic model and subject-specific calibration, analysis of joint kinematics, and quality assessment. We have avoided an overly prescriptive set of recommendations for algorithms and protocols, and instead offer reporting guidelines to facilitate reproducibility and comparability across studies. In addition to a conceptual framework and reporting guidelines, we provide a checklist to guide the design and review of research using inertial measurement units for joint kinematics.

  View details for DOI 10.1016/j.jbiomech.2024.112225

  View details for PubMedID 39032224

• ShapeMed-Knee: A Dataset and Neural Shape Model Benchmark for Modeling 3D Femurs. medRxiv : the preprint server for health sciences Gatti, A. A., Blankemeier, L., Van Veen, D., Hargreaves, B., Delp, S. L., Gold, G. E., Kogan, F., Chaudhari, A. S. 2024

  Abstract

  Analyzing anatomic shapes of tissues and organs is pivotal for accurate disease diagnostics and clinical decision-making. One prominent disease that depends on anatomic shape analysis is osteoarthritis, which affects 30 million Americans. To advance osteoarthritis diagnostics and prognostics, we introduce ShapeMed-Knee, a 3D shape dataset with 9,376 high-resolution, medical-imaging-based 3D shapes of both femur bone and cartilage. Besides data, ShapeMed-Knee includes two benchmarks for assessing reconstruction accuracy and five clinical prediction tasks that assess the utility of learned shape representations. Leveraging ShapeMed-Knee, we develop and evaluate a novel hybrid explicit-implicit neural shape model which achieves up to 40% better reconstruction accuracy than a statistical shape model and implicit neural shape model. Our hybrid models achieve state-of-the-art performance for preserving cartilage biomarkers; they're also the first models to successfully predict localized structural features of osteoarthritis, outperforming shape models and convolutional neural networks applied to raw magnetic resonance images and segmentations. The ShapeMed-Knee dataset provides medical evaluations to reconstruct multiple anatomic surfaces and embed meaningful disease-specific information. ShapeMed-Knee reduces barriers to applying 3D modeling in medicine, and our benchmarks highlight that advancements in 3D modeling can enhance the diagnosis and risk stratification for complex diseases. The dataset, code, and benchmarks will be made freely accessible.

  View details for DOI 10.1101/2024.05.06.24306965

  View details for PubMedID 38766040

• A DIGITAL MINDSET INTERVENTION TO IMPROVE PAIN AND EXERCISE PARTICIPATION IN INDIVIDUALS WITH KNEE OSTEOARTHRITIS: A RANDOMIZED CLINICAL TRIAL Boswell, M., Evans, K., Ghnadwani, D., Hastie, T., Zion, S., Moya, P., Giori, N., Crum, A., Delp, S. ELSEVIER SCI LTD. 2024: S7-S8

  View details for Web of Science ID 001280544200005

• REDUCED COMPRESSIVE & SHEAR FORCES FROM GAIT RETRAINING RELATE TO SLOWED CARTILAGE DEGENERATION Seagers, K., Kolesar, J. A., Mazzoli, V., Halilaj, E., Delp, S., Uhlrich, S. ELSEVIER SCI LTD. 2024: S28-S29

  View details for Web of Science ID 001280544200033

• Hamstrings are stretched more and faster during accelerative running compared to speed-matched constant speed running. bioRxiv : the preprint server for biology Gurchiek, R. D., Teplin, Z., Falisse, A., Hicks, J. L., Delp, S. L. 2024

  Abstract

  Hamstring strain injuries are associated with significant time away from sport and high reinjury rates. Recent evidence suggests that hamstring injuries often occur during accelerative running, but investigations of hamstring mechanics have primarily examined constant speed running on a treadmill. To help fill this gap in knowledge, this study compares hamstring lengths and lengthening velocities between accelerative running and constant speed overground running.We recorded 2 synchronized videos of 10 participants (5 female, 5 male) during 6 accelerative running trials and 6 constant speed running trials. We used OpenCap (a markerless motion capture system) to estimate body segment kinematics for each trial and a 3-dimensional musculoskeletal model to compute peak length and step-average lengthening velocity of the biceps femoris (long head) muscle-tendon unit. To compare running conditions, we used linear mixed regression models with running speed (normalized by the subject-specific maximum) as the independent variable.At running speeds below 75% of top speed accelerative running resulted in greater peak lengths than constant speed running. For example, the peak hamstring muscle-tendon length when a person accelerated from running at only 50% of top speed was equivalent to running at a constant 88% of top speed. Lengthening velocities were greater during accelerative running at all running speeds. Differences in hip flexion kinematics primarily drove the greater peak muscle-tendon lengths and lengthening velocities observed in accelerative running.Hamstrings are subjected to longer muscle-tendon lengths and faster lengthening velocities in accelerative running compared to constant speed running. This provides a biomechanical explanation for the observation that hamstring strain injuries often occur during acceleration. Our results suggest coaches who monitor exposure to high-risk circumstances (long lengths, fast lengthening velocities) should consider the accelerative nature of running in addition to running speed.

  View details for DOI 10.1101/2024.03.25.586659

  View details for PubMedID 38585841

  View details for PubMedCentralID PMC10996654

• A simulation framework to determine optimal strength training and musculoskeletal geometry for sprinting and distance running. PLoS computational biology Van Wouwe, T., Hicks, J., Delp, S., Liu, K. C. 2024; 20 (2): e1011410

  Abstract

  Musculoskeletal geometry and muscle volumes vary widely in the population and are intricately linked to the performance of tasks ranging from walking and running to jumping and sprinting. As an alternative to experimental approaches, where it is difficult to isolate factors and establish causal relationships, simulations can be used to independently vary musculoskeletal geometry and muscle volumes, and develop a fundamental understanding. However, our ability to understand how these parameters affect task performance has been limited due to the high computational cost of modelling the necessary complexity of the musculoskeletal system and solving the requisite multi-dimensional optimization problem. For example, sprinting and running are fundamental to many forms of sport, but past research on the relationships between musculoskeletal geometry, muscle volumes, and running performance has been limited to observational studies, which have not established cause-effect relationships, and simulation studies with simplified representations of musculoskeletal geometry. In this study, we developed a novel musculoskeletal simulator that is differentiable with respect to musculoskeletal geometry and muscle volumes. This simulator enabled us to find the optimal body segment dimensions and optimal distribution of added muscle volume for sprinting and marathon running. Our simulation results replicate experimental observations, such as increased muscle mass in sprinters, as well as a mass in the lower end of the healthy BMI range and a higher leg-length-to-height ratio in marathon runners. The simulations also reveal new relationships, for example showing that hip musculature is vital to both sprinting and marathon running. We found hip flexor and extensor moment arms were maximized to optimize sprint and marathon running performance, and hip muscles the main target when we simulated strength training for sprinters. Our simulation results provide insight to inspire future studies to examine optimal strength training. Our simulator can be extended to other athletic tasks, such as jumping, or to non-athletic applications, such as designing interventions to improve mobility in older adults or individuals with movement disorders.

  View details for DOI 10.1371/journal.pcbi.1011410

  View details for PubMedID 38394308

• Lower-Limb Exoskeletons Appeal to Both Clinicians and Older Adults, Especially for Fall Prevention and Joint Pain Reduction. IEEE transactions on neural systems and rehabilitation engineering : a publication of the IEEE Engineering in Medicine and Biology Society Raitor, M., Ruggles, S. W., Delp, S. L., Liu, C. K., Collins, S. H. 2024; 32: 1577-1585

  Abstract

  Exoskeletons are a burgeoning technology with many possible applications to improve human life; focusing the effort of exoskeleton research and development on the most important features is essential for facilitating adoption and maximizing positive societal impact. To identify important focus areas for exoskeleton research and development, we conducted a survey with 154 potential users (older adults) and another survey with 152 clinicians. The surveys were conducted online and to ensure a consistent concept of an exoskeleton across respondents, an image of a hip exoskeleton was shown during exoskeleton-related prompts. The survey responses indicate that both older adults and clinicians are open to using exoskeletons, fall prevention and joint pain reduction are especially important features, and users are likely to wear an exoskeleton in the scenarios when it has the greatest opportunity to help prevent a fall. These findings can help inform future exoskeleton research and guide the development of devices that are accepted, used, and provide meaningful benefit to users.

  View details for DOI 10.1109/TNSRE.2024.3381979

  View details for PubMedID 38536680

• Muscle-driven simulations and experimental data of cycling. Scientific reports Clancy, C. E., Gatti, A. A., Ong, C. F., Maly, M. R., Delp, S. L. 2023; 13 (1): 21534

  Abstract

  Muscle-driven simulations have provided valuable insights in studies of walking and running, but a set of freely available simulations and corresponding experimental data for cycling do not exist. The aim of this work was to develop a set of muscle-driven simulations of cycling and to validate them by comparison with experimental data. We used direct collocation to generate simulations of 16 participants cycling over a range of powers (40-216 W) and cadences (75-99 RPM) using two optimization objectives: a baseline objective that minimized muscle effort and a second objective that additionally minimized tibiofemoral joint forces. We tested the accuracy of the simulations by comparing the timing of active muscle forces in our baseline simulation to timing in experimental electromyography data. Adding a term in the objective function to minimize tibiofemoral forces preserved cycling power and kinematics, improved similarity between active muscle force timing and experimental electromyography, and decreased tibiofemoral joint reaction forces, which better matched previously reported in vivo measurements. The musculoskeletal models, muscle-driven simulations, simulation software, and experimental data are freely shared at https://simtk.org/projects/cycling_sim for others to reproduce these results and build upon this research.

  View details for DOI 10.1038/s41598-023-47945-5

  View details for PubMedID 38057337

• From Skin to Skeleton: Towards Biomechanically Accurate 3D Digital Humans ACM TRANSACTIONS ON GRAPHICS Keller, M., Werling, K., Shin, S., Delp, S., Pujades, S., Liu, C., Black, M. J. 2023; 42 (6)

  View details for DOI 10.1145/3618381

  View details for Web of Science ID 001139790400081

• AddBiomechanics: Automating model scaling, inverse kinematics, and inverse dynamics from human motion data through sequential optimization. PloS one Werling, K., Bianco, N. A., Raitor, M., Stingel, J., Hicks, J. L., Collins, S. H., Delp, S. L., Liu, C. K. 2023; 18 (11): e0295152

  Abstract

  Creating large-scale public datasets of human motion biomechanics could unlock data-driven breakthroughs in our understanding of human motion, neuromuscular diseases, and assistive devices. However, the manual effort currently required to process motion capture data and quantify the kinematics and dynamics of movement is costly and limits the collection and sharing of large-scale biomechanical datasets. We present a method, called AddBiomechanics, to automate and standardize the quantification of human movement dynamics from motion capture data. We use linear methods followed by a non-convex bilevel optimization to scale the body segments of a musculoskeletal model, register the locations of optical markers placed on an experimental subject to the markers on a musculoskeletal model, and compute body segment kinematics given trajectories of experimental markers during a motion. We then apply a linear method followed by another non-convex optimization to find body segment masses and fine tune kinematics to minimize residual forces given corresponding trajectories of ground reaction forces. The optimization approach requires approximately 3-5 minutes to determine a subject's skeleton dimensions and motion kinematics, and less than 30 minutes of computation to also determine dynamically consistent skeleton inertia properties and fine-tuned kinematics and kinetics, compared with about one day of manual work for a human expert. We used AddBiomechanics to automatically reconstruct joint angle and torque trajectories from previously published multi-activity datasets, achieving close correspondence to expert-calculated values, marker root-mean-square errors less than 2 cm, and residual force magnitudes smaller than 2% of peak external force. Finally, we confirmed that AddBiomechanics accurately reproduced joint kinematics and kinetics from synthetic walking data with low marker error and residual loads. We have published the algorithm as an open source cloud service at AddBiomechanics.org, which is available at no cost and asks that users agree to share processed and de-identified data with the community. As of this writing, hundreds of researchers have used the prototype tool to process and share about ten thousand motion files from about one thousand experimental subjects. Reducing the barriers to processing and sharing high-quality human motion biomechanics data will enable more people to use state-of-the-art biomechanical analysis, do so at lower cost, and share larger and more accurate datasets.

  View details for DOI 10.1371/journal.pone.0295152

  View details for PubMedID 38033114

• OpenCap: Human movement dynamics from smartphone videos. PLoS computational biology Uhlrich, S. D., Falisse, A., Kidziński, Ł., Muccini, J., Ko, M., Chaudhari, A. S., Hicks, J. L., Delp, S. L. 2023; 19 (10): e1011462

  Abstract

  Measures of human movement dynamics can predict outcomes like injury risk or musculoskeletal disease progression. However, these measures are rarely quantified in large-scale research studies or clinical practice due to the prohibitive cost, time, and expertise required. Here we present and validate OpenCap, an open-source platform for computing both the kinematics (i.e., motion) and dynamics (i.e., forces) of human movement using videos captured from two or more smartphones. OpenCap leverages pose estimation algorithms to identify body landmarks from videos; deep learning and biomechanical models to estimate three-dimensional kinematics; and physics-based simulations to estimate muscle activations and musculoskeletal dynamics. OpenCap's web application enables users to collect synchronous videos and visualize movement data that is automatically processed in the cloud, thereby eliminating the need for specialized hardware, software, and expertise. We show that OpenCap accurately predicts dynamic measures, like muscle activations, joint loads, and joint moments, which can be used to screen for disease risk, evaluate intervention efficacy, assess between-group movement differences, and inform rehabilitation decisions. Additionally, we demonstrate OpenCap's practical utility through a 100-subject field study, where a clinician using OpenCap estimated musculoskeletal dynamics 25 times faster than a laboratory-based approach at less than 1% of the cost. By democratizing access to human movement analysis, OpenCap can accelerate the incorporation of biomechanical metrics into large-scale research studies, clinical trials, and clinical practice.

  View details for DOI 10.1371/journal.pcbi.1011462

  View details for PubMedID 37856442

  View details for PubMedCentralID PMC10586693

• Simulating Muscle-Level Energetic Cost Savings When Humans Run with a Passive Assistive Device. IEEE robotics and automation letters Stingel, J. P., Hicks, J. L., Uhlrich, S. D., Delp, S. L. 2023; 8 (10): 6267-6274

  Abstract

  Connecting the legs with a spring attached to the shoelaces, called an exotendon, can reduce the energetic cost of running, but how the exotendon reduces the energetic burden of individual muscles remains unknown. We generated muscle-driven simulations of seven individuals running with and without the exotendon to discern whether savings occurred during the stance phase or the swing phase, and to identify which muscles contributed to energy savings. We computed differences in muscle-level energy consumption, muscle activations, and changes in muscle-fiber velocity and force between running with and without the exotendon. The seven of nine participants who reduced energy cost when running with the exotendon reduced their measured energy expenditure rate by 0.9 W/kg (8.3%). Simulations predicted a 1.4 W/kg (12.0%) reduction in the average rate of energy expenditure and correctly identified that the exotendon reduced rates of energy expenditure for all seven individuals. Simulations showed most of the savings occurred during stance (1.5 W/kg), though the rate of energy expenditure was also reduced during swing (0.3 W/kg). The energetic savings were distributed across the quadriceps, hip flexor, hip abductor, hamstring, hip adductor, and hip extensor muscle groups, whereas no changes were observed in the plantarflexor or dorsiflexor muscles. Energetic savings were facilitated by reductions in the rate of mechanical work performed by muscles and their estimated rate of heat production. By modeling muscle-level energetics, this simulation framework accurately captured measured changes in whole-body energetics when using an assistive device. This is a useful first step towards using simulation to accelerate device design by predicting how humans will interact with assistive devices that have yet to be built.

  View details for DOI 10.1109/lra.2023.3303094

  View details for PubMedID 37745177

  View details for PubMedCentralID PMC10512759

• The History and Future of Neuromusculoskeletal Biomechanics. Journal of applied biomechanics Lloyd, D. G., Jonkers, I., Delp, S. L., Modenese, L. 2023: 1-11

  Abstract

  The Executive Council of the International Society of Biomechanics has initiated and overseen the commemorations of the Society's 50th Anniversary in 2023. This included multiple series of lectures at the ninth World Congress of Biomechanics in 2022 and XXIXth Congress of the International Society of Biomechanics in 2023, all linked to special issues of International Society of Biomechanics' affiliated journals. This special issue of the Journal of Applied Biomechanics is dedicated to the biomechanics of the neuromusculoskeletal system. The reader is encouraged to explore this special issue which comprises 6 papers exploring the current state-of the-art, and future directions and roles for neuromusculoskeletal biomechanics. This editorial presents a very brief history of the science of the neuromusculoskeletal system's 4 main components: the central nervous system, musculotendon units, the musculoskeletal system, and joints, and how they biomechanically integrate to enable an understanding of the generation and control of human movement. This also entails a quick exploration of contemporary neuromusculoskeletal biomechanics and its future with new fields of application.

  View details for DOI 10.1123/jab.2023-0165

  View details for PubMedID 37751904

• AddBiomechanics: Automating model scaling, inverse kinematics, and inverse dynamics from human motion data through sequential optimization. bioRxiv : the preprint server for biology Werling, K., Bianco, N. A., Raitor, M., Stingel, J., Hicks, J. L., Collins, S. H., Delp, S. L., Liu, C. K. 2023

  Abstract

  Creating large-scale public datasets of human motion biomechanics could unlock data-driven breakthroughs in our understanding of human motion, neuromuscular diseases, and assistive devices. However, the manual effort currently required to process motion capture data and quantify the kinematics and dynamics of movement is costly and limits the collection and sharing of large-scale biomechanical datasets. We present a method, called AddBiomechanics, to automate and standardize the quantification of human movement dynamics from motion capture data. We use linear methods followed by a non-convex bilevel optimization to scale the body segments of a musculoskeletal model, register the locations of optical markers placed on an experimental subject to the markers on a musculoskeletal model, and compute body segment kinematics given trajectories of experimental markers during a motion. We then apply a linear method followed by another non-convex optimization to find body segment masses and fine tune kinematics to minimize residual forces given corresponding trajectories of ground reaction forces. The optimization approach requires approximately 3-5 minutes to determine a subjecťs skeleton dimensions and motion kinematics, and less than 30 minutes of computation to also determine dynamically consistent skeleton inertia properties and fine-tuned kinematics and kinetics, compared with about one day of manual work for a human expert. We used AddBiomechanics to automatically reconstruct joint angle and torque trajectories from previously published multi-activity datasets, achieving close correspondence to expert-calculated values, marker root-mean-square errors less than 2cm, and residual force magnitudes smaller than 2% of peak external force. Finally, we confirmed that AddBiomechanics accurately reproduced joint kinematics and kinetics from synthetic walking data with low marker error and residual loads. We have published the algorithm as an open source cloud service at AddBiomechanics.org, which is available at no cost and asks that users agree to share processed and de-identified data with the community. As of this writing, hundreds of researchers have used the prototype tool to process and share about ten thousand motion files from about one thousand experimental subjects. Reducing the barriers to processing and sharing high-quality human motion biomechanics data will enable more people to use state-of-the-art biomechanical analysis, do so at lower cost, and share larger and more accurate datasets.

  View details for DOI 10.1101/2023.06.15.545116

  View details for PubMedID 37398034

  View details for PubMedCentralID PMC10312696

• Simulating the effect of ankle plantarflexion and inversion-eversion exoskeleton torques on center of mass kinematics during walking. PLoS computational biology Bianco, N. A., Collins, S. H., Liu, K., Delp, S. L. 2023; 19 (8): e1010712

  Abstract

  Walking balance is central to independent mobility, and falls due to loss of balance are a leading cause of death for people 65 years of age and older. Bipedal gait is typically unstable, but healthy humans use corrective torques to counteract perturbations and stabilize gait. Exoskeleton assistance could benefit people with neuromuscular deficits by providing stabilizing torques at lower-limb joints to replace lost muscle strength and sensorimotor control. However, it is unclear how applied exoskeleton torques translate to changes in walking kinematics. This study used musculoskeletal simulation to investigate how exoskeleton torques applied to the ankle and subtalar joints alter center of mass kinematics during walking. We first created muscle-driven walking simulations using OpenSim Moco by tracking experimental kinematics and ground reaction forces recorded from five healthy adults. We then used forward integration to simulate the effect of exoskeleton torques applied to the ankle and subtalar joints while keeping muscle excitations fixed based on our previous tracking simulation results. Exoskeleton torque lasted for 15% of the gait cycle and was applied between foot-flat and toe-off during the stance phase, and changes in center of mass kinematics were recorded when the torque application ended. We found that changes in center of mass kinematics were dependent on both the type and timing of exoskeleton torques. Plantarflexion torques produced upward and backward changes in velocity of the center of mass in mid-stance and upward and smaller forward velocity changes near toe-off. Eversion and inversion torques primarily produced lateral and medial changes in velocity in mid-stance, respectively. Intrinsic muscle properties reduced kinematic changes from exoskeleton torques. Our results provide mappings between ankle plantarflexion and inversion-eversion torques and changes in center of mass kinematics which can inform designers building exoskeletons aimed at stabilizing balance during walking. Our simulations and software are freely available and allow researchers to explore the effects of applied torques on balance and gait.

  View details for DOI 10.1371/journal.pcbi.1010712

  View details for PubMedID 37549183

• Ten steps to becoming a musculoskeletal simulation expert: A half-century of progress and outlook for the future. Journal of biomechanics Uhlrich, S. D., Uchida, T. K., Lee, M. R., Delp, S. L. 2023; 154: 111623

  Abstract

  Over the past half-century, musculoskeletal simulations have deepened our knowledge of human and animal movement. This article outlines ten steps to becoming a musculoskeletal simulation expert so you can contribute to the next half-century of technical innovation and scientific discovery. We advocate looking to the past, present, and future to harness the power of simulations that seek to understand and improve mobility. Instead of presenting a comprehensive literature review, we articulate a set of ideas intended to help researchers use simulations effectively and responsibly by understanding the work on which today's musculoskeletal simulations are built, following established modeling and simulation principles, and branching out in new directions.

  View details for DOI 10.1016/j.jbiomech.2023.111623

  View details for PubMedID 37210923

• How Connecting the Legs with a Spring Improves Human Running Economy. bioRxiv : the preprint server for biology Stingel, J. P., Hicks, J. L., Uhlrich, S. D., Delp, S. L. 2023

  Abstract

  Connecting the legs with a spring attached to the shoelaces reduces the energy cost of running, but how the spring reduces the energy burden of individual muscles remains unknown. We generated muscle-driven simulations of seven individuals running with and without the spring to discern whether savings occurred during the stance phase or the swing phase, and to identify which muscles contributed to energy savings. We computed differences in muscle-level energy consumption, muscle activations, and changes in muscle-fiber velocity and force between running with and without the spring. Across participants, running with the spring reduced the measured rate of energy expenditure by 0.9 W/kg (8.3%). Simulations predicted a 1.4 W/kg (12.0%) reduction in the average rate of energy expenditure and correctly identified that the spring reduced rates of energy expenditure for all participants. Simulations showed most of the savings occurred during stance (1.5 W/kg), though the rate of energy expenditure was also reduced during swing (0.3 W/kg). The energetic savings were distributed across the quadriceps, hip flexor, hip abductor, hamstring, hip adductor, and hip extensor muscle groups, whereas no changes in the rate of energy expenditure were observed in the plantarflexor or dorsiflexor muscles. Energetic savings were facilitated by reductions in the rate of mechanical work performed by muscles and their estimated rate of heat production. The simulations provide insight into muscle-level changes that occur when utilizing an assistive device and the mechanisms by which a spring connecting the legs improves running economy.

  View details for DOI 10.1101/2023.04.03.535498

  View details for PubMedID 37066206

  View details for PubMedCentralID PMC10104051

• Can static optimization detect changes in peak medial knee contact forces induced by gait modifications? Journal of biomechanics Kaneda, J. M., Seagers, K. A., Uhlrich, S. D., Kolesar, J. A., Thomas, K. A., Delp, S. L. 2023; 152: 111569

  Abstract

  Medial knee contact force (MCF) is related to the pathomechanics of medial knee osteoarthritis. However, MCF cannot be directly measured in the native knee, making it difficult for therapeutic gait modifications to target this metric. Static optimization, a musculoskeletal simulation technique, can estimate MCF, but there has been little work validating its ability to detect changes in MCF induced by gait modifications. In this study, we quantified the error in MCF estimates from static optimization compared to measurements from instrumented knee replacements during normal walking and seven different gait modifications. We then identified minimum magnitudes of simulated MCF changes for which static optimization correctly identified the direction of change (i.e., whether MCF increased or decreased) at least 70% of the time. A full-body musculoskeletal model with a multi-compartment knee and static optimization was used to estimate MCF. Simulations were evaluated using experimental data from three subjects with instrumented knee replacements who walked with various gait modifications for a total of 115 steps. Static optimization underpredicted the first peak (mean absolute error = 0.16 bodyweights) and overpredicted the second peak (mean absolute error = 0.31 bodyweights) of MCF. Average root mean square error in MCF over stance phase was 0.32 bodyweights. Static optimization detected the direction of change with at least 70% accuracy for early-stance reductions, late-stance reductions, and early-stance increases in peak MCF of at least 0.10 bodyweights. These results suggest that a static optimization approach accurately detects the direction of change in early-stance medial knee loading, potentially making it a valuable tool for evaluating the biomechanical efficacy of gait modifications for knee osteoarthritis.

  View details for DOI 10.1016/j.jbiomech.2023.111569

  View details for PubMedID 37058768

• A scoping review of portable sensing for out-of-lab anterior cruciate ligament injury prevention and rehabilitation. NPJ digital medicine Tan, T., Gatti, A. A., Fan, B., Shea, K. G., Sherman, S. L., Uhlrich, S. D., Hicks, J. L., Delp, S. L., Shull, P. B., Chaudhari, A. S. 2023; 6 (1): 46

  Abstract

  Anterior cruciate ligament (ACL) injury and ACL reconstruction (ACLR) surgery are common. Laboratory-based biomechanical assessment can evaluate ACL injury risk and rehabilitation progress after ACLR; however, lab-based measurements are expensive and inaccessible to most people. Portable sensors such as wearables and cameras can be deployed during sporting activities, in clinics, and in patient homes. Although many portable sensing approaches have demonstrated promising results during various assessments related to ACL injury, they have not yet been widely adopted as tools for out-of-lab assessment. The purpose of this review is to summarize research on out-of-lab portable sensing applied to ACL and ACLR and offer our perspectives on new opportunities for future research and development. We identified 49 original research articles on out-of-lab ACL-related assessment; the most common sensing modalities were inertial measurement units, depth cameras, and RGB cameras. The studies combined portable sensors with direct feature extraction, physics-based modeling, or machine learning to estimate a range of biomechanical parameters (e.g., knee kinematics and kinetics) during jump-landing tasks, cutting, squats, and gait. Many of the reviewed studies depict proof-of-concept methods for potential future clinical applications including ACL injury risk screening, injury prevention training, and rehabilitation assessment. By synthesizing these results, we describe important opportunities that exist for clinical validation of existing approaches, using sophisticated modeling techniques, standardization of data collection, and creation of large benchmark datasets. If successful, these advances will enable widespread use of portable-sensing approaches to identify ACL injury risk factors, mitigate high-risk movements prior to injury, and optimize rehabilitation paradigms.

  View details for DOI 10.1038/s41746-023-00782-2

  View details for PubMedID 36934194

• Smartphone videos of the sit-to-stand test predict osteoarthritis and health outcomes in a nationwide study. NPJ digital medicine Boswell, M. A., Kidziński, Ł., Hicks, J. L., Uhlrich, S. D., Falisse, A., Delp, S. L. 2023; 6 (1): 32

  Abstract

  Physical function decline due to aging or disease can be assessed with quantitative motion analysis, but this currently requires expensive laboratory equipment. We introduce a self-guided quantitative motion analysis of the widely used five-repetition sit-to-stand test using a smartphone. Across 35 US states, 405 participants recorded a video performing the test in their homes. We found that the quantitative movement parameters extracted from the smartphone videos were related to a diagnosis of osteoarthritis, physical and mental health, body mass index, age, and ethnicity and race. Our findings demonstrate that at-home movement analysis goes beyond established clinical metrics to provide objective and inexpensive digital outcome metrics for nationwide studies.

  View details for DOI 10.1038/s41746-023-00775-1

  View details for PubMedID 36871119

  View details for PubMedCentralID 5009047

• PREDICTING CHRONIC KNEE PAIN USING AN AUTOMATED MRIBASED BONE AND CARTILAGE STATISTICAL SHAPE MODEL: DATA FROM THE OSTEOARTHRITIS INITIATIVE Gatti, A. A., Kogan, F., Delp, S. L., Gold, G. E., Chaudhari, A. S. ELSEVIER SCI LTD. 2023: S78-S79

  View details for Web of Science ID 001013148000088

• Effects of Wearable Fitness Trackers and Activity Adequacy Mindsets on Affect, Behavior, and Health: Longitudinal Randomized Controlled Trial. Journal of medical Internet research Zahrt, O. H., Evans, K., Murnane, E., Santoro, E., Baiocchi, M., Landay, J., Delp, S., Crum, A. 2023; 25: e40529

  Abstract

  There is some initial evidence suggesting that mindsets about the adequacy and health consequences of one's physical activity (activity adequacy mindsets [AAMs]) can shape physical activity behavior, health, and well-being. However, it is unknown how to leverage these mindsets using wearable technology and other interventions.This research examined how wearable fitness trackers and meta-mindset interventions influence AAMs, affect, behavior, and health.A total of 162 community-dwelling adults were recruited via flyers and web-based platforms (ie, Craigslist and Nextdoor; final sample size after attrition or exclusion of 45 participants). Participants received an Apple Watch (Apple Inc) to wear for 5 weeks, which was equipped with an app that recorded step count and could display a (potentially manipulated) step count on the watch face. After a baseline week of receiving no feedback about step count, participants were randomly assigned to 1 of 4 experimental groups: they received either accurate step count (reference group; 41/162, 25.3%), 40% deflated step count (40/162, 24.7%), 40% inflated step count (40/162, 24.7%), or accurate step count+a web-based meta-mindset intervention teaching participants the value of adopting more positive AAMs (41/162, 25.3%). Participants were blinded to the condition. Outcome measures were taken in the laboratory by an experimenter at the beginning and end of participation and via web-based surveys in between. Longitudinal analysis examined changes within the accurate step count condition from baseline to treatment and compared them with changes in the deflated step count, inflated step count, and meta-mindset conditions.Participants receiving accurate step counts perceived their activity as more adequate and healthier, adopted a healthier diet, and experienced improved mental health (Patient-Reported Outcomes Measurement Information System [PROMIS]-29) and aerobic capacity but also reduced functional health (PROMIS-29; compared with their no-step-count baseline). Participants exposed to deflated step counts perceived their activity as more inadequate; ate more unhealthily; and experienced more negative affect, reduced self-esteem and mental health, and increased blood pressure and heart rate (compared with participants receiving accurate step counts). Inflated step counts did not change AAM or most other outcomes (compared with accurate step counts). Participants receiving the meta-mindset intervention experienced improved AAM, affect, functional health, and self-reported physical activity (compared with participants receiving accurate step counts only). Actual step count did not change in either condition.AAMs--induced by trackers or adopted deliberately--can influence affect, behavior, and health independently of actual physical activity.ClinicalTrials.gov NCT03939572; https://www.clinicaltrials.gov/ct2/show/NCT03939572.

  View details for DOI 10.2196/40529

  View details for PubMedID 36696172

• Digital medicine Digitising tremor LANCET Delp, S. L., Topol, E. J. 2023; 401 (10372): 187

  View details for Web of Science ID 000926827800001

• Digitising tremor. Lancet (London, England) Delp, S. L., Topol, E. J. 2023; 401 (10372): 187

  View details for DOI 10.1016/S0140-6736(23)00055-7

  View details for PubMedID 36681408

• Leveraging Mobile Technology for Public Health Promotion: A Multidisciplinary Perspective. Annual review of public health Hicks, J. L., Boswell, M. A., Althoff, T., Crum, A. J., Ku, J. P., Landay, J. A., Moya, P. M., Murnane, E. L., Snyder, M. P., King, A. C., Delp, S. L. 2022

  Abstract

  Health behaviors are inextricably linked to health and well-being, yet issues such as physical inactivity and insufficient sleep remain significant global public health problems. Mobile technology-and the unprecedented scope and quantity of data it generates-has a promising but largely untapped potential to promote health behaviors at the individual and population levels. This perspective article provides multidisciplinary recommendations on the design and use of mobile technology, and the concomitant wealth of data, to promote behaviors that support overall health. Using physical activity as an exemplar health behavior, we review emerging strategies for health behavior change interventions. We describe progress on personalizing interventions to an individual and their social, cultural, and built environments, as well as on evaluating relationships between mobile technology data and health to establish evidence-based guidelines. In reviewing these strategies and highlighting directions for future research, we advance the use of theory-based, personalized, and human-centered approaches in promoting health behaviors. Expected final online publication date for the Annual Review of Public Health, Volume 44 is April 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

  View details for DOI 10.1146/annurev-publhealth-060220-041643

  View details for PubMedID 36542772

• Confounds in neuroimaging: A clear case of sex as a confound in brain-based prediction. Frontiers in neurology Weber, K. A., Teplin, Z. M., Wager, T. D., Law, C. S., Prabhakar, N. K., Ashar, Y. K., Gilam, G., Banerjee, S., Delp, S. L., Glover, G. H., Hastie, T. J., Mackey, S. 2022; 13: 960760

  Abstract

  Muscle weakness is common in many neurological, neuromuscular, and musculoskeletal conditions. Muscle size only partially explains muscle strength as adaptions within the nervous system also contribute to strength. Brain-based biomarkers of neuromuscular function could provide diagnostic, prognostic, and predictive value in treating these disorders. Therefore, we sought to characterize and quantify the brain's contribution to strength by developing multimodal MRI pipelines to predict grip strength. However, the prediction of strength was not straightforward, and we present a case of sex being a clear confound in brain decoding analyses. While each MRI modality-structural MRI (i.e., gray matter morphometry), diffusion MRI (i.e., white matter fractional anisotropy), resting state functional MRI (i.e., functional connectivity), and task-evoked functional MRI (i.e., left or right hand motor task activation)-and a multimodal prediction pipeline demonstrated significant predictive power for strength (R 2 = 0.108-0.536, p ≤ 0.001), after correcting for sex, the predictive power was substantially reduced (R 2 = -0.038-0.075). Next, we flipped the analysis and demonstrated that each MRI modality and a multimodal prediction pipeline could significantly predict sex (accuracy = 68.0%-93.3%, AUC = 0.780-0.982, p < 0.001). However, correcting the brain features for strength reduced the accuracy for predicting sex (accuracy = 57.3%-69.3%, AUC = 0.615-0.780). Here we demonstrate the effects of sex-correlated confounds in brain-based predictive models across multiple brain MRI modalities for both regression and classification models. We discuss implications of confounds in predictive modeling and the development of brain-based MRI biomarkers, as well as possible strategies to overcome these barriers.

  View details for DOI 10.3389/fneur.2022.960760

  View details for PubMedID 36601297

  View details for PubMedCentralID PMC9806266

• Independently ambulatory children with spina bifida experience near-typical knee and ankle joint moments and forces during walking. Gait & posture Lee, M. R., Hicks, J. L., Wren, T. A., Delp, S. L. 2022; 99: 1-8

  Abstract

  BACKGROUND: Spina bifida, a neurological defect, can result in lower-limb muscle weakness. Altered ambulation and reduced musculoskeletal loading can yield decreased bone strength in individuals with spina bifida, yet individuals who remain ambulatory can exhibit normal bone outcomes.RESEARCH QUESTION: During walking, how do lower-limb joint kinematics and moments and tibial forces in independently ambulatory children with spina bifida differ from those of children with typical development?METHODS: We retrospectively analyzed data from 16 independently ambulatory children with spina bifida and 16 children with typical development and confirmed that tibial bone strength was similar between the two groups. Plantar flexor muscle strength was measured by manual muscle testing, and 14 of the children with spina bifida wore activity monitors for an average of 5 days. We estimated tibial forces at the knee and ankle using motion capture data and musculoskeletal simulations. We used Statistical Parametric Mapping t-tests to compare lower-limb joint kinematic and kinetic waveforms between the groups with spina bifida and typical development. Within the group with spina bifida, we examined relationships between plantar flexor muscle strength and peak tibial forces by calculating Spearman correlations.RESULTS: Activity monitors from the children with spina bifida reported typical daily steps (9656 [SD 3095]). Despite slower walking speeds (p=0.004) and altered lower-body kinematics (p<0.001), children with spina bifida had knee and ankle joint moments and forces similar to those of children with typical development, with no detectable differences during stance. Plantar flexor muscle weakness was associated with increased compressive knee force (p=0.002) and shear ankle force (p=0.009).SIGNIFICANCE: High-functioning, independently ambulatory children with spina bifida exhibited near-typical tibial bone strength and near-typical step counts and tibial load magnitudes. Our results suggest that the tibial forces in this group are of sufficient magnitudes to support the development of normal tibial bone strength.

  View details for DOI 10.1016/j.gaitpost.2022.10.010

  View details for PubMedID 36283301

• Personalizing exoskeleton assistance while walking in the real world. Nature Slade, P., Kochenderfer, M. J., Delp, S. L., Collins, S. H. 2022; 610 (7931): 277-282

  Abstract

  Personalized exoskeleton assistance provides users with the largest improvements in walking speed1 and energy economy2-4 but requires lengthy tests under unnatural laboratory conditions. Here we show that exoskeleton optimization can be performed rapidly and under real-world conditions. We designed a portable ankle exoskeleton based on insights from tests with a versatile laboratory testbed. We developed a data-driven method for optimizing exoskeleton assistance outdoors using wearable sensors and found that it was equally effective as laboratory methods, but identified optimal parameters four times faster. We performed real-world optimization using data collected during many short bouts of walking at varying speeds. Assistance optimized during one hour of naturalistic walking in a public setting increased self-selected speed by 9±4% and reduced the energy used to travel a given distance by 17±5% compared with normal shoes. This assistance reduced metabolic energy consumption by 23±8% when participants walked on a treadmill at a standard speed of 1.5ms-1. Human movements encode information that can be used to personalize assistive devices and enhance performance.

  View details for DOI 10.1038/s41586-022-05191-1

  View details for PubMedID 36224415

• Personalization improves the biomechanical efficacy of foot progression angle modifications in individuals with medial knee osteoarthritis. Journal of biomechanics Uhlrich, S. D., Kolesar, J. A., Kidzinski, L., Boswell, M. A., Silder, A., Gold, G. E., Delp, S. L., Beaupre, G. S. 2022; 144: 111312

  Abstract

  Modifying the foot progression angle during walking can reduce the knee adduction moment, a surrogate measure of medial knee loading. However, not all individuals reduce their knee adduction moment with the same modification. This study evaluates whether a personalized approach to prescribing foot progression angle modifications increases the proportion of individuals with medial knee osteoarthritis who reduce their knee adduction moment, compared to a non-personalized approach. Individuals with medial knee osteoarthritis (N=107) walked with biofeedback instructing them to toe-in and toe-out by 5° and 10° relative to their self-selected angle. We selected individuals' personalized foot progression angle as the modification that maximally reduced their larger knee adduction moment peak. Additionally, we used lasso regression to identify which secondary kinematic changes made a 10° toe-in gait modification more effective at reducing the first knee adduction moment peak. Seventy percent of individuals reduced their larger knee adduction moment peak by at least 5% with a personalized foot progression angle modification, which was more than (p≤0.002) the 23-57% of individuals who reduced it with a uniformly assigned 5° or 10° toe-in or toe-out modification. When toeing-in, greater reductions in the first knee adduction moment peak were related to an increased frontal-plane tibia angle (knee more medial than ankle), a more valgus knee abduction angle, reduced contralateral pelvic drop, and a more medialized center of pressure in the foot reference frame. In summary, personalization increases the proportion of individuals with medial knee osteoarthritis who may benefit from a foot progression angle modification.

  View details for DOI 10.1016/j.jbiomech.2022.111312

  View details for PubMedID 36191434

• Botulinum neurotoxin type A improves vasti muscle balance, patellar tracking, and pain in patients with chronic patellofemoral pain. Journal of orthopaedic research : official publication of the Orthopaedic Research Society Pal, S., Choi, J., Delp, S. L., Fredericson, M. 2022

  Abstract

  The purpose of this study was to determine the effects of botulinum neurotoxin type A (BoNT-A) on vastus lateralis:vastus medialis (VL:VM) muscle balance, patellar tracking, and pain in patients with chronic patellofemoral (PF) pain. We recruited 13 participants (9 females, 4 males) with recalcitrant PF pain who underwent ultrasound-guided BoNT-A injections into the distal third of the VL muscle, followed by a 6-week home exercise program to strengthen their VM muscle. We imaged the participants in a C-arm computed tomography (CT) scanner before and after the intervention. We calculated VL:VM ratios from CT images from a supine, non-weightbearing condition. We obtained patellar tilt and bisect offset values from CT images from an upright, weight-bearing condition. We recorded functional pain scores before, immediately after, and 2 to 4 years after the intervention. We classified the participants into normal tracking and maltracking groups based on their patellar tilt and bisect offset values. BoNT-A with home exercise reduced VL:VM ratio (18%; p < 0.001), patellar tilt (19%; p = 0.020), and bisect offset (5%; p = 0.025). Four participants classified as maltrackers prior to the intervention transitioned to normal tracking after the intervention. Functional pain scores improved immediately after the intervention (13%, p < 0.001) and remained improved at 2 year follow up (12%, p = 0.011). Statement of Clinical Significance: This study provides new evidence in support of BoNT-A for treatment of PF pain. Classification of patients under weight-bearing condition may identify individuals who will most benefit from a BoNT-A treatment. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/jor.25435

  View details for PubMedID 36031589

• Muscle coordination retraining inspired by musculoskeletal simulations reduces knee contact force. Scientific reports Uhlrich, S. D., Jackson, R. W., Seth, A., Kolesar, J. A., Delp, S. L. 2022; 12 (1): 9842

  Abstract

  Humans typically coordinate their muscles to meet movement objectives like minimizing energy expenditure. In the presence of pathology, new objectives gain importance, like reducing loading in an osteoarthritic joint, but people often do not change their muscle coordination patterns to meet these new objectives. Here we use musculoskeletal simulations to identify simple changes in coordination that can be taught using electromyographic biofeedback, achieving the therapeutic goal of reducing joint loading. Our simulations predicted that changing the relative activation of two redundant ankle plantarflexor muscles-the gastrocnemius and soleus-could reduce knee contact force during walking, but it was unclear whether humans could re-coordinate redundant muscles during a complex task like walking. Our experiments showed that after a single session of walking with biofeedback of summary measures of plantarflexor muscle activation, healthy individuals reduced the ratio of gastrocnemius-to-soleus muscle activation by 25±15% (p=0.004, paired t test, n=10). Participants who walked with this "gastrocnemius avoidance" gait pattern reduced late-stance knee contact force by 12±12% (p=0.029, paired t test, n=8). Simulation-informed coordination retraining could be a promising treatment for knee osteoarthritis and a powerful tool for optimizing coordination for a variety of rehabilitation and performance applications.

  View details for DOI 10.1038/s41598-022-13386-9

  View details for PubMedID 35798755

• Changes in foot progression angle during gait reduce the knee adduction moment and do not increase hip moments in individuals with knee osteoarthritis. Journal of biomechanics Seagers, K., Uhlrich, S. D., Kolesar, J. A., Berkson, M., Kaneda, J. M., Beaupre, G. S., Delp, S. L. 2022; 141: 111204

  Abstract

  People with knee osteoarthritis who adopt a modified foot progression angle (FPA) during gait often benefit from a reduction in the knee adduction moment. It is unknown, however, whether changes in the FPA increase hip moments, a surrogate measure of hip loading, which will increase the mechanical demand on the joint. This study examined how altering the FPA affects hip moments. Individuals with knee osteoarthritis walked on an instrumented treadmill with their baseline gait, 10° toe-in gait, and 10° toe-out gait. A musculoskeletal modeling package was used to compute joint moments from the experimental data. Fifty participants were selected from a larger study who reduced their peak knee adduction moment with a modified FPA. In this group, participants reduced the first peak of the knee adduction moment by 7.6% with 10° toe-in gait and reduced the second peak by 11.0% with 10° toe-out gait. Modifying the FPA reduced the early-stance hip abduction moment, at the time of peak hip contact force, by 4.3% ± 1.3% for 10° toe-in gait (p = 0.005, d = 0.49) and by 4.6% ± 1.1% for 10° toe-out gait (p < 0.001, d = 0.59) without increasing the flexion and internal rotation moments (p > 0.15). Additionally, 74% of individuals reduced their total hip moment at time of peak hip contact force with a modified FPA. In summary, when adopting a FPA modification that reduced the knee adduction moment, participants, on average, did not increase surrogate measures of hip loading.

  View details for DOI 10.1016/j.jbiomech.2022.111204

  View details for PubMedID 35772243

• Medical and Biomechanical Risk Factors for Incident Bone Stress Injury in Collegiate Runners: Can Plantar Pressure Predict Injury? Orthopaedic journal of sports medicine Wilzman, A. R., Tenforde, A. S., Troy, K. L., Hunt, K., Fogel, N., Roche, M. D., Kraus, E., Trikha, R., Delp, S., Fredericson, M. 2022; 10 (6): 23259671221104793

  Abstract

  Background: Bone stress injury (BSI) is a common reason for missed practices and competitions in elite track and field runners.Hypothesis: It was hypothesized that, after accounting for medical risk factors, higher plantar loading during running, walking, and athletic movements would predict the risk of future BSI in elite collegiate runners.Study Design: Cohort study; Level of evidence, 2.Methods: A total of 39 elite collegiate runners (24 male, 15 female) were evaluated during the 2014-2015 academic year to determine the degree to which plantar pressure data and medical history (including Female and Male Athlete Triad risk factors) could predict subsequent BSI. Runners completed athletic movements while plantar pressures and contact areas in 7 key areas of the foot were recorded, and the measurements were reported overall and by specific foot area. Regression models were constructed to determine factors related to incident BSI.Results: Twenty-one runners (12 male, 9 female) sustained ≥1 incident BSI during the study period. Four regression models incorporating both plantar pressure measurements and medical risk factors were able to predict the subsequent occurrence of (A) BSIs in female runners, (B) BSIs in male runners, (C) multiple BSIs in either male or female runners, and (D) foot BSIs in female runners. Model A used maximum mean pressure (MMP) under the first metatarsal during a jump takeoff and only misclassified 1 female with no BSI. Model B used increased impulses under the hindfoot and second through fifth distal metatarsals while walking, and under the lesser toes during a cutting task, correctly categorizing 83.3% of male runners. Model C used higher medial midfoot peak pressure during a shuttle run and triad cumulative risk scores and correctly categorized 93.3% of runners who did not incur multiple BSIs and 66.7% of those who did. Model D included lower hindfoot impulses in the shuttle run and higher first metatarsal MMP during treadmill walking to correctly predict the subsequent occurrence of a foot BSI for 75% of women and 100% without.Conclusion: The models collectively suggested that higher plantar pressure may contribute to risk for BSI.

  View details for DOI 10.1177/23259671221104793

  View details for PubMedID 35734769

• Mindset is associated with future physical activity and management strategies in individuals with knee osteoarthritis. Annals of physical and rehabilitation medicine Boswell, M. A., Evans, K. M., Zion, S. R., Boles, D. Z., Hicks, J. L., Delp, S. L., Crum, A. J. 2022; 65 (6): 101634

  Abstract

  Despite the benefits of physical activity for individuals with knee osteoarthritis (KOA), physical activity levels are low in this population.We conducted a repeated cross-sectional study to compare mindset about physical activity among individuals with and without KOA and to investigate whether mindset relates to physical activity.Participants with (n = 150) and without (n = 152) KOA completed an online survey at enrollment (T1). Participants with KOA repeated the survey 3 weeks later (T2; n = 62). The mindset questionnaire, scored from 1 to 4, assessed the extent to which individuals associate the process of exercising with less appeal-focused qualities (e.g., boring, painful, isolating, and depriving) versus appeal-focused (e.g., fun, pleasurable, social, and indulgent). Using linear regression, we examined the relationship between mindset and having KOA, and, in the subgroup of KOA participants, the relationship between mindset at T1 and self-reported physical activity at T2. We also compared mindset between people who use medication for management and those who use exercise.Within the KOA group, a more appeal-focused mindset was associated with higher future physical activity (β=38.72, p = 0.006) when controlling for demographics, health, and KOA symptoms. Individuals who used exercise with or without pain medication or injections had a more appeal-focused mindset than those who used medication or injections without exercise (p<0.001). A less appeal-focused mindset regarding physical activity was not significantly associated with KOA (β = -0.14, p = 0.067). Further, the mindset score demonstrated strong internal consistency (α = 0.92; T1; n = 150 and α = 0.92; T2; n = 62) and test-retest reliability (intraclass correlation coefficient (ICC) > 0.84, p < 0.001) within the KOA sample.In individuals with KOA, mindset is associated with future physical activity levels and relates to the individual's management strategy. Mindset is a reliable and malleable construct and may be a valuable target for increasing physical activity and improving adherence to rehabilitation strategies involving exercise among individuals with KOA.

  View details for DOI 10.1016/j.rehab.2022.101634

  View details for PubMedID 35091113

• Running in the wild: Energetics explain ecological running speeds. Current biology : CB Selinger, J. C., Hicks, J. L., Jackson, R. W., Wall-Scheffler, C. M., Chang, D., Delp, S. L. 2022

  Abstract

  Human runners have long been thought to have the ability to consume a near-constant amount of energy per distance traveled, regardless of speed, allowing speed to be adapted to particular task demands with minimal energetic consequence.1-3 However, recent and more precise laboratory measures indicate that humans may in fact have an energy-optimal running speed.4-6 Here, we characterize runners' speeds in a free-living environment and determine if preferred speed is consistent with task- or energy-dependent objectives. We analyzed a large-scale dataset of free-living runners, which was collected via a commercial fitness tracking device, and found that individual runners preferred a particular speed that did not change across commonly run distances. We compared the data from lab experiments that measured participants' energy-optimal running speeds with the free-living preferred speeds of age- and gender-matched runners in our dataset and found the speeds to be indistinguishable. Human runners prefer a particular running speed that is independent of task distance and is consistent with the objective of minimizing energy expenditure. Our findings offer an insight into the biological objectives that shape human running preferences in the real world-an important consideration when examining human ecology or creating training strategies to improve performance and prevent injury.

  View details for DOI 10.1016/j.cub.2022.03.076

  View details for PubMedID 35487220

• OpenSense: An open-source toolbox for inertial-measurement-unit-based measurement of lower extremity kinematics over long durations. Journal of neuroengineering and rehabilitation Al Borno, M., O'Day, J., Ibarra, V., Dunne, J., Seth, A., Habib, A., Ong, C., Hicks, J., Uhlrich, S., Delp, S. 2022; 19 (1): 22

  Abstract

  BACKGROUND: The ability to measure joint kinematics in natural environments over long durations using inertial measurement units (IMUs) could enable at-home monitoring and personalized treatment of neurological and musculoskeletal disorders. However, drift, or the accumulation of error over time, inhibits the accurate measurement of movement over long durations. We sought to develop an open-source workflow to estimate lower extremity joint kinematics from IMU data that was accurate and capable of assessing and mitigating drift.METHODS: We computed IMU-based estimates of kinematics using sensor fusion and an inverse kinematics approach with a constrained biomechanical model. We measured kinematics for 11 subjects as they performed two 10-min trials: walking and a repeated sequence of varied lower-extremity movements. To validate the approach, we compared the joint angles computed with IMU orientations to the joint angles computed from optical motion capture using root mean square (RMS) difference and Pearson correlations, and estimated drift using a linear regression on each subject's RMS differences over time.RESULTS: IMU-based kinematic estimates agreed with optical motion capture; median RMS differences over all subjects and all minutes were between 3 and 6 degrees for all joint angles except hip rotation and correlation coefficients were moderate to strong (r=0.60-0.87). We observed minimal drift in the RMS differences over 10min; the average slopes of the linear fits to these data were near zero (- 0.14-0.17deg/min).CONCLUSIONS: Our workflow produced joint kinematics consistent with those estimated by optical motion capture, and could mitigate kinematic drift even in the trials of continuous walking without rest, which may obviate the need for explicit sensor recalibration (e.g. sitting or standing still for a few seconds or zero-velocity updates) used in current drift-mitigation approaches when studying similar activities. This could enable long-duration measurements, bringing the field one step closer to estimating kinematics in natural environments.

  View details for DOI 10.1186/s12984-022-01001-x

  View details for PubMedID 35184727

• Non-invasive electrical stimulation of peripheral nerves for the management of tremor. Journal of the neurological sciences Pascual-Valdunciel, A., Rajagopal, A., Pons, J. L., Delp, S. 2022; 435: 120195

  Abstract

  Pathological tremor in patients with essential tremor and Parkinsons disease is typically treated using medication or neurosurgical interventions. There is a widely recognized need for new treatments that avoid the side effects of current medications and do not carry the risks of surgical interventions. Building on decades of research and engineering development, non-invasive electrical stimulation of peripheral nerves has emerged as a safe and effective strategy for reducing pathologic tremor in essential tremor. This review surveys the peripheral electrical stimulation (PES) literature and summarizes effectiveness, safety, clinical translatability, and hypothesized tremor-reduction mechanisms of various PES approaches. The review also proposes guidelines for assessing tremor in the context of evaluating new therapies that combine the strengths of clinician assessments, patient evaluations, and novel motion sensing technology. The review concludes with a summary of future directions for PES, including expanding clinical access for patients with Parkinson's disease and leveraging large, at-home datasets to learn more about tremor physiology and treatment effect that will better characterize the state of tremor management and accelerate discovery of new therapies. Growing evidence suggests that non-invasive electrical stimulation of afferent neural pathways provides a viable new option for management of pathological tremor, with one specific PES therapy cleared for prescription and home use, suggesting that PES be considered along with medication and neurosurgical interventions for treatment of tremor. This article is part of the Special Issue "Tremor" edited by Daniel D. Truong, Mark Hallett, and Aasef Shaikh.

  View details for DOI 10.1016/j.jns.2022.120195

  View details for PubMedID 35220113

• Assessing inertial measurement unit locations for freezing of gait detection and patient preference. Journal of neuroengineering and rehabilitation O'Day, J., Lee, M., Seagers, K., Hoffman, S., Jih-Schiff, A., Kidzinski, L., Delp, S., Bronte-Stewart, H. 2022; 19 (1): 20

  Abstract

  BACKGROUND: Freezing of gait, a common symptom of Parkinson's disease, presents as sporadic episodes in which an individual's feet suddenly feel stuck to the ground. Inertial measurement units (IMUs) promise to enable at-home monitoring and personalization of therapy, but there is a lack of consensus on the number and location of IMUs for detecting freezing of gait. The purpose of this study was to assess IMU sets in the context of both freezing of gait detection performance and patient preference.METHODS: Sixteen people with Parkinson's disease were surveyed about sensor preferences. Raw IMU data from seven people with Parkinson's disease, wearing up to eleven sensors, were used to train convolutional neural networks to detect freezing of gait. Models trained with data from different sensor sets were assessed for technical performance; a best technical set and minimal IMU set were identified. Clinical utility was assessed by comparing model- and human-rater-determined percent time freezing and number of freezing events.RESULTS: The best technical set consisted of three IMUs (lumbar and both ankles, AUROC=0.83), all of which were rated highly wearable. The minimal IMU set consisted of a single ankle IMU (AUROC=0.80). Correlations between these models and human raters were good to excellent for percent time freezing (ICC=0.93, 0.89) and number of freezing events (ICC=0.95, 0.86) for the best technical set and minimal IMU set, respectively.CONCLUSIONS: Several IMU sets consisting of three IMUs or fewer were highly rated for both technical performance and wearability, and more IMUs did not necessarily perform better in FOG detection. We openly share our data and software to further the development and adoption of a general, open-source model that uses raw signals and a standard sensor set for at-home monitoring of freezing of gait.

  View details for DOI 10.1186/s12984-022-00992-x

  View details for PubMedID 35152881

• Coupled exoskeleton assistance simplifies control and maintains metabolic benefits: A simulation study. PloS one Bianco, N. A., Franks, P. W., Hicks, J. L., Delp, S. L. 1800; 17 (1): e0261318

  Abstract

  Assistive exoskeletons can reduce the metabolic cost of walking, and recent advances in exoskeleton device design and control have resulted in large metabolic savings. Most exoskeleton devices provide assistance at either the ankle or hip. Exoskeletons that assist multiple joints have the potential to provide greater metabolic savings, but can require many actuators and complicated controllers, making it difficult to design effective assistance. Coupled assistance, when two or more joints are assisted using one actuator or control signal, could reduce control dimensionality while retaining metabolic benefits. However, it is unknown which combinations of assisted joints are most promising and if there are negative consequences associated with coupled assistance. Since designing assistance with human experiments is expensive and time-consuming, we used musculoskeletal simulation to evaluate metabolic savings from multi-joint assistance and identify promising joint combinations. We generated 2D muscle-driven simulations of walking while simultaneously optimizing control strategies for simulated lower-limb exoskeleton assistive devices to minimize metabolic cost. Each device provided assistance either at a single joint or at multiple joints using massless, ideal actuators. To assess if control could be simplified for multi-joint exoskeletons, we simulated different control strategies in which the torque provided at each joint was either controlled independently or coupled between joints. We compared the predicted optimal torque profiles and changes in muscle and total metabolic power consumption across the single joint and multi-joint assistance strategies. We found multi-joint devices-whether independent or coupled-provided 50% greater metabolic savings than single joint devices. The coupled multi-joint devices were able to achieve most of the metabolic savings produced by independently-controlled multi-joint devices. Our results indicate that device designers could simplify multi-joint exoskeleton designs by reducing the number of torque control parameters through coupling, while still maintaining large reductions in metabolic cost.

  View details for DOI 10.1371/journal.pone.0261318

  View details for PubMedID 34986191

• Biceps femoris long head sarcomere and fascicle length adaptations after three weeks of eccentric exercise training. Journal of sport and health science Pincheira, P. A., Boswell, M. A., Franchi, M. V., Delp, S. L., Lichtwark, G. A. 2021

  Abstract

  BACKGROUND: Eccentric exercise increases muscle fascicle lengths; however, the mechanisms behind this adaptation are still unknown. This study aimed to determine whether biceps femoris long head (BFlh)1 fascicle length increases in response to 3 weeks of eccentric exercise training are the result of an in-series addition of sarcomeres within the muscle fibers.METHODS: Ten recreationally active participants (age: 27 ± 3 years; mass: 70 ± 14 kg; height: 174 ± 9 cm; mean ± SD) completed 3 weeks of Nordic hamstring exercise (NHE)1 training on a custom exercise device that was instrumented with load cells. We collected in vivo sarcomere and muscle fascicle images of the BFlh in 2 regions (central and distal), using microendoscopy and 3D ultrasonography. We then estimated sarcomere length, sarcomere number, and fascicle length before and after the training intervention.RESULTS: Eccentric knee flexion strength increased after the training (15%; p < 0.001; etap2 = 0.75). Further, we found a significant increase in fascicle length (21%; p < 0.001; etap2 = 0.81) and sarcomere length (17%; p < 0.001; etap2 = 0.90) in the distal but not in the central portion of the muscle. The estimated number of sarcomeres in series did not change in either region.CONCLUSION: Fascicle length adaptations appear to be heterogeneous in the BFlh in response to 3 weeks of NHE training. An increase in sarcomere length, rather than the addition of sarcomeres in series, appears to underlie increases in fascicle length in the distal region of the BFlh. The mechanism driving regional increases in fascicle and sarcomere length remains unknown, but we speculate it may be driven by regional changes in the passive tension of muscle or connective tissue adaptations.

  View details for DOI 10.1016/j.jshs.2021.09.002

  View details for PubMedID 34509714

• Open Source Software for Automatic Subregional Assessment of Knee Cartilage Degradation Using Quantitative T2 Relaxometry and Deep Learning. Cartilage Thomas, K. A., Krzeminski, D., Kidzinski, L., Paul, R., Rubin, E. B., Halilaj, E., Black, M. S., Chaudhari, A., Gold, G. E., Delp, S. L. 2021: 19476035211042406

  Abstract

  OBJECTIVE: We evaluated a fully automated femoral cartilage segmentation model for measuring T2 relaxation values and longitudinal changes using multi-echo spin-echo (MESE) magnetic resonance imaging (MRI). We open sourced this model and developed a web app available at https://kl.stanford.edu into which users can drag and drop images to segment them automatically.DESIGN: We trained a neural network to segment femoral cartilage from MESE MRIs. Cartilage was divided into 12 subregions along medial-lateral, superficial-deep, and anterior-central-posterior boundaries. Subregional T2 values and four-year changes were calculated using a radiologist's segmentations (Reader 1) and the model's segmentations. These were compared using 28 held-out images. A subset of 14 images were also evaluated by a second expert (Reader 2) for comparison.RESULTS: Model segmentations agreed with Reader 1 segmentations with a Dice score of 0.85 ± 0.03. The model's estimated T2 values for individual subregions agreed with those of Reader 1 with an average Spearman correlation of 0.89 and average mean absolute error (MAE) of 1.34 ms. The model's estimated four-year change in T2 for individual subregions agreed with Reader 1 with an average correlation of 0.80 and average MAE of 1.72 ms. The model agreed with Reader 1 at least as closely as Reader 2 agreed with Reader 1 in terms of Dice score (0.85 vs. 0.75) and subregional T2 values.CONCLUSIONS: Assessments of cartilage health using our fully automated segmentation model agreed with those of an expert as closely as experts agreed with one another. This has the potential to accelerate osteoarthritis research.

  View details for DOI 10.1177/19476035211042406

  View details for PubMedID 34496667

• Deep reinforcement learning for modeling human locomotion control in neuromechanical simulation. Journal of neuroengineering and rehabilitation Song, S., Kidzinski, L., Peng, X. B., Ong, C., Hicks, J., Levine, S., Atkeson, C. G., Delp, S. L. 2021; 18 (1): 126

  Abstract

  Modeling human motor control and predicting how humans will move in novel environments is a grand scientific challenge. Researchers in the fields of biomechanics and motor control have proposed and evaluated motor control models via neuromechanical simulations, which produce physically correct motions of a musculoskeletal model. Typically, researchers have developed control models that encode physiologically plausible motor control hypotheses and compared the resulting simulation behaviors to measurable human motion data. While such plausible control models were able to simulate and explain many basic locomotion behaviors (e.g. walking, running, and climbing stairs), modeling higher layer controls (e.g. processing environment cues, planning long-term motion strategies, and coordinating basic motor skills to navigate in dynamic and complex environments) remains a challenge. Recent advances in deep reinforcement learning lay a foundation for modeling these complex control processes and controlling a diverse repertoire of human movement; however, reinforcement learning has been rarely applied in neuromechanical simulation to model human control. In this paper, we review the current state of neuromechanical simulations, along with the fundamentals of reinforcement learning, as it applies to human locomotion. We also present a scientific competition and accompanying software platform, which we have organized to accelerate the use of reinforcement learning in neuromechanical simulations. This "Learn to Move" competition was an official competition at the NeurIPS conference from 2017 to 2019 and attracted over 1300 teams from around the world. Top teams adapted state-of-the-art deep reinforcement learning techniques and produced motions, such as quick turning and walk-to-stand transitions, that have not been demonstrated before in neuromechanical simulations without utilizing reference motion data. We close with a discussion of future opportunities at the intersection of human movement simulation and reinforcement learning and our plans to extend the Learn to Move competition to further facilitate interdisciplinary collaboration in modeling human motor control for biomechanics and rehabilitation research.

  View details for DOI 10.1186/s12984-021-00919-y

  View details for PubMedID 34399772

• An open-source and wearable system for measuring 3D human motion in real-time. IEEE transactions on bio-medical engineering Slade, P., Habib, A., Hicks, J. L., Delp, S. L. 2021; PP

  Abstract

  OBJECTIVE: Analyzing human motion is essential for diagnosing movement disorders and guiding rehabilitation for conditions like osteoarthritis, stroke, and Parkinson's disease. Optical motion capture systems are the standard for estimating kinematics but require expensive equipment located in a predefined space. While wearable sensor systems can estimate kinematics in any environment, existing systems are generally less accurate than optical motion capture. Many wearable sensor systems require a computer in close proximity and use proprietary software, limiting experimental reproducibility.METHODS: Here, we present OpenSenseRT, an open-source and wearable system that estimates upper and lower extremity kinematics in real time by using inertial measurement units and a portable microcontroller.RESULTS: We compared the OpenSenseRT system to optical motion capture and found an average RMSE of 4.4 degrees across 5 lower-limb joint angles during three minutes of walking and an average RMSE of 5.6 degrees across 8 upper extremity joint angles during a Fugl-Meyer task. The open-source software and hardware are scalable, tracking 1 to 14 body segments, with one sensor per segment. A musculoskeletal model and inverse kinematics solver estimate Kinematics in real-time. The computation frequency depends on the number of tracked segments, but is sufficient for real-time measurement for many tasks of interest; for example, the system can track 7 segments at 30 Hz in real-time. The system uses off-the-shelf parts costing approximately 100 USD plus 20 for each tracked segment.SIGNIFICANCE: The OpenSenseRT system is validated against optical motion capture, low-cost, and simple to replicate, enabling movement analysis in clinics, homes, and free-living settings.

  View details for DOI 10.1109/TBME.2021.3103201

  View details for PubMedID 34383640

• Wearable sensors enable personalized predictions of clinical laboratory measurements. Nature medicine Dunn, J., Kidzinski, L., Runge, R., Witt, D., Hicks, J. L., Schussler-Fiorenza Rose, S. M., Li, X., Bahmani, A., Delp, S. L., Hastie, T., Snyder, M. P. 2021

  Abstract

  Vital signs, including heart rate and body temperature, are useful in detecting or monitoring medical conditions, but are typically measured in the clinic and require follow-up laboratory testing for more definitive diagnoses. Here we examined whether vital signs as measured by consumer wearable devices (that is, continuously monitored heart rate, body temperature, electrodermal activity and movement) can predict clinical laboratory test results using machine learning models, including random forest and Lasso models. Our results demonstrate that vital sign data collected from wearables give a more consistent and precise depiction of resting heart rate than do measurements taken in the clinic. Vital sign data collected from wearables can also predict several clinical laboratory measurements with lower prediction error than predictions made using clinically obtained vital sign measurements. The length of time over which vital signs are monitored and the proximity of the monitoring period to the date of prediction play a critical role in the performance of the machine learning models. These results demonstrate the value of commercial wearable devices for continuous and longitudinal assessment of physiological measurements that today can be measured only with clinical laboratory tests.

  View details for DOI 10.1038/s41591-021-01339-0

  View details for PubMedID 34031607

• IDENTIFYING CONCERNS OF YOUNG ADULTS WITH OSTEOARTHRITIS ON REDDIT Song, A. J., Boswell, M. A., Hicks, J. L., Delp, S. L. ELSEVIER SCI LTD. 2021: S227-S229

  View details for Web of Science ID 000642588500293

• MINDSETS PREDICT PHYSICAL ACTIVITY AND MANAGEMENT STRATEGIES IN INDIVIDUALS WITH KNEE OSTEOARTHRITIS Boswell, M. A., Hicks, J. L., Evans, K. M., Zion, S. R., Boles, D. Z., Delp, S. L., Crum, A. J. ELSEVIER SCI LTD. 2021: S222-S224

  View details for Web of Science ID 000642588500286

• OPEN SOURCE AND AUTOMATIC SUBREGIONAL ASSESSMENT OF KNEE CARTILAGE DEGRADATION USING QUANTITATIVE T2 RELAXOMETRY AND DEEP LEARNING Thomas, K. A., Krzeminski, D., Kidzinski, L., Paul, R., Rubin, E. B., Halilaj, E., Black, M. S., Chaudhari, A., Gold, G. E., Delp, S. L. ELSEVIER SCI LTD. 2021: S43-S44

  View details for Web of Science ID 000642588500050

• Assessment of Extractability and Accuracy of Electronic Health Record Data for Joint Implant Registries. JAMA network open Giori, N. J., Radin, J., Callahan, A., Fries, J. A., Halilaj, E., Re, C., Delp, S. L., Shah, N. H., Harris, A. H. 2021; 4 (3): e211728

  Abstract

  Importance: Implant registries provide valuable information on the performance of implants in a real-world setting, yet they have traditionally been expensive to establish and maintain. Electronic health records (EHRs) are widely used and may include the information needed to generate clinically meaningful reports similar to a formal implant registry.Objectives: To quantify the extractability and accuracy of registry-relevant data from the EHR and to assess the ability of these data to track trends in implant use and the durability of implants (hereafter referred to as implant survivorship), using data stored since 2000 in the EHR of the largest integrated health care system in the United States.Design, Setting, and Participants: Retrospective cohort study of a large EHR of veterans who had 45 351 total hip arthroplasty procedures in Veterans Health Administration hospitals from 2000 to 2017. Data analysis was performed from January 1, 2000, to December 31, 2017.Exposures: Total hip arthroplasty.Main Outcomes and Measures: Number of total hip arthroplasty procedures extracted from the EHR, trends in implant use, and relative survivorship of implants.Results: A total of 45 351 total hip arthroplasty procedures were identified from 2000 to 2017 with 192 805 implant parts. Data completeness improved over the time. After 2014, 85% of prosthetic heads, 91% of shells, 81% of stems, and 85% of liners used in the Veterans Health Administration health care system were identified by part number. Revision burden and trends in metal vs ceramic prosthetic femoral head use were found to reflect data from the American Joint Replacement Registry. Recalled implants were obvious negative outliers in implant survivorship using Kaplan-Meier curves.Conclusions and Relevance: Although loss to follow-up remains a challenge that requires additional attention to improve the quantitative nature of calculated implant survivorship, we conclude that data collected during routine clinical care and stored in the EHR of a large health system over 18 years were sufficient to provide clinically meaningful data on trends in implant use and to identify poor implants that were subsequently recalled. This automated approach was low cost and had no reporting burden. This low-cost, low-overhead method to assess implant use and performance within a large health care setting may be useful to internal quality assurance programs and, on a larger scale, to postmarket surveillance of implant performance.

  View details for DOI 10.1001/jamanetworkopen.2021.1728

  View details for PubMedID 33720372

• A neural network to predict the knee adduction moment in patients with osteoarthritis using anatomical landmarks obtainable from 2D video analysis. Osteoarthritis and cartilage Boswell, M. A., Uhlrich, S. D., Kidzinski, L., Thomas, K., Kolesar, J. A., Gold, G. E., Beaupre, G. S., Delp, S. L. 2021

  Abstract

  OBJECTIVE: The knee adduction moment (KAM) can inform treatment of medial knee osteoarthritis; however, measuring the KAM requires an expensive gait analysis laboratory. We evaluated the feasibility of predicting the peak KAM during natural and modified walking patterns using the positions of anatomical landmarks that could be identified from video analysis.METHOD: Using inverse dynamics, we calculated the KAM for 86 individuals (64 with knee osteoarthritis, 22 without) walking naturally and with foot progression angle modifications. We trained a neural network to predict the peak KAM using the 3-dimensional positions of 13 anatomical landmarks measured with motion capture (3D neural network). We also trained models to predict the peak KAM using 2-dimensional subsets of the dataset to simulate 2-dimensional video analysis (frontal and sagittal plane neural networks). Model performance was evaluated on a held-out, 8-person test set that included steps from all trials.RESULTS: The 3D neural network predicted the peak KAM for all test steps with r2=0.78. This model predicted individuals' average peak KAM during natural walking with r2=0.86 and classified which 15° foot progression angle modifications reduced the peak KAM with accuracy=0.85. The frontal plane neural network predicted peak KAM with similar accuracy (r2=0.85) to the 3D neural network, but the sagittal plane neural network did not (r2=0.14).CONCLUSION: Using the positions of anatomical landmarks from motion capture, a neural network accurately predicted the peak KAM during natural and modified walking. This study demonstrates the feasibility of measuring the peak KAM using positions obtainable from 2D video analysis.

  View details for DOI 10.1016/j.joca.2020.12.017

  View details for PubMedID 33422707

• Sensing leg movement enhances wearable monitoring of energy expenditure. Nature communications Slade, P., Kochenderfer, M. J., Delp, S. L., Collins, S. H. 2021; 12 (1): 4312

  Abstract

  Physical inactivity is the fourth leading cause of global mortality. Health organizations have requested a tool to objectively measure physical activity. Respirometry and doubly labeled water accurately estimate energy expenditure, but are infeasible for everyday use. Smartwatches are portable, but have significant errors. Existing wearable methods poorly estimate time-varying activity, which comprises 40% of daily steps. Here, we present a Wearable System that estimates metabolic energy expenditure in real-time during common steady-state and time-varying activities with substantially lower error than state-of-the-art methods. We perform experiments to select sensors, collect training data, and validate the Wearable System with new subjects and new conditions for walking, running, stair climbing, and biking. The Wearable System uses inertial measurement units worn on the shank and thigh as they distinguish lower-limb activity better than wrist or trunk kinematics and converge more quickly than physiological signals. When evaluated with a diverse group of new subjects, the Wearable System has a cumulative error of 13% across common activities, significantly less than 42% for a smartwatch and 44% for an activity-specific smartwatch. This approach enables accurate physical activity monitoring which could enable new energy balance systems for weight management or large-scale activity monitoring.

  View details for DOI 10.1038/s41467-021-24173-x

  View details for PubMedID 34257310

• A marker registration method to improve joint angles computed by constrained inverse kinematics. PloS one Dunne, J. J., Uchida, T. K., Besier, T. F., Delp, S. L., Seth, A. 2021; 16 (5): e0252425

  Abstract

  Accurate computation of joint angles from optical marker data using inverse kinematics methods requires that the locations of markers on a model match the locations of experimental markers on participants. Marker registration is the process of positioning the model markers so that they match the locations of the experimental markers. Markers are typically registered using a graphical user interface (GUI), but this method is subjective and may introduce errors and uncertainty to the calculated joint angles and moments. In this investigation, we use OpenSim to isolate and quantify marker registration-based error from other sources of error by analyzing the gait of a bipedal humanoid robot for which segment geometry, mass properties, and joint angles are known. We then propose a marker registration method that is informed by the orientation of anatomical reference frames derived from surface-mounted optical markers as an alternative to user registration using a GUI. The proposed orientation registration method reduced the average root-mean-square error in both joint angles and joint moments by 67% compared to the user registration method, and eliminated variability among users. Our results show that a systematic method for marker registration that reduces subjective user input can make marker registration more accurate and repeatable.

  View details for DOI 10.1371/journal.pone.0252425

  View details for PubMedID 34048476

• Transcutaneous Afferent Patterned Stimulation Therapy Reduces Hand Tremor for One Hour in Essential Tremor Patients FRONTIERS IN NEUROSCIENCE Yu, J. Y., Rajagopal, A., Syrkin-Nikolau, J., Shin, S., Rosenbluth, K. H., Khosla, D., Ross, E. K., Delp, S. L. 2020; 14: 530300

  Abstract

  Essential tremor (ET) patients often experience hand tremor that impairs daily activities. Non-invasive electrical stimulation of median and radial nerves in the wrist using a recently developed therapy called transcutaneous afferent patterned stimulation (TAPS) has been shown to provide symptomatic tremor relief in ET patients and improve patients' ability to perform functional tasks, but the duration of tremor reduction is unknown. In this single-arm, open-label study, fifteen ET patients performed four hand tremor-specific tasks (postural hold, spiral drawing, finger-to-nose reach, and pouring) from the Fahn-Tolosa-Marin Clinical Rating Scale (FTM-CRS) prior to, during, and 0, 30, and 60 min following TAPS. At each time point, tremor severity was visually rated according to the FTM-CRS and simultaneously measured by wrist-worn accelerometers. The duration of tremor reduction was assessed using (1) improvement in the mean FTM-CRS score across all four tasks relative to baseline, and (2) reduction in accelerometer-measured tremor power relative to baseline for each task. Patients were labeled as having at least 60 min of therapeutic benefit from TAPS with respect to each specified metric if all three (i.e., 0, 30, and 60 min) post-therapy measurements were better than that metric's baseline value. The mean FTM-CRS scores improved for at least 60 min beyond the end of TAPS for 80% (12 of 15, p = 4.6e-9) of patients. Similarly, for each assessed task, tremor power improved for at least 60 min beyond the end of TAPS for over 70% of patients. The postural hold task had the largest reduction in tremor power (median 5.9-fold peak reduction in tremor power) and had at least 60 min of improvement relative to baseline beyond the end of TAPS therapy for 73% (11 of 15, p = 9.8e-8) of patients. Clinical ratings of tremor severity were correlated to simultaneously recorded accelerometer-measured tremor power (r = 0.33-0.76 across the four tasks), suggesting tremor power is a valid, objective tremor assessment metric that can be used to track tremor symptoms outside the clinic. These results suggest TAPS can provide reductions in upper limb tremor symptoms for at least 1 h post-therapy in some patients, which may improve patients' ability to perform tasks of daily living.

  View details for DOI 10.3389/fnins.2020.530300

  View details for Web of Science ID 000592232200001

  View details for PubMedID 33281539

  View details for PubMedCentralID PMC7689107

• Brain Strength: Multi-Modal Brain MRI Predicts Grip Strength Weber, K. A., Wager, T. D., Upadhyayula, P. A., Law, C. S., Ashar, Y. K., Prabhakar, N. K., Zhu, S., Gilam, G., Banerjee, S., Delp, S. L., Glover, G. H., Hastie, T. J., Mackey, S. WILEY. 2020: S223–S224

  View details for Web of Science ID 000572509100411

• Prospective Home-use Study on Non-invasive Neuromodulation Therapy for Essential Tremor TREMOR AND OTHER HYPERKINETIC MOVEMENTS Isaacson, S. H., Peckham, E., Tse, W., Waln, O., Way, C., Petrossian, M. T., Dahodwala, N., Soileau, M. J., Lew, M., Dietiker, C., Luthra, N., Agarwal, P., Dhall, R., Morgan, J., Calakos, N., Zesiewicz, T. A., Shamim, E. A., Kumar, R., LeWitt, P., Shill, H. A., Simmons, A., Pagan, F. L., Khemani, P., Tate, J., Maddux, B., Luo, L., Ondo, W., Hallett, M., Rajagopal, A., Chidester, P., Rosenbluth, K. H., Delp, S. L., Pahwa, R. 2020; 10

  View details for DOI 10.5334/tohm.59

  View details for Web of Science ID 000560144300001

• Prospective Home-use Study on Non-invasive Neuromodulation Therapy for Essential Tremor. Tremor and other hyperkinetic movements (New York, N.Y.) Isaacson, S. H., Peckham, E., Tse, W., Waln, O., Way, C., Petrossian, M. T., Dahodwala, N., Soileau, M. J., Lew, M., Dietiker, C., Luthra, N., Agarwal, P., Dhall, R., Morgan, J., Calakos, N., Zesiewicz, T. A., Shamim, E. A., Kumar, R., LeWitt, P., Shill, H. A., Simmons, A., Pagan, F. L., Khemani, P., Tate, J., Maddux, B., Luo, L., Ondo, W., Hallett, M., Rajagopal, A., Chidester, P., Rosenbluth, K. H., Delp, S. L., Pahwa, R. 2020; 10: 29

  Abstract

  This prospective study is one of the largest clinical trials in essential tremor to date. Study findings suggest that individualized non-invasive neuromodulation therapy used repeatedly at home over three months results in safe and effective hand tremor reduction and improves quality of life for many essential tremor patients.Two previous randomized, controlled, single-session trials demonstrated efficacy of non-invasive neuromodulation therapy targeting the median and radial nerves for reducing hand tremor. This current study evaluated efficacy and safety of the therapy over three months of repeated home use.This was a prospective, open-label, post-clearance, single-arm study with 263 patients enrolled across 26 sites. Patients were instructed to use the therapy twice daily for three months. Pre-specified co-primary endpoints were improvements on clinician-rated Tremor Research Group Essential Tremor Rating Assessment Scale (TETRAS) and patient-rated Bain & Findley Activities of Daily Living (BF-ADL) dominant hand scores. Other endpoints included improvement in the tremor power detected by an accelerometer on the therapeutic device, Clinical and Patient Global Impression scores (CGI-I, PGI-I), and Quality of Life in Essential Tremor (QUEST) survey.205 patients completed the study. The co-primary endpoints were met (p≪0.0001), with 62% (TETRAS) and 68% (BF-ADL) of 'severe' or 'moderate' patients improving to 'mild' or 'slight'. Clinicians (CGI-I) reported improvement in 68% of patients, 60% (PGI-I) of patients reported improvement, and QUEST improved (p = 0.0019). Wrist-worn accelerometer recordings before and after 21,806 therapy sessions showed that 92% of patients improved, and 54% of patients experienced ≥50% improvement in tremor power. Device-related adverse events (e.g., wrist discomfort, skin irritation, pain) occurred in 18% of patients. No device-related serious adverse events were reported.This study suggests that non-invasive neuromodulation therapy used repeatedly at home over three months results in safe and effective hand tremor reduction in many essential tremor patients.

  View details for DOI 10.5334/tohm.59

  View details for PubMedID 32864188

  View details for PubMedCentralID PMC7427656

• The effects of motor modularity on performance, learning and generalizability in upper-extremity reaching: a computational analysis. Journal of the Royal Society, Interface Al Borno, M., Hicks, J. L., Delp, S. L. 2020; 17 (167): 20200011

  Abstract

  It has been hypothesized that the central nervous system simplifies the production of movement by limiting motor commands to a small set of modules known as muscle synergies. Recently, investigators have questioned whether a low-dimensional controller can produce the rich and flexible behaviours seen in everyday movements. To study this issue, we implemented muscle synergies in a biomechanically realistic model of the human upper extremity and performed computational experiments to determine whether synergies introduced task performance deficits, facilitated the learning of movements, and generalized to different movements. We derived sets of synergies from the muscle excitations our dynamic optimizations computed for a nominal task (reaching in a plane). Then we compared the performance and learning rates of a controller that activated all muscles independently to controllers that activated the synergies derived from the nominal reaching task. We found that a controller based on synergies had errors within 1 cm of a full-dimensional controller and achieved faster learning rates (as estimated from computational time to converge). The synergy-based controllers could also accomplish new tasks-such as reaching to targets on a higher or lower plane, and starting from alternative initial poses-with average errors similar to a full-dimensional controller.

  View details for DOI 10.1098/rsif.2020.0011

  View details for PubMedID 32486950

• Automated Classification of Radiographic Knee Osteoarthritis Severity Using Deep Neural Networks. Radiology. Artificial intelligence Thomas, K. A., Kidzinski, L., Halilaj, E., Fleming, S. L., Venkataraman, G. R., Oei, E. H., Gold, G. E., Delp, S. L. 2020; 2 (2): e190065

  Abstract

  Purpose: To develop an automated model for staging knee osteoarthritis severity from radiographs and to compare its performance to that of musculoskeletal radiologists.Materials and Methods: Radiographs from the Osteoarthritis Initiative staged by a radiologist committee using the Kellgren-Lawrence (KL) system were used. Before using the images as input to a convolutional neural network model, they were standardized and augmented automatically. The model was trained with 32116 images, tuned with 4074 images, evaluated with a 4090-image test set, and compared to two individual radiologists using a 50-image test subset. Saliency maps were generated to reveal features used by the model to determine KL grades.Results: With committee scores used as ground truth, the model had an average F1 score of 0.70 and an accuracy of 0.71 for the full test set. For the 50-image subset, the best individual radiologist had an average F1 score of 0.60 and an accuracy of 0.60; the model had an average F1 score of 0.64 and an accuracy of 0.66. Cohen weighted kappa between the committee and model was 0.86, comparable to intraexpert repeatability. Saliency maps identified sites of osteophyte formation as influential to predictions.Conclusion: An end-to-end interpretable model that takes full radiographs as input and predicts KL scores with state-of-the-art accuracy, performs as well as musculoskeletal radiologists, and does not require manual image preprocessing was developed. Saliency maps suggest the model's predictions were based on clinically relevant information. Supplemental material is available for this article. © RSNA, 2020.

  View details for DOI 10.1148/ryai.2020190065

  View details for PubMedID 32280948

• Optogenetic chronic neuromodulation of the diabetic cystopathy mouse model - histology and bladder tissue analysis Wallace, S., Tran, D., Briggs, M., Wen, Y., Montgomery, K., Zhuang, G., Dibberfuhl, A., Delp, S., Chen, B. WILEY. 2020: S48–S49

  View details for Web of Science ID 000519661200055

• Optogenetic chronic neuromodulation of the diabetic cystopathy mouse model - functional effect Wallace, S., Briggs, M., Wen, Y., Tran, D., Montgomery, K., Zhuang, G., Dobberfuhl, A., Delp, S., Chen, B. WILEY. 2020: S47–S48

  View details for Web of Science ID 000519661200054

• Optogenetic chronic neuromodulation of the diabetic cystopathy mouse model - study design Wallace, S., Wan, Y., Briggs, M., Tran, D., Montgomery, K., Zhuang, G., Dobberfuhl, A., Delp, S., Chen, B. WILEY. 2020: S44–S45

  View details for Web of Science ID 000519661200050

• Microendoscopy detects altered muscular contractile dynamics in a mouse model of amyotrophic lateral sclerosis. Scientific reports Chen, X., Sanchez, G. N., Schnitzer, M. J., Delp, S. L. 2020; 10 (1): 457

  Abstract

  Amyotrophic lateral sclerosis (ALS) is a fatal disease involving motor neuron degeneration. Effective diagnosis of ALS and quantitative monitoring of its progression are crucial to the success of clinical trials. Second harmonic generation (SHG) microendoscopy is an emerging technology for imaging single motor unit contractions. To assess the potential value of microendoscopy for diagnosing and tracking ALS, we monitored motor unit dynamics in a B6.SOD1G93A mouse model of ALS for several weeks. Prior to overt symptoms, muscle twitch rise and relaxation time constants both increased, consistent with a loss of fast-fatigable motor units. These effects became more pronounced with disease progression, consistent with the death of fast fatigue-resistant motor units and superior survival of slow motor units. From these measurements we constructed a physiological metric that reflects the changing distributions of measured motor unit time constants and effectively diagnoses mice before symptomatic onset and tracks disease state. These results indicate that SHG microendoscopy provides a means for developing a quantitative, physiologic characterization of ALS progression.

  View details for DOI 10.1038/s41598-019-56555-z

  View details for PubMedID 31949214

• OpenSim Moco: Musculoskeletal optimal control. PLoS computational biology Dembia, C. L., Bianco, N. A., Falisse, A. n., Hicks, J. L., Delp, S. L. 2020; 16 (12): e1008493

  Abstract

  Musculoskeletal simulations are used in many different applications, ranging from the design of wearable robots that interact with humans to the analysis of patients with impaired movement. Here, we introduce OpenSim Moco, a software toolkit for optimizing the motion and control of musculoskeletal models built in the OpenSim modeling and simulation package. OpenSim Moco uses the direct collocation method, which is often faster and can handle more diverse problems than other methods for musculoskeletal simulation. Moco frees researchers from implementing direct collocation themselves-which typically requires extensive technical expertise-and allows them to focus on their scientific questions. The software can handle a wide range of problems that interest biomechanists, including motion tracking, motion prediction, parameter optimization, model fitting, electromyography-driven simulation, and device design. Moco is the first musculoskeletal direct collocation tool to handle kinematic constraints, which enable modeling of kinematic loops (e.g., cycling models) and complex anatomy (e.g., patellar motion). To show the abilities of Moco, we first solved for muscle activity that produced an observed walking motion while minimizing squared muscle excitations and knee joint loading. Next, we predicted how muscle weakness may cause deviations from a normal walking motion. Lastly, we predicted a squat-to-stand motion and optimized the stiffness of an assistive device placed at the knee. We designed Moco to be easy to use, customizable, and extensible, thereby accelerating the use of simulations to understand the movement of humans and other animals.

  View details for DOI 10.1371/journal.pcbi.1008493

  View details for PubMedID 33370252

• Testing Simulated Assistance Strategies on a Hip-Knee-Ankle Exoskeleton: a Case Study Franks, P. W., Bianco, N. A., Bryan, G. M., Hicks, J. L., Delp, S. L., Collins, S. H., IEEE IEEE. 2020: 700-707

  View details for Web of Science ID 000636920600112

• The turning and barrier course reveals gait parameters for detecting freezing of gait and measuring the efficacy of deep brain stimulation. PloS one O'Day, J. n., Syrkin-Nikolau, J. n., Anidi, C. n., Kidzinski, L. n., Delp, S. n., Bronte-Stewart, H. n. 2020; 15 (4): e0231984

  Abstract

  Freezing of gait (FOG) is a devastating motor symptom of Parkinson's disease that leads to falls, reduced mobility, and decreased quality of life. Reliably eliciting FOG has been difficult in the clinical setting, which has limited discovery of pathophysiology and/or documentation of the efficacy of treatments, such as different frequencies of subthalamic deep brain stimulation (STN DBS). In this study we validated an instrumented gait task, the turning and barrier course (TBC), with the international standard FOG questionnaire question 3 (FOG-Q3, r = 0.74, p < 0.001). The TBC is easily assembled and mimics real-life environments that elicit FOG. People with Parkinson's disease who experience FOG (freezers) spent more time freezing during the TBC compared to during forward walking (p = 0.007). Freezers also exhibited greater arrhythmicity during non-freezing gait when performing the TBC compared to forward walking (p = 0.006); this difference in gait arrhythmicity between tasks was not detected in non-freezers or controls. Freezers' non-freezing gait was more arrhythmic than that of non-freezers or controls during all walking tasks (p < 0.05). A logistic regression model determined that a combination of gait arrhythmicity, stride time, shank angular range, and asymmetry had the greatest probability of classifying a step as FOG (area under receiver operating characteristic curve = 0.754). Freezers' percent time freezing and non-freezing gait arrhythmicity decreased, and their shank angular velocity increased in the TBC during both 60 Hz and 140 Hz STN DBS (p < 0.05) to non-freezer values. The TBC is a standardized tool for eliciting FOG and demonstrating the efficacy of 60 Hz and 140 Hz STN DBS for gait impairment and FOG. The TBC revealed gait parameters that differentiated freezers from non-freezers and best predicted FOG; these may serve as relevant control variables for closed loop neurostimulation for FOG in Parkinson's disease.

  View details for DOI 10.1371/journal.pone.0231984

  View details for PubMedID 32348346

• Foot strike pattern during running alters muscle-tendon dynamics of the gastrocnemius and the soleus. Scientific reports Yong, J. R., Dembia, C. L., Silder, A. n., Jackson, R. W., Fredericson, M. n., Delp, S. L. 2020; 10 (1): 5872

  Abstract

  Running is thought to be an efficient gait due, in part, to the behavior of the plantar flexor muscles and elastic energy storage in the Achilles tendon. Although plantar flexor muscle mechanics and Achilles tendon energy storage have been explored during rearfoot striking, they have not been fully characterized during forefoot striking. This study examined how plantar flexor muscle-tendon mechanics during running differs between rearfoot and forefoot striking. We used musculoskeletal simulations, driven by joint angles and electromyography recorded from runners using both rearfoot and forefoot striking running patterns, to characterize plantar flexor muscle-tendon mechanics. The simulations revealed that foot strike pattern affected the soleus and gastrocnemius differently. For the soleus, forefoot striking decreased tendon energy storage and fiber work done while the muscle fibers were shortening compared to rearfoot striking. For the gastrocnemius, forefoot striking increased muscle activation and fiber work done while the muscle fibers were lengthening compared to rearfoot striking. These changes in gastrocnemius mechanics suggest that runners planning to convert to forefoot striking might benefit from a progressive eccentric gastrocnemius strengthening program to avoid injury.

  View details for DOI 10.1038/s41598-020-62464-3

  View details for PubMedID 32245985

• Pre-operative gastrocnemius lengths in gait predict outcomes following gastrocnemius lengthening surgery in children with cerebral palsy. PloS one Rajagopal, A., Kidzinski, L., McGlaughlin, A. S., Hicks, J. L., Delp, S. L., Schwartz, M. H. 2020; 15 (6): e0233706

  Abstract

  Equinus deformity is one of the most common gait deformities in children with cerebral palsy. We examined whether estimates of gastrocnemius length in gait could identify limbs likely to have short-term and long-term improvements in ankle kinematics following gastrocnemius lengthening surgery to correct equinus. We retrospectively analyzed data of 891 limbs that underwent a single-event multi-level surgery (SEMLS), and categorized outcomes based on the normalcy of ankle kinematics. Limbs with short gastrocnemius lengths that received a gastrocnemius lengthening surgery as part of a SEMLS (case limbs) were 2.2 times more likely than overtreated limbs (i.e., limbs who did not have short lengths, but still received a lengthening surgery) to have a good surgical outcome at the follow-up gait visit (good outcome rate of 71% vs. 33%). Case limbs were 1.2 times more likely than control limbs (i.e., limbs that had short gastrocnemius lengths but no lengthening surgery) to have a good outcome (71% vs. 59%). Three-fourths of the case limbs with a good outcome at the follow-up gait visit maintained this outcome over time, compared to only one-half of the overtreated limbs. Our results caution against over-prescription of gastrocnemius lengthening surgery and suggest gastrocnemius lengths can be used to identify good surgical candidates.

  View details for DOI 10.1371/journal.pone.0233706

  View details for PubMedID 32502157

• Deep neural networks enable quantitative movement analysis using single-camera videos. Nature communications Kidziński, Ł. n., Yang, B. n., Hicks, J. L., Rajagopal, A. n., Delp, S. L., Schwartz, M. H. 2020; 11 (1): 4054

  Abstract

  Many neurological and musculoskeletal diseases impair movement, which limits people's function and social participation. Quantitative assessment of motion is critical to medical decision-making but is currently possible only with expensive motion capture systems and highly trained personnel. Here, we present a method for predicting clinically relevant motion parameters from an ordinary video of a patient. Our machine learning models predict parameters include walking speed (r = 0.73), cadence (r = 0.79), knee flexion angle at maximum extension (r = 0.83), and Gait Deviation Index (GDI), a comprehensive metric of gait impairment (r = 0.75). These correlation values approach the theoretical limits for accuracy imposed by natural variability in these metrics within our patient population. Our methods for quantifying gait pathology with commodity cameras increase access to quantitative motion analysis in clinics and at home and enable researchers to conduct large-scale studies of neurological and musculoskeletal disorders.

  View details for DOI 10.1038/s41467-020-17807-z

  View details for PubMedID 32792511

• High-fidelity musculoskeletal modeling reveals a motor planning contribution to the speed-accuracy tradeoff. eLife Al Borno, M. n., Vyas, S. n., Shenoy, K. V., Delp, S. L. 2020; 9

  Abstract

  A long-standing challenge in motor neuroscience is to understand the relationship between movement speed and accuracy, known as the speed-accuracy tradeoff. Here, we introduce a biomechanically realistic computational model of three-dimensional upper extremity movements that reproduces well-known features of reaching movements. This model revealed that the speed-accuracy tradeoff, as described by Fitts' law, emerges even without the presence of motor noise, which is commonly believed to underlie the speed-accuracy tradeoff. Next, we analyzed motor cortical neural activity from monkeys reaching to targets of different sizes. We found that the contribution of preparatory neural activity to movement duration variability is greater for smaller targets than larger targets, and that movements to smaller targets exhibit less variability in population-level preparatory activity, but greater movement duration variability. These results propose a new theory underlying the speed-accuracy tradeoff: Fitts' law emerges from greater task demands constraining the optimization landscape in a fashion that reduces the number of 'good' control solutions (i.e., faster reaches). Thus, contrary to current beliefs, the speed-accuracy tradeoff could be a consequence of motor planning variability and not exclusively signal-dependent noise.

  View details for DOI 10.7554/eLife.57021

  View details for PubMedID 33325369

• Rapid volumetric gagCEST imaging of knee articular cartilage at 3 T: evaluation of improved dynamic range and an osteoarthritic population. NMR in biomedicine Watkins, L. E., Rubin, E. B., Mazzoli, V. n., Uhlrich, S. D., Desai, A. D., Black, M. n., Ho, G. K., Delp, S. L., Levenston, M. E., Beaupré, G. S., Gold, G. E., Kogan, F. n. 2020: e4310

  Abstract

  Chemical exchange saturation transfer of glycosaminoglycans, gagCEST, is a quantitative MR technique that has potential for assessing cartilage proteoglycan content at field strengths of 7 T and higher. However, its utility at 3 T remains unclear. The objective of this work was to implement a rapid volumetric gagCEST sequence with higher gagCEST asymmetry at 3 T to evaluate its sensitivity to osteoarthritic changes in knee articular cartilage and in comparison with T2 and T1ρ measures. We hypothesize that gagCEST asymmetry at 3 T decreases with increasing severity of osteoarthritis (OA). Forty-two human volunteers, including 10 healthy subjects and 32 subjects with medial OA, were included in the study. Knee Injury and Osteoarthritis Outcome Scores (KOOS) were assessed for all subjects, and Kellgren-Lawrence grading was performed for OA volunteers. Healthy subjects were scanned consecutively at 3 T to assess the repeatability of the volumetric gagCEST sequence at 3 T. For healthy and OA subjects, gagCEST asymmetry and T2 and T1ρ relaxation times were calculated for the femoral articular cartilage to assess sensitivity to OA severity. Volumetric gagCEST imaging had higher gagCEST asymmetry than single-slice acquisitions (p = 0.015). The average scan-rescan coefficient of variation was 6.8%. There were no significant differences in average gagCEST asymmetry between younger and older healthy controls (p = 0.655) or between healthy controls and OA subjects (p = 0.310). T2 and T1ρ relaxation times were elevated in OA subjects (p < 0.001 for both) compared with healthy controls and both were moderately correlated with total KOOS scores (rho = -0.181 and rho = -0.332 respectively). The gagCEST technique developed here, with volumetric scan times under 10 min and high gagCEST asymmetry at 3 T, did not vary significantly between healthy subjects and those with mild-moderate OA. This further supports a limited utility for gagCEST imaging at 3 T for assessment of early changes in cartilage composition in OA.

  View details for DOI 10.1002/nbm.4310

  View details for PubMedID 32445515

• OPTOGENETIC NEUROMODULATION IN THE DIABETIC CYSTOPATHY MOUSE MODEL Wallace, S. L., Briggs, M. A., Tran, D. T., Wen, Y., Montgomery, K., Zhuang, G., Dobberfuhl, A. D., Delp, S., Chen, B. H. SPRINGER LONDON LTD. 2019: S272

  View details for Web of Science ID 000488930400503

• Connecting the legs with a spring improves human running economy. The Journal of experimental biology Simpson, C. S., Welker, C. G., Uhlrich, S. D., Sketch, S. M., Jackson, R. W., Delp, S. L., Collins, S. H., Selinger, J. C., Hawkes, E. W. 2019

  Abstract

  Human running is inefficient. For every ten calories burned, less than one is needed to maintain a constant forward velocity-the remaining energy is, in a sense, wasted. The majority of this wasted energy is expended to support the bodyweight and redirect the center of mass during the stance phase of gait. An order of magnitude less energy is expended to brake and accelerate the swinging leg. Accordingly, most devices designed to increase running efficiency have targeted the costlier stance phase of gait. An alternative approach is seen in nature: spring-like tissues in some animals and humans are believed to assist leg swing. While it has been assumed that such a spring simply offloads the muscles that swing the legs, thus saving energy, this mechanism has not been experimentally investigated. Here we show that a spring, or 'exotendon', connecting the legs of a human reduces the energy required for running by 6.4±2.8%, and does so through a complex mechanism that produces savings beyond those associated with leg swing. The exotendon applies assistive forces to the swinging legs, increasing the energy optimal stride frequency. Runners then adopt this frequency, taking faster and shorter strides, and reduce the joint mechanical work to redirect their center of mass. Our study shows how a simple spring improves running economy through a complex interaction between the changing dynamics of the body and the adaptive strategies of the runner, highlighting the importance of considering each when designing systems that couple human and machine.

  View details for DOI 10.1242/jeb.202895

  View details for PubMedID 31395676

• An Acute Randomized Controlled Trial of Noninvasive Peripheral Nerve Stimulation in Essential Tremor NEUROMODULATION Pahwa, R., Dhall, R., Ostrem, J., Gwinn, R., Lyons, K., Ro, S., Dietiker, C., Luthra, N., Chidester, P., Hamner, S., Ross, E., Delp, S. 2019; 22 (5): 537–45

  View details for DOI 10.1111/ner.12930

  View details for Web of Science ID 000475988900004

• Best practices for analyzing large-scale health data from wearables and smartphone apps. NPJ digital medicine Hicks, J. L., Althoff, T., Sosic, R., Kuhar, P., Bostjancic, B., King, A. C., Leskovec, J., Delp, S. L. 2019; 2: 45

  Abstract

  Smartphone apps and wearable devices for tracking physical activity and other health behaviors have become popular in recent years and provide a largely untapped source of data about health behaviors in the free-living environment. The data are large in scale, collected at low cost in the "wild", and often recorded in an automatic fashion, providing a powerful complement to traditional surveillance studies and controlled trials. These data are helping to reveal, for example, new insights about environmental and social influences on physical activity. The observational nature of the datasets and collection via commercial devices and apps pose challenges, however, including the potential for measurement, population, and/or selection bias, as well as missing data. In this article, we review insights gleaned from these datasets and propose best practices for addressing the limitations of large-scale data from apps and wearables. Our goal is to enable researchers to effectively harness the data from smartphone apps and wearable devices to better understand what drives physical activity and other health behaviors.

  View details for DOI 10.1038/s41746-019-0121-1

  View details for PubMedID 31304391

  View details for PubMedCentralID PMC6550237

• Best practices for analyzing large-scale health data from wearables and smartphone apps NPJ DIGITAL MEDICINE Hicks, J. L., Althoff, T., Sosic, R., Kuhar, P., Bostjancic, B., King, A. C., Leskovec, J., Delp, S. L. 2019; 2

  View details for DOI 10.1038/s41746-019-0121-1

  View details for Web of Science ID 000470039200002

• GAIT RETRAINING AS A CONSERVATIVE TREATMENT FOR MEDIAL KNEE OSTEOARTHRITIS Mazzoli, V., Uhlrich, S., Rubin, E., Kogan, F., Heargraves, B., Delp, S., Beaupre, G. S., Gold, G. E. ELSEVIER SCI LTD. 2019: S349

  View details for DOI 10.1016/j.joca.2019.02.758

  View details for Web of Science ID 000466779302071

• GAGCEST MRI AT 3T CAN DETECT CARTILAGE DIFFERENCES BETWEEN HEALTHY AND OSTEOARTHRITIC SUBJECTS Rubin, E., Watkins, L., Mazzoli, V., Desai, A. D., Ho, G., Kogan, F., Ulrich, S., Kolesar, J., Delp, S., Beaupre, G., Gold, G. E. ELSEVIER SCI LTD. 2019: S355–S356

  View details for DOI 10.1016/j.joca.2019.02.767

  View details for Web of Science ID 000466779302080

• SIX WEEKS OF PERSONALIZED GAIT RETRAINING TO OFFLOAD THE MEDIAL COMPARTMENT OF THE KNEE REDUCES PAIN MORE THAN SHAM GAIT RETRAINING Uhlrich, S. D., Kolesar, J. A., Silder, A., Berkson, M. Z., Presten, B., Montague-Alamin, H. A., Edouard, N., Willoughby, D., Finlay, A. K., Gold, G. E., Delp, S. L., Beaupre, G. S. ELSEVIER SCI LTD. 2019: S28

  View details for DOI 10.1016/j.joca.2019.02.043

  View details for Web of Science ID 000466779300038

• A murine model of chronic sacral neuromodulation using the optogenetic technique Wallace, S. L., Briggs, M. A., Wen, Y., Montgomery, K., Dobberfuhl, A. D., Zhuang, G., Diaz, E. C., Delp, S., Chen, B. WILEY. 2019: S15

  View details for Web of Science ID 000462357800015

• An Acute Randomized Controlled Trial of Noninvasive Peripheral Nerve Stimulation in Essential Tremor. Neuromodulation : journal of the International Neuromodulation Society Pahwa, R., Dhall, R., Ostrem, J., Gwinn, R., Lyons, K., Ro, S., Dietiker, C., Luthra, N., Chidester, P., Hamner, S., Ross, E., Delp, S. 2019

  Abstract

  OBJECTIVE: To evaluate the safety and effectiveness of a wrist-worn peripheral nerve stimulation device in patients with essential tremor (ET) in a single in-office session.METHODS: This was a randomized controlled study of 77 ET patients who received either treatment stimulation (N = 40) or sham stimulation (N = 37) on the wrist of the hand with more severe tremor. Tremor was evaluated before and immediately after the end of a single 40-minute stimulation session. The primary endpoint compared spiral drawing in the stimulated hand using the Tremor Research Group Essential Tremor Rating Assessment Scale (TETRAS) Archimedes spiral scores in treatment and sham groups. Additional endpoints included TETRAS upper limb tremor scores, subject-rated tasks from the Bain and Findley activities of daily living (ADL) scale before and after stimulation as well as clinical global impression-improvement (CGI-I) rating after stimulation.RESULTS: Subjects who received peripheral nerve stimulation did not show significantly larger improvement in the Archimedes spiral task compared to sham but did show significantly greater improvement in upper limb TETRAS tremor scores (p = 0.017) compared to sham. Subject-rated improvements in ADLs were significantly greater with treatment (49% reduction) than with sham (27% reduction; p = 0.001). A greater percentage of ET patients (88%) reported improvement in the stimulation group as compared to the sham group (62%) according to CGI-I ratings (p = 0.019). No significant adverse events were reported; 3% of subjects experienced mild adverse events.CONCLUSIONS: Peripheral nerve stimulation in ET may provide a safe, well-tolerated, and effective treatment for transient relief of hand tremor symptoms.

  View details for PubMedID 30701655

• Patellofemoral cartilage stresses are most sensitive to variations in vastus medialis muscle forces COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING Pal, S., Besier, T. F., Gold, G. E., Fredericson, M., Delp, S. L., Beaupre, G. S. 2019; 22 (2): 206–16

  View details for DOI 10.1080/10255842.2018.1544629

  View details for Web of Science ID 000465032000008

• Medical device surveillance with electronic health records. NPJ digital medicine Callahan, A. n., Fries, J. A., Ré, C. n., Huddleston, J. I., Giori, N. J., Delp, S. n., Shah, N. H. 2019; 2: 94

  Abstract

  Post-market medical device surveillance is a challenge facing manufacturers, regulatory agencies, and health care providers. Electronic health records are valuable sources of real-world evidence for assessing device safety and tracking device-related patient outcomes over time. However, distilling this evidence remains challenging, as information is fractured across clinical notes and structured records. Modern machine learning methods for machine reading promise to unlock increasingly complex information from text, but face barriers due to their reliance on large and expensive hand-labeled training sets. To address these challenges, we developed and validated state-of-the-art deep learning methods that identify patient outcomes from clinical notes without requiring hand-labeled training data. Using hip replacements-one of the most common implantable devices-as a test case, our methods accurately extracted implant details and reports of complications and pain from electronic health records with up to 96.3% precision, 98.5% recall, and 97.4% F1, improved classification performance by 12.8-53.9% over rule-based methods, and detected over six times as many complication events compared to using structured data alone. Using these additional events to assess complication-free survivorship of different implant systems, we found significant variation between implants, including for risk of revision surgery, which could not be detected using coded data alone. Patients with revision surgeries had more hip pain mentions in the post-hip replacement, pre-revision period compared to patients with no evidence of revision surgery (mean hip pain mentions 4.97 vs. 3.23; t = 5.14; p < 0.001). Some implant models were associated with higher or lower rates of hip pain mentions. Our methods complement existing surveillance mechanisms by requiring orders of magnitude less hand-labeled training data, offering a scalable solution for national medical device surveillance using electronic health records.

  View details for DOI 10.1038/s41746-019-0168-z

  View details for PubMedID 31583282

  View details for PubMedCentralID PMC6761113

• The Interaction of Compliance and Activation on the Force-Length Operating Range and Force Generating Capacity of Skeletal Muscle: A Computational Study using a Guinea Fowl Musculoskeletal Model. Integrative organismal biology (Oxford, England) Cox, S. M., Easton, K. L., Lear, M. C., Marsh, R. L., Delp, S. L., Rubenson, J. 2019; 1 (1): obz022

  Abstract

  A muscle's performance is influenced by where it operates on its force-length (F-L) curve. Here we explore how activation and tendon compliance interact to influence muscle operating lengths and force-generating capacity. To study this, we built a musculoskeletal model of the lower limb of the guinea fowl and simulated the F-L operating range during fixed-end fixed-posture contractions for 39 actuators under thousands of combinations of activation and posture using three different muscle models: Muscles with non-compliant tendons, muscles with compliant tendons but no activation-dependent shift in optimal fiber length (L0), and muscles with both compliant tendons and activation-dependent shifts in L0. We found that activation-dependent effects altered muscle fiber lengths up to 40% and increased or decreased force capacity by up to 50% during fixed-end contractions. Typically, activation-compliance effects reduce muscle force and are dominated by the effects of tendon compliance at high activations. At low activation, however, activation-dependent shifts in L0 are equally important and can result in relative force changes for low compliance muscles of up to 60%. There are regions of the F-L curve in which muscles are most sensitive to compliance and there are troughs of influence where these factors have little effect. These regions are hard to predict, though, because the magnitude and location of these areas of high and low sensitivity shift with compliance level. In this study we provide a map for when these effects will meaningfully influence force capacity and an example of their contributions to force production during a static task, namely standing.

  View details for DOI 10.1093/iob/obz022

  View details for PubMedID 32510037

• Learning one's genetic risk changes physiology independent of actual genetic risk Nature Human Behaviour Turnwald, B. P., Goyer, J. P., Boles, D. Z., Silder, A., Delp, S. L., Crum, A. J. 2019; 3: 48-56

  View details for DOI 10.1038/s41562-018-0483-4

• Learning one's genetic risk changes physiology independent of actual genetic risk. Nature human behaviour Turnwald, B. P., Goyer, J. P., Boles, D. Z., Silder, A., Delp, S. L., Crum, A. J. 2019; 3 (1): 48-56

  Abstract

  Millions of people now access personal genetic risk estimates for diseases such as Alzheimer's, cancer and obesity1. While this information can be informative2-4, research on placebo and nocebo effects5-8 suggests that learning of one's genetic risk may evoke physiological changes consistent with the expected risk profile. Here we tested whether merely learning of one's genetic risk for disease alters one's actual risk by making people more likely to exhibit the expected changes in gene-related physiology, behaviour and subjective experience. Individuals were genotyped for actual genetic risk and then randomly assigned to receive either a 'high-risk' or 'protected' genetic test result for obesity via cardiorespiratory exercise capacity (experiment 1, N = 116) or physiological satiety (experiment 2, N = 107) before engaging in a task in which genetic risk was salient. Merely receiving genetic risk information changed individuals' cardiorespiratory physiology, perceived exertion and running endurance during exercise, and changed satiety physiology and perceived fullness after food consumption in a self-fulfilling manner. Effects of perceived genetic risk on outcomes were sometimes greater than the effects associated with actual genetic risk. If simply conveying genetic risk information can alter actual risk, clinicians and ethicists should wrestle with appropriate thresholds for when revealing genetic risk is warranted.

  View details for DOI 10.1038/s41562-018-0483-4

  View details for PubMedID 30932047

• Predicting gait adaptations due to ankle plantarflexor muscle weakness and contracture using physics-based musculoskeletal simulations. PLoS computational biology Ong, C. F., Geijtenbeek, T. n., Hicks, J. L., Delp, S. L. 2019; 15 (10): e1006993

  Abstract

  Deficits in the ankle plantarflexor muscles, such as weakness and contracture, occur commonly in conditions such as cerebral palsy, stroke, muscular dystrophy, Charcot-Marie-Tooth disease, and sarcopenia. While these deficits likely contribute to observed gait pathologies, determining cause-effect relationships is difficult due to the often co-occurring biomechanical and neural deficits. To elucidate the effects of weakness and contracture, we systematically introduced isolated deficits into a musculoskeletal model and generated simulations of walking to predict gait adaptations due to these deficits. We trained a planar model containing 9 degrees of freedom and 18 musculotendon actuators to walk using a custom optimization framework through which we imposed simple objectives, such as minimizing cost of transport while avoiding falling and injury, and maintaining head stability. We first generated gaits at prescribed speeds between 0.50 m/s and 2.00 m/s that reproduced experimentally observed kinematic, kinetic, and metabolic trends for walking. We then generated a gait at self-selected walking speed; quantitative comparisons between our simulation and experimental data for joint angles, joint moments, and ground reaction forces showed root-mean-squared errors of less than 1.6 standard deviations and normalized cross-correlations above 0.8 except for knee joint moment trajectories. Finally, we applied mild, moderate, and severe levels of muscle weakness or contracture to either the soleus (SOL) or gastrocnemius (GAS) or both of these major plantarflexors (PF) and retrained the model to walk at a self-selected speed. The model was robust to all deficits, finding a stable gait in all cases. Severe PF weakness caused the model to adopt a slower, "heel-walking" gait. Severe contracture of only SOL or both PF yielded similar results: the model adopted a "toe-walking" gait with excessive hip and knee flexion during stance. These results highlight how plantarflexor weakness and contracture may contribute to observed gait patterns.

  View details for DOI 10.1371/journal.pcbi.1006993

  View details for PubMedID 31589597

• Rapid energy expenditure estimation for ankle assisted and inclined loaded walking. Journal of neuroengineering and rehabilitation Slade, P. n., Troutman, R. n., Kochenderfer, M. J., Collins, S. H., Delp, S. L. 2019; 16 (1): 67

  Abstract

  Estimating energy expenditure with indirect calorimetry requires expensive equipment and several minutes of data collection for each condition of interest. While several methods estimate energy expenditure using correlation to data from wearable sensors, such as heart rate monitors or accelerometers, their accuracy has not been evaluated for activity conditions or subjects not included in the correlation process. The goal of our study was to develop data-driven models to estimate energy expenditure at intervals of approximately one second and demonstrate their ability to predict energetic cost for new conditions and subjects. Model inputs were muscle activity and vertical ground reaction forces, which are measurable by wearable electromyography electrodes and pressure sensing insoles.We developed models that estimated energy expenditure while walking (1) with ankle exoskeleton assistance and (2) while carrying various loads and walking on inclines. Estimates were made each gait cycle or four second interval. We evaluated the performance of the models for three use cases. The first estimated energy expenditure (in Watts) during walking conditions for subjects with some subject specific training data available. The second estimated all conditions in the dataset for a new subject not included in the training data. The third estimated new conditions for a new subject.The mean absolute percent errors in estimated energy expenditure during assisted walking conditions were 4.4%, 8.0%, and 8.1% for the three use cases, respectively. The average errors in energy expenditure estimation during inclined and loaded walking conditions were 6.1%, 9.7%, and 11.7% for the three use cases. For models not using subject-specific data, we evaluated the ability to order the magnitude of energy expenditure across conditions. The average percentage of correctly ordered conditions was 63% for assisted walking and 87% for incline and loaded walking.We have determined the accuracy of estimating energy expenditure with data-driven models that rely on ground reaction forces and muscle activity for three use cases. For experimental use cases where the accuracy of a data-driven model is sufficient and similar training data is available, standard indirect calorimetry could be replaced. The models, code, and datasets are provided for reproduction and extension of our results.

  View details for DOI 10.1186/s12984-019-0535-7

  View details for PubMedID 31171003

• Muscle Contributions to Upper-Extremity Movement and Work From a Musculoskeletal Model of the Human Shoulder. Frontiers in neurorobotics Seth, A. n., Dong, M. n., Matias, R. n., Delp, S. n. 2019; 13: 90

  Abstract

  Musculoskeletal models enable movement scientists to examine muscle function by computing the mechanical work done by muscles during motor tasks. To estimate muscle work accurately requires a model that is physiologically plausible. Previous models of the human shoulder have coupled scapula movement to humeral movement. While coupled movement produces a stereotypical scapulohumeral rhythm, it cannot model shrugging or independent movement of the scapula and humerus. The artificial coupling of humeral elevation to scapular rotation permits muscles that cross the glenohumeral joint, such as the rotator-cuff muscles and deltoids, to do implausible work to elevate and rotate the scapula. In reality, the motion of the scapula is controlled by thoracoscapular muscles, yet the roles of these muscles in shoulder function remains unclear. To elucidate the roles of the thoracoscapular muscles, we developed a shoulder model with an accurate scapulothoracic joint and includes scapular muscles to drive its motion. We used the model to compute the work done by the thoracoscapular muscles during shrugging and arm elevation. We found that the bulk of the work done in upper-extremity tasks is performed by the largest muscles of the shoulder: trapezius, deltoids, pectoralis major, and serratus-anterior. Trapezius and serratus anterior prove to be important synergists in performing upward-rotation of the scapula. We show that the large thoracoscapular muscles do more work than glenohumeral muscles during arm-elevation tasks. The model, experimental data and simulation results are freely available on SimTK.org to enable anyone to explore our results and to perform further studies in OpenSim 4.0.

  View details for DOI 10.3389/fnbot.2019.00090

  View details for PubMedID 31780916

  View details for PubMedCentralID PMC6856649

• Weakly supervised classification of rare aortic valve malformations using unlabeled cardiac MRI sequences Nature Communications Fries, J. A., Varma, P., Chen, V. S., Xiao, K., Tejeda, H., Saha, P., Dunnmon, J., Chubb, H., Maskatia, S., Fiterau, M., Delp, S., Ashley, E., Ré, C., Priest, J. R. 2019; 10

  View details for DOI 10.1038/s41467-019-11012-3

• Automatic real-time gait event detection in children using deep neural networks. PloS one Kidzinski, L., Delp, S., Schwartz, M. 2019; 14 (1): e0211466

  Abstract

  Annotation of foot-contact and foot-off events is the initial step in post-processing for most quantitative gait analysis workflows. If clean force plate strikes are present, the events can be automatically detected. Otherwise, annotation of gait events is performed manually, since reliable automatic tools are not available. Automatic annotation methods have been proposed for normal gait, but are usually based on heuristics of the coordinates and velocities of motion capture markers placed on the feet. These heuristics do not generalize to pathological gait due to greater variability in kinematics and anatomy of patients, as well as the presence of assistive devices. In this paper, we use a data-driven approach to predict foot-contact and foot-off events from kinematic and marker time series in children with normal and pathological gait. Through analysis of 9092 gait cycle measurements we build a predictive model using Long Short-Term Memory (LSTM) artificial neural networks. The best-performing model identifies foot-contact and foot-off events with an average error of 10 and 13 milliseconds respectively, outperforming popular heuristic-based approaches. We conclude that the accuracy of our approach is sufficient for most clinical and research applications in the pediatric population. Moreover, the LSTM architecture enables real-time predictions, enabling applications for real-time control of active assistive devices, orthoses, or prostheses. We provide the model, usage examples, and the training code in an open-source package.

  View details for PubMedID 30703141

• Patellofemoral cartilage stresses are most sensitive to variations in vastus medialis muscle forces. Computer methods in biomechanics and biomedical engineering Pal, S., Besier, T. F., Gold, G. E., Fredericson, M., Delp, S. L., Beaupre, G. S. 2018: 1–11

  Abstract

  The purpose of this study was to evaluate the effects of variations in quadriceps muscle forces on patellofemoral stress. We created subject-specific finite element models for 21 individuals with chronic patellofemoral pain and 16 pain-free control subjects. We extracted three-dimensional geometries from high resolution magnetic resonance images and registered the geometries to magnetic resonance images from an upright weight bearing squat with the knees flexed at 60°. We estimated quadriceps muscle forces corresponding to 60° knee flexion during a stair climb task from motion analysis and electromyography-driven musculoskeletal modelling. We applied the quadriceps muscle forces to our finite element models and evaluated patellofemoral cartilage stress. We quantified cartilage stress using an energy-based effective stress, a scalar quantity representing the local stress intensity in the tissue. We used probabilistic methods to evaluate the effects of variations in quadriceps muscle forces from five trials of the stair climb task for each subject. Patellofemoral effective stress was most sensitive to variations in forces in the two branches of the vastus medialis muscle. Femur cartilage effective stress was most sensitive to variations in vastus medialis forces in 29/37 (78%) subjects, and patella cartilage effective stress was most sensitive to variations in vastus medialis forces in 21/37 (57%) subjects. Femur cartilage effective stress was more sensitive to variations in vastus medialis longus forces in subjects classified as maltrackers compared to normal tracking subjects (p=0.006). This study provides new evidence of the importance of the vastus medialis muscle in the treatment of patellofemoral pain.

  View details for PubMedID 30596523

• Robust Physics-based Motion Retargeting with Realistic Body Shapes COMPUTER GRAPHICS FORUM Al Borno, M., Righetti, L., Black, M. J., Delp, S. L., Fiume, E., Romero, J. 2018; 37 (8): 81–92

  View details for DOI 10.1111/cgf.13514

  View details for Web of Science ID 000444412400008

• Microendoscopy reveals positive correlation in multiscale length changes and variable sarcomere lengths across different regions of human muscle JOURNAL OF APPLIED PHYSIOLOGY Lichtwark, G. A., Farris, D. J., Chen, X., Hodges, P. W., Delp, S. L. 2018; 125 (6): 1812–20

  Abstract

  Sarcomere length is a key physiological parameter that affects muscle force output; however, our understanding of the scaling of human muscle from sarcomere to whole muscle is based primarily on cadaveric data. The aims of this study were to explore the in vivo relationship between passive fascicle length and passive sarcomere length at different muscle-tendon unit lengths and determine whether sarcomere and fascicle length relationships are the same in different regions of muscle. A microendoscopy needle probe capable of in vivo sarcomere imaging was inserted into a proximal location of the human tibialis anterior muscle at three different ankle positions (5° dorsiflexion [DF], 5° plantar flexion [PF], 15° PF) and one distal location at a constant ankle position (5° PF distal). Ultrasound imaging of tibialis anterior fascicles, centred on the location of the needle probe, was performed for each condition to estimate fascicle length. Sarcomere length and fascicle length increased with increasing muscle-tendon unit length, although the correlation between sarcomere length change and muscle fascicle length change was only moderate (r2 = 0.45). Passive sarcomere length was longer at the distal imaging site than the proximal site (P = 0.01). When sarcomere number was estimated from sarcomere length and fascicle length, there were fewer sarcomeres in the fibres of distal location than the proximal location (P = 0.01). These data demonstrate that fascicle length changes are representative of sarcomere length changes, although significant variability in sarcomere length exists within a muscle, and sarcomere number per fibre is region dependent.

  View details for PubMedID 30212307

• Machine learning in human movement biomechanics: Best practices, common pitfalls, and new opportunities JOURNAL OF BIOMECHANICS Halilaj, E., Rajagopal, A., Fiterau, M., Hicks, J. L., Hastie, T. J., Delp, S. L. 2018; 81: 1–11

  View details for DOI 10.1016/j.jbiomech.2018.09.009

  View details for Web of Science ID 000451104300001

• Estimating the effect size of surgery to improve walking in children with cerebral palsy from retrospective observational clinical data. Scientific reports Rajagopal, A., Kidzinski, L., McGlaughlin, A. S., Hicks, J. L., Delp, S. L., Schwartz, M. H. 2018; 8 (1): 16344

  Abstract

  Single-event multilevel surgery (SEMLS) is a standard treatment approach aimed at improving gait for patients with cerebral palsy, but the effect of this approach compared to natural progression without surgical intervention is unclear. In this study, we used retrospective patient history, physical exam, and three-dimensional gait analysis data from 2,333 limbs to build regression models estimating the effect of SEMLS on gait, while controlling for expected natural progression. Post-hoc classifications using the regression model results identified which limbs would exhibit gait within two standard deviations of typical gait at the follow-up visit with or without a SEMLS with 73% and 77% accuracy, respectively. Using these models, we found that, while surgery was expected to have a positive effect on 93% of limbs compared to natural progression, in only 37% of limbs was this expected effect a clinically meaningful improvement. We identified 26% of the non-surgically treated limbs that may have shown a clinically meaningful improvement in gait had they received surgery. Our models suggest that pre-operative physical therapy focused on improving biomechanical characteristics, such as walking speed and strength, may improve likelihood of positive surgical outcomes. These models are shared with the community to use as an evaluation tool when considering whether or not a patient should undergo a SEMLS.

  View details for PubMedID 30397268

• Machine learning in human movement biomechanics: Best practices, common pitfalls, and new opportunities. Journal of biomechanics Halilaj, E., Rajagopal, A., Fiterau, M., Hicks, J. L., Hastie, T. J., Delp, S. L. 2018

  Abstract

  Traditional laboratory experiments, rehabilitation clinics, and wearable sensors offer biomechanists a wealth of data on healthy and pathological movement. To harness the power of these data and make research more efficient, modern machine learning techniques are starting to complement traditional statistical tools. This survey summarizes the current usage of machine learning methods in human movement biomechanics and highlights best practices that will enable critical evaluation of the literature. We carried out a PubMed/Medline database search for original research articles that used machine learning to study movement biomechanics in patients with musculoskeletal and neuromuscular diseases. Most studies that met our inclusion criteria focused on classifying pathological movement, predicting risk of developing a disease, estimating the effect of an intervention, or automatically recognizing activities to facilitate out-of-clinic patient monitoring. We found that research studies build and evaluate models inconsistently, which motivated our discussion of best practices. We provide recommendations for training and evaluating machine learning models and discuss the potential of several underutilized approaches, such as deep learning, to generate new knowledge about human movement. We believe that cross-training biomechanists in data science and a cultural shift toward sharing of data and tools are essential to maximize the impact of biomechanics research.

  View details for PubMedID 30279002

• Modeling and Predicting Osteoarthritis Progression: Data from the Osteoarthritis Initiative. Osteoarthritis and cartilage Halilaj, E., Le, Y., Hicks, J. L., Hastie, T. J., Delp, S. L. 2018

  Abstract

  OBJECTIVE: The goal of this study was to model the longitudinal progression of knee osteoarthritis (OA) and build a prognostic tool that uses data collected in one year to predict disease progression over eight years.DESIGN: To model OA progression, we used a mixed-effects mixture model and eight-year data from the Osteoarthritis Initiative-specifically, joint space width measurements from X-rays and pain scores from the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire. We included 1243 subjects who at enrollment were classified as being at high risk of developing OA based on age, body mass index, and medical and occupational histories. After clustering subjects based on radiographic and pain progression, we used clinical variables collected within the first year to build LASSO regression models for predicting the probabilities of belonging to each cluster. Areas under the receiver operating characteristic curve (AUC) represent predictive performance on held-out data.RESULTS: Based on joint space narrowing, subjects clustered as progressing or non-progressing. Based on pain scores, they clustered as stable, improving, or worsening. Radiographic progression could be predicted with high accuracy (AUC = .86) using data from two visits spanning one year, whereas pain progression could be predicted with high accuracy (AUC = .95) using data from a single visit. Joint space narrowing and pain progression were not associated.CONCLUSION: Statistical models for characterizing and predicting OA progression promise to improve clinical trial design and OA prevention efforts in the future.

  View details for PubMedID 30130590

• OpenSim: Simulating musculoskeletal dynamics and neuromuscular control to study human and animal movement. PLoS computational biology Seth, A., Hicks, J. L., Uchida, T. K., Habib, A., Dembia, C. L., Dunne, J. J., Ong, C. F., DeMers, M. S., Rajagopal, A., Millard, M., Hamner, S. R., Arnold, E. M., Yong, J. R., Lakshmikanth, S. K., Sherman, M. A., Ku, J. P., Delp, S. L. 2018; 14 (7): e1006223

  Abstract

  Movement is fundamental to human and animal life, emerging through interaction of complex neural, muscular, and skeletal systems. Study of movement draws from and contributes to diverse fields, including biology, neuroscience, mechanics, and robotics. OpenSim unites methods from these fields to create fast and accurate simulations of movement, enabling two fundamental tasks. First, the software can calculate variables that are difficult to measure experimentally, such as the forces generated by muscles and the stretch and recoil of tendons during movement. Second, OpenSim can predict novel movements from models of motor control, such as kinematic adaptations of human gait during loaded or inclined walking. Changes in musculoskeletal dynamics following surgery or due to human-device interaction can also be simulated; these simulations have played a vital role in several applications, including the design of implantable mechanical devices to improve human grasping in individuals with paralysis. OpenSim is an extensible and user-friendly software package built on decades of knowledge about computational modeling and simulation of biomechanical systems. OpenSim's design enables computational scientists to create new state-of-the-art software tools and empowers others to use these tools in research and clinical applications. OpenSim supports a large and growing community of biomechanics and rehabilitation researchers, facilitating exchange of models and simulations for reproducing and extending discoveries. Examples, tutorials, documentation, and an active user forum support this community. The OpenSim software is covered by the Apache License 2.0, which permits its use for any purpose including both nonprofit and commercial applications. The source code is freely and anonymously accessible on GitHub, where the community is welcomed to make contributions. Platform-specific installers of OpenSim include a GUI and are available on simtk.org.

  View details for PubMedID 30048444

• Noninvasive Neuromodulation in Essential Tremor Demonstrates Relief in a Sham-Controlled Pilot Trial MOVEMENT DISORDERS Lin, P. T., Ross, E. K., Chidester, P., Rosenbluth, K. H., Hamner, S. R., Wong, S. H., Sanger, T. D., Hallett, M., Delp, S. L. 2018; 33 (7): 1182–83

  View details for DOI 10.1002/mds.27350

  View details for Web of Science ID 000442851800020

  View details for PubMedID 29663525

• Physical activity is associated with changes in knee cartilage microstructure OSTEOARTHRITIS AND CARTILAGE Halilaj, E., Hastie, T. J., Gold, G. E., Delp, S. L. 2018; 26 (6): 770–74

  Abstract

  The purpose of this study was to determine if there is an association between objectively measured physical activity and longitudinal changes in knee cartilage microstructure.We used accelerometry and T2-weighted magnetic resonance imaging (MRI) data from the Osteoarthritis Initiative, restricting the analysis to men aged 45-60 years, with a body mass index (BMI) of 25-27 kg/m2 and no radiographic evidence of knee osteoarthritis. After computing 4-year changes in mean T2 relaxation time for six femoral cartilage regions and mean daily times spent in the sedentary, light, moderate, and vigorous activity ranges, we performed canonical correlation analysis (CCA) to find a linear combination of times spent in different activity intensity ranges (Activity Index) that was maximally correlated with a linear combination of regional changes in cartilage microstructure (Cartilage Microstructure Index). We used leave-one-out pre-validation to test the robustness of the model on new data.Nineteen subjects satisfied the inclusion criteria. CCA identified an Activity Index and a Cartilage Microstructure Index that were significantly correlated (r = .82, P < .0001 on test data). Higher levels of sedentary time and vigorous activity were associated with greater medial-lateral differences in longitudinal T2 changes, whereas light activity was associated with smaller differences.Physical activity is better associated with an index that contrasts microstructural changes in different cartilage regions than it is with univariate or cumulative changes, likely because this index separates the effect of activity, which is greater in the medial loadbearing region, from that of patient-specific natural aging.

  View details for PubMedID 29605382

• Acute changes in foot strike pattern and cadence affect running parameters associated with tibial stress fractures. Journal of biomechanics Yong, J. R., Silder, A., Montgomery, K. L., Fredericson, M., Delp, S. L. 2018

  Abstract

  Tibial stress fractures are a common and debilitating injury that occur in distance runners. Runners may be able to decrease tibial stress fracture risk by adopting a running pattern that reduces biomechanical parameters associated with a history of tibial stress fracture. The purpose of this study was to test the hypothesis that converting to a forefoot striking pattern or increasing cadence without focusing on changing foot strike type would reduce injury risk parameters in recreational runners. Running kinematics, ground reaction forces and tibial accelerations were recorded from seventeen healthy, habitual rearfoot striking runners while running in their natural running pattern and after two acute retraining conditions: (1) converting to forefoot striking without focusing on cadence and (2) increasing cadence without focusing on foot strike. We found that converting to forefoot striking decreased two risk factors for tibial stress fracture: average and peak loading rates. Increasing cadence decreased one risk factor: peak hip adduction angle. Our results demonstrate that acute adaptation to forefoot striking reduces different injury risk parameters than acute adaptation to increased cadence and suggest that both modifications may reduce the risk of tibial stress fractures.

  View details for PubMedID 29866518

• AUTOMATED STAGING OF KNEE OSTEOARTHRITIS SEVERITY USING DEEP NEURAL NETWORKS Suresha, S., Kidzinski, L., Halilaj, E., Gold, G. E., Delp, S. L. ELSEVIER SCI LTD. 2018: S441

  View details for Web of Science ID 000432189700835

• Age Influences Biomechanical Changes After Participation in an Anterior Cruciate Ligament Injury Prevention Program AMERICAN JOURNAL OF SPORTS MEDICINE Thompson-Kolesar, J. A., Gatewood, C. T., Tran, A. A., Silder, A., Shultz, R., Delpt, S. L., Dragoo, J. L. 2018; 46 (3): 598–606

  Abstract

  The prevalence of anterior cruciate ligament (ACL) injuries increases during maturation and peaks during late adolescence. Previous studies suggested an age-related association between participation in injury prevention programs and reduction of ACL injury. However, few studies have investigated differences in biomechanical changes after injury prevention programs between preadolescent and adolescent athletes. Purpose/Hypothesis: The purpose was to investigate the influence of age on the effects of the FIFA Medical and Research Centre (F-MARC) 11+ injury prevention warm-up program on differences in biomechanical risk factors for ACL injury between preadolescent and adolescent female soccer players. It was hypothesized that the ACL injury risk factors of knee valgus angle and moment would be greater at baseline but would improve more after training for preadolescent athletes than adolescent athletes. It was further hypothesized that flexor-extensor muscle co-contraction would increase after training for both preadolescent and adolescent athletes.Controlled laboratory study.Institutional Review Board-approved written consent was obtained for 51 preadolescent female athletes aged 10 to 12 years (intervention: n = 28, 11.8 ± 0.8 years; control: n = 23, 11.2 ± 0.6 years) and 43 adolescent female athletes aged 14 to 18 years (intervention: n = 22, 15.9 ± 0.9 years; control: n = 21, 15.7 ± 1.1 years). The intervention groups participated in 15 in-season sessions of the F-MARC 11+ program 2 times per week. Pre- and postseason motion capture data were collected during 4 tasks: preplanned cutting, unanticipated cutting, double-legged jump, and single-legged jump. Lower extremity joint angles and moments were estimated through biomechanical modeling. Knee flexor-extensor muscle co-contraction was estimated from surface electromyography.At baseline, preadolescent athletes displayed greater initial contact and peak knee valgus angles during all activities when compared with the adolescent athletes, but knee valgus moment was not significantly different between age groups. After intervention training, preadolescent athletes improved and decreased their initial contact knee valgus angle (-1.24° ± 0.36°; P = .036) as well as their peak knee valgus moment (-0.57 ± 0.27 percentage body weight × height; P = .033) during the double-legged jump task, as compared with adolescent athletes in the intervention. Compared with adolescent athletes, preadolescent athletes displayed higher weight acceptance flexor-extensor muscle co-contraction at baseline during all activities ( P < .05). After intervention training, preadolescent athletes displayed an increase in precontact flexor-extensor muscle co-contraction during preplanned cutting as compared with adolescent intervention athletes (0.07 ± 0.02 vs -0.30 ± 0.27, respectively; P = .002).The F-MARC 11+ program may be more effective at improving some risk factors for ACL injury among preadolescent female athletes than adolescent athletes, notably by reducing knee valgus angle and moment during a double-legged jump landing.ACL prevention programs may be more effective if administered early in an athlete's career, as younger athletes may be more likely to adapt new biomechanical movement patterns.

  View details for PubMedID 29281799

• Perspectives on Sharing Models and Related Resources in Computational Biomechanics Research JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME Erdemir, A., Hunter, P. J., Holzapfel, G. A., Loew, L. M., Middleton, J., Jacobs, C. R., Nithiarasu, P., Lohner, R., Wei, G., Winkelstein, B. A., Barocas, V. H., Guilak, F., Ku, J. P., Hicks, J. L., Delp, S. L., Sacks, M. S., Weiss, J. A., Ateshian, G. A., Maas, S. A., McCulloch, A. D., Peng, G. Y. 2018; 140 (2)

  Abstract

  The role of computational modeling for biomechanics research and related clinical care will be increasingly prominent. The biomechanics community has been developing computational models routinely for exploration of the mechanics and mechanobiology of diverse biological structures. As a result, a large array of models, data, and discipline-specific simulation software has emerged to support endeavors in computational biomechanics. Sharing computational models and related data and simulation software has first become a utilitarian interest, and now, it is a necessity. Exchange of models, in support of knowledge exchange provided by scholarly publishing, has important implications. Specifically, model sharing can facilitate assessment of reproducibility in computational biomechanics and can provide an opportunity for repurposing and reuse, and a venue for medical training. The community's desire to investigate biological and biomechanical phenomena crossing multiple systems, scales, and physical domains, also motivates sharing of modeling resources as blending of models developed by domain experts will be a required step for comprehensive simulation studies as well as the enhancement of their rigor and reproducibility. The goal of this paper is to understand current perspectives in the biomechanics community for the sharing of computational models and related resources. Opinions on opportunities, challenges, and pathways to model sharing, particularly as part of the scholarly publishing workflow, were sought. A group of journal editors and a handful of investigators active in computational biomechanics were approached to collect short opinion pieces as a part of a larger effort of the IEEE EMBS Computational Biology and the Physiome Technical Committee to address model reproducibility through publications. A synthesis of these opinion pieces indicates that the community recognizes the necessity and usefulness of model sharing. There is a strong will to facilitate model sharing, and there are corresponding initiatives by the scientific journals. Outside the publishing enterprise, infrastructure to facilitate model sharing in biomechanics exists, and simulation software developers are interested in accommodating the community's needs for sharing of modeling resources. Encouragement for the use of standardized markups, concerns related to quality assurance, acknowledgement of increased burden, and importance of stewardship of resources are noted. In the short-term, it is advisable that the community builds upon recent strategies and experiments with new pathways for continued demonstration of model sharing, its promotion, and its utility. Nonetheless, the need for a long-term strategy to unify approaches in sharing computational models and related resources is acknowledged. Development of a sustainable platform supported by a culture of open model sharing will likely evolve through continued and inclusive discussions bringing all stakeholders at the table, e.g., by possibly establishing a consortium.

  View details for PubMedID 29247253

  View details for PubMedCentralID PMC5821103

• Subject-specific toe-in or toe-out gait modifications reduce the larger knee adduction moment peak more than a non-personalized approach JOURNAL OF BIOMECHANICS Uhlrich, S. D., Slider, A., Beaupre, G. S., Shull, P. B., Delp, S. L. 2018; 66: 103–10

  Abstract

  The knee adduction moment (KAM) is a surrogate measure for medial compartment knee loading and is related to the progression of knee osteoarthritis. Toe-in and toe-out gait modifications typically reduce the first and second KAM peaks, respectively. We investigated whether assigning a subject-specific foot progression angle (FPA) modification reduces the peak KAM by more than assigning the same modification to everyone. To explore the effects of motor learning on muscle coordination and kinetics, we also evaluated the peak knee flexion moment and quadriceps-hamstring co-contraction during normal walking, when subjects first learned their subject-specific FPA, and following 20 min of training. Using vibrotactile feedback, we trained 20 healthy adults to toe-in and toe-out by 5° and 10° relative to their natural FPA, then identified the subject-specific FPA as the angle where each subject maximally reduced their larger KAM peak. When walking at their subject-specific FPA, 18 subjects significantly reduced their larger KAM peak; 8 by toeing-in and 10 by toeing-out. On average, subjects reduced their larger KAM peak by 18.6 ± 16.2% when walking at their subject-specific FPA, which was more than the reductions achieved when all subjects toed-in by 10° (10.0 ± 17.1%, p = .013) or toed-out by 10° (11.0 ± 18.3%, p = .002). Quadriceps-hamstring co-contraction and the peak knee flexion moment increased when subjects first learned their subject-specific FPA, but only co-contraction returned to baseline levels following training. These findings demonstrate that subject-specific gait modifications reduce the peak KAM more than uniformly assigned modifications and have the potential to slow the progression of medial compartment knee osteoarthritis.

  View details for PubMedID 29174534

  View details for PubMedCentralID PMC5859947

• Large-scale physical activity data reveal worldwide activity inequality NATURE Althoff, T., Sosic, R., Hicks, J. L., King, A. C., Delp, S. L., Leskovec, J. 2017; 547 (7663): 336-+

  Abstract

  To be able to curb the global pandemic of physical inactivity and the associated 5.3 million deaths per year, we need to understand the basic principles that govern physical activity. However, there is a lack of large-scale measurements of physical activity patterns across free-living populations worldwide. Here we leverage the wide usage of smartphones with built-in accelerometry to measure physical activity at the global scale. We study a dataset consisting of 68 million days of physical activity for 717,527 people, giving us a window into activity in 111 countries across the globe. We find inequality in how activity is distributed within countries and that this inequality is a better predictor of obesity prevalence in the population than average activity volume. Reduced activity in females contributes to a large portion of the observed activity inequality. Aspects of the built environment, such as the walkability of a city, are associated with a smaller gender gap in activity and lower activity inequality. In more walkable cities, activity is greater throughout the day and throughout the week, across age, gender, and body mass index (BMI) groups, with the greatest increases in activity found for females. Our findings have implications for global public health policy and urban planning and highlight the role of activity inequality and the built environment in improving physical activity and health.

  View details for PubMedID 28693034

  View details for PubMedCentralID PMC5774986

• Muscle-tendon mechanics explain unexpected effects of exoskeleton assistance on metabolic rate during walking. journal of experimental biology Jackson, R. W., Dembia, C. L., Delp, S. L., Collins, S. H. 2017; 220: 2082-2095

  Abstract

  The goal of this study was to gain insight into how ankle exoskeletons affect the behavior of the plantarflexor muscles during walking. Using data from previous experiments, we performed electromyography-driven simulations of musculoskeletal dynamics to explore how changes in exoskeleton assistance affected plantarflexor muscle-tendon mechanics, particularly for the soleus. We used a model of muscle energy consumption to estimate individual muscle metabolic rate. As average exoskeleton torque was increased, while no net exoskeleton work was provided, a reduction in tendon recoil led to an increase in positive mechanical work performed by the soleus muscle fibers. As net exoskeleton work was increased, both soleus muscle fiber force and positive mechanical work decreased. Trends in the sum of the metabolic rates of the simulated muscles correlated well with trends in experimentally observed whole-body metabolic rate (R(2)=0.9), providing confidence in our model estimates. Our simulation results suggest that different exoskeleton behaviors can alter the functioning of the muscles and tendons acting at the assisted joint. Furthermore, our results support the idea that the series tendon helps reduce positive work done by the muscle fibers by storing and returning energy elastically. We expect the results from this study to promote the use of electromyography-driven simulations to gain insight into the operation of muscle-tendon units and to guide the design and control of assistive devices.

  View details for DOI 10.1242/jeb.150011

  View details for PubMedID 28341663

• A Brainstem-Spinal Cord Inhibitory Circuit for Mechanical Pain Modulation by GABA and Enkephalins. Neuron François, A., Low, S. A., Sypek, E. I., Christensen, A. J., Sotoudeh, C., Beier, K. T., Ramakrishnan, C., Ritola, K. D., Sharif-Naeini, R., Deisseroth, K., Delp, S. L., Malenka, R. C., Luo, L., Hantman, A. W., Scherrer, G. 2017; 93 (4): 822-839 e6

  Abstract

  Pain thresholds are, in part, set as a function of emotional and internal states by descending modulation of nociceptive transmission in the spinal cord. Neurons of the rostral ventromedial medulla (RVM) are thought to critically contribute to this process; however, the neural circuits and synaptic mechanisms by which distinct populations of RVM neurons facilitate or diminish pain remain elusive. Here we used in vivo opto/chemogenetic manipulations and trans-synaptic tracing of genetically identified dorsal horn and RVM neurons to uncover an RVM-spinal cord-primary afferent circuit controlling pain thresholds. Unexpectedly, we found that RVM GABAergic neurons facilitate mechanical pain by inhibiting dorsal horn enkephalinergic/GABAergic interneurons. We further demonstrate that these interneurons gate sensory inputs and control pain through temporally coordinated enkephalin- and GABA-mediated presynaptic inhibition of somatosensory neurons. Our results uncover a descending disynaptic inhibitory circuit that facilitates mechanical pain, is engaged during stress, and could be targeted to establish higher pain thresholds. VIDEO ABSTRACT.

  View details for DOI 10.1016/j.neuron.2017.01.008

  View details for PubMedID 28162807

• Preparatory co-activation of the ankle muscles may prevent ankle inversion injuries. Journal of biomechanics Demers, M. S., Hicks, J. L., Delp, S. L. 2017; 52: 17-23

  Abstract

  Ankle inversion sprains are the most frequent acute musculoskeletal injuries occurring in physical activity. Interventions that retrain muscle coordination have helped rehabilitate injured ankles, but it is unclear which muscle coordination strategies, if any, can prevent ankle sprains. The purpose of this study was to determine whether coordinated activity of the ankle muscles could prevent excessive ankle inversion during a simulated landing on a 30° incline. We used a set of musculoskeletal simulations to evaluate the efficacy of two strategies for coordinating the ankle evertor and invertor muscles during simulated landing scenarios: planned co-activation and stretch reflex activation with physiologic latency (60-ms delay). A full-body musculoskeletal model of landing was used to generate simulations of a subject dropping onto an inclined surface with each coordination condition. Within each condition, the intensity of evertor and invertor co-activity or stretch reflexes were varied systematically. The simulations revealed that strong preparatory co-activation of the ankle evertors and invertors prior to ground contact prevented ankle inversion from exceeding injury thresholds by rapidly generating eversion moments after initial contact. Conversely, stretch reflexes were too slow to generate eversion moments before the simulations reached the threshold for inversion injury. These results suggest that training interventions to protect the ankle should focus on stiffening the ankle with muscle co-activation prior to landing. The musculoskeletal models, controllers, software, and simulation results are freely available online at http://simtk.org/home/ankle-sprains, enabling others to reproduce the results and explore new injury scenarios and interventions.

  View details for DOI 10.1016/j.jbiomech.2016.11.002

  View details for PubMedID 28057351

• Sanativo Wound Healing Product Does Not Accelerate Reepithelialization in a Mouse Cutaneous Wound Healing Model. Plastic and reconstructive surgery Marshall, C. D., Hu, M. S., Leavitt, T., Barnes, L. A., Cheung, A. T., Malhotra, S., Lorenz, H. P., Delp, S. L., Quake, S. R., Longaker, M. T. 2017; 139 (2): 343-352

  Abstract

  Sanativo is an over-the-counter Brazilian product derived from Amazon rainforest plant extract that is purported to improve the healing of skin wounds. Two experimental studies have shown accelerated closure of nonsplinted excisional wounds in rat models. However, these models allow for significant contraction of the wound and do not approximate healing in the tight skin of humans.Full-thickness excisional wounds were created on the dorsal skin of mice and were splinted with silicone rings, a model that forces the wound to heal by granulation and reepithelialization. Sanativo or a control solution was applied either daily or every other day to the wounds. Photographs were taken every other day, and the degree of reepithelialization of the wounds was determined.With both daily and every-other-day applications, Sanativo delayed reepithelialization of the wounds. Average time to complete healing was faster with control solution versus Sanativo in the daily application group (9.4 versus 15.2 days; p < 0.0001) and the every-other-day application group (11 versus 13 days; p = 0.017). The size of visible scar at the last time point of the study was not significantly different between the groups, and no differences were found on histologic examination.Sanativo wound healing compound delayed wound reepithelialization in a mouse splinted excisional wound model that approximates human wound healing. The size of visible scar after complete healing was not improved with the application of Sanativo. These results should cast doubt on claims that this product can improve wound healing in humans.

  View details for DOI 10.1097/PRS.0000000000003013

  View details for PubMedID 28121865

• Biomechanical Effects of an Injury Prevention Program in Preadolescent Female Soccer Athletes AMERICAN JOURNAL OF SPORTS MEDICINE Thompson, J. A., Tran, A. A., Gatewood, C. T., Shultz, R., Silder, A., Delp, S. L., Dragoo, J. L. 2017; 45 (2): 294-301

  Abstract

  Anterior cruciate ligament (ACL) injuries are common, and children as young as 10 years of age exhibit movement patterns associated with an ACL injury risk. Prevention programs have been shown to reduce injury rates, but the mechanisms behind these programs are largely unknown. Few studies have investigated biomechanical changes after injury prevention programs in children.To investigate the effects of the F-MARC 11+ injury prevention warm-up program on changes to biomechanical risk factors for an ACL injury in preadolescent female soccer players. We hypothesized that the primary ACL injury risk factor of peak knee valgus moment would improve after training. In addition, we explored other kinematic and kinetic variables associated with ACL injuries.Controlled laboratory study.A total of 51 female athletes aged 10 to 12 years were recruited from soccer clubs and were placed into an intervention group (n = 28; mean [±SD] age, 11.8 ± 0.8 years) and a control group (n = 23; mean age, 11.2 ± 0.6 years). The intervention group participated in 15 in-season sessions of the F-MARC 11+ program (2 times/wk). Pre- and postseason motion capture data were collected during preplanned cutting, unanticipated cutting, double-leg jump, and single-leg jump tasks. Lower extremity joint angles and moments were estimated using OpenSim, a biomechanical modeling system.Athletes in the intervention group reduced their peak knee valgus moment compared with the control group during the double-leg jump (mean [±standard error of the mean] pre- to posttest change, -0.57 ± 0.27 %BW×HT vs 0.25 ± 0.25 %BW×HT, respectively; P = .034). No significant differences in the change in peak knee valgus moment were found between the groups for any other activity; however, the intervention group displayed a significant pre- to posttest increase in peak knee valgus moment during unanticipated cutting (P = .044). Additional analyses revealed an improvement in peak ankle eversion moment after training during preplanned cutting (P = .015), unanticipated cutting (P = .004), and the double-leg jump (P = .016) compared with the control group. Other secondary risk factors did not significantly improve after training, although the peak knee valgus angle improved in the control group compared with the intervention group during unanticipated cutting (P = .018).The F-MARC 11+ program may be effective in improving some risk factors for an ACL injury during a double-leg jump in preadolescent athletes, most notably by reducing peak knee valgus moment.This study provides motivation for enhancing injury prevention programs to produce improvement in other ACL risk factors, particularly during cutting and single-leg tasks.

  View details for DOI 10.1177/0363546516669326

  View details for Web of Science ID 000394776900004

• ShortFuse: Biomedical Time Series Representations in the Presence of Structured Information. Proceedings of machine learning research Fiterau, M. n., Bhooshan, S. n., Fries, J. n., Bournhonesque, C. n., Hicks, J. n., Halilaj, E. n., Ré, C. n., Delp, S. n. 2017; 68: 59–74

  Abstract

  In healthcare applications, temporal variables that encode movement, health status and longitudinal patient evolution are often accompanied by rich structured information such as demographics, diagnostics and medical exam data. However, current methods do not jointly optimize over structured covariates and time series in the feature extraction process. We present ShortFuse, a method that boosts the accuracy of deep learning models for time series by explicitly modeling temporal interactions and dependencies with structured covariates. ShortFuse introduces hybrid convolutional and LSTM cells that incorporate the covariates via weights that are shared across the temporal domain. ShortFuse outperforms competing models by 3% on two biomedical applications, forecasting osteoarthritis-related cartilage degeneration and predicting surgical outcomes for cerebral palsy patients, matching or exceeding the accuracy of models that use features engineered by domain experts.

  View details for PubMedID 30882086

• Prostaglandin E2 is essential for efficacious skeletal muscle stem-cell function, augmenting regeneration and strength. Proceedings of the National Academy of Sciences of the United States of America Ho, A. T., Palla, A. R., Blake, M. R., Yucel, N. D., Wang, Y. X., Magnusson, K. E., Holbrook, C. A., Kraft, P. E., Delp, S. L., Blau, H. M. 2017; 114 (26): 6675–84

  Abstract

  Skeletal muscles harbor quiescent muscle-specific stem cells (MuSCs) capable of tissue regeneration throughout life. Muscle injury precipitates a complex inflammatory response in which a multiplicity of cell types, cytokines, and growth factors participate. Here we show that Prostaglandin E2 (PGE2) is an inflammatory cytokine that directly targets MuSCs via the EP4 receptor, leading to MuSC expansion. An acute treatment with PGE2 suffices to robustly augment muscle regeneration by either endogenous or transplanted MuSCs. Loss of PGE2 signaling by specific genetic ablation of the EP4 receptor in MuSCs impairs regeneration, leading to decreased muscle force. Inhibition of PGE2 production through nonsteroidal anti-inflammatory drug (NSAID) administration just after injury similarly hinders regeneration and compromises muscle strength. Mechanistically, the PGE2 EP4 interaction causes MuSC expansion by triggering a cAMP/phosphoCREB pathway that activates the proliferation-inducing transcription factor, Nurr1 Our findings reveal that loss of PGE2 signaling to MuSCs during recovery from injury impedes muscle repair and strength. Through such gain- or loss-of-function experiments, we found that PGE2 signaling acts as a rheostat for muscle stem-cell function. Decreased PGE2 signaling due to NSAIDs or increased PGE2 due to exogenous delivery dictates MuSC function, which determines the outcome of regeneration. The markedly enhanced and accelerated repair of damaged muscles following intramuscular delivery of PGE2 suggests a previously unrecognized indication for this therapeutic agent.

  View details for PubMedID 28607093

• Simulating ideal assistive devices to reduce the metabolic cost of walking with heavy loads. PloS one Dembia, C. L., Silder, A. n., Uchida, T. K., Hicks, J. L., Delp, S. L. 2017; 12 (7): e0180320

  Abstract

  Wearable robotic devices can restore and enhance mobility. There is growing interest in designing devices that reduce the metabolic cost of walking; however, designers lack guidelines for which joints to assist and when to provide the assistance. To help address this problem, we used musculoskeletal simulation to predict how hypothetical devices affect muscle activity and metabolic cost when walking with heavy loads. We explored 7 massless devices, each providing unrestricted torque at one degree of freedom in one direction (hip abduction, hip flexion, hip extension, knee flexion, knee extension, ankle plantarflexion, or ankle dorsiflexion). We used the Computed Muscle Control algorithm in OpenSim to find device torque profiles that minimized the sum of squared muscle activations while tracking measured kinematics of loaded walking without assistance. We then examined the metabolic savings provided by each device, the corresponding device torque profiles, and the resulting changes in muscle activity. We found that the hip flexion, knee flexion, and hip abduction devices provided greater metabolic savings than the ankle plantarflexion device. The hip abduction device had the greatest ratio of metabolic savings to peak instantaneous positive device power, suggesting that frontal-plane hip assistance may be an efficient way to reduce metabolic cost. Overall, the device torque profiles generally differed from the corresponding net joint moment generated by muscles without assistance, and occasionally exceeded the net joint moment to reduce muscle activity at other degrees of freedom. Many devices affected the activity of muscles elsewhere in the limb; for example, the hip flexion device affected muscles that span the ankle joint. Our results may help experimentalists decide which joint motions to target when building devices and can provide intuition for how devices may interact with the musculoskeletal system. The simulations are freely available online, allowing others to reproduce and extend our work.

  View details for PubMedID 28700630

  View details for PubMedCentralID PMC5507502

• Human soleus sarcomere lengths measured using in vivo microendoscopy at two ankle flexion angles JOURNAL OF BIOMECHANICS Chen, X., Delp, S. L. 2016; 49 (16): 4164-4167

  Abstract

  The forces generated by the soleus muscle play an important role in standing and locomotion. The lengths of the sarcomeres of the soleus affect its force-generating capacity, yet it is unknown how sarcomere lengths in the soleus change as a function of ankle flexion angle. In this study, we used microendoscopy to measure resting sarcomere lengths at 10° plantarflexion and 20° dorsiflexion in 7 healthy individuals. Mean sarcomere lengths at 10° plantarflexion were 2.84±0.09µm (mean±S.E.M.), near the optimal length for sarcomere force generation. Sarcomere lengths were 3.43±0.09µm at 20° dorsiflexion, indicating that they were longer than optimal length when the ankle was in dorsiflexion and the muscle was inactive. Our results indicate a smaller sarcomere length difference between two ankle flexion angles compared to estimates from musculoskeletal models and suggest why these models frequently underestimate the force-generating capacity of the soleus.

  View details for DOI 10.1016/j.jbiomech.2016.11.010

  View details for Web of Science ID 000390971300061

  View details for PubMedID 27866676

• In Vivo Interrogation of Spinal Mechanosensory Circuits. Cell reports Christensen, A. J., Iyer, S. M., François, A., Vyas, S., Ramakrishnan, C., Vesuna, S., Deisseroth, K., Scherrer, G., Delp, S. L. 2016; 17 (6): 1699-1710

  Abstract

  Spinal dorsal horn circuits receive, process, and transmit somatosensory information. To understand how specific components of these circuits contribute to behavior, it is critical to be able to directly modulate their activity in unanesthetized in vivo conditions. Here, we develop experimental tools that enable optogenetic control of spinal circuitry in freely moving mice using commonly available materials. We use these tools to examine mechanosensory processing in the spinal cord and observe that optogenetic activation of somatostatin-positive interneurons facilitates both mechanosensory and itch-related behavior, while reversible chemogenetic inhibition of these neurons suppresses mechanosensation. These results extend recent findings regarding the processing of mechanosensory information in the spinal cord and indicate the potential for activity-induced release of the somatostatin neuropeptide to affect processing of itch. The spinal implant approach we describe here is likely to enable a wide range of studies to elucidate spinal circuits underlying pain, touch, itch, and movement.

  View details for DOI 10.1016/j.celrep.2016.10.010

  View details for PubMedID 27806306

• Biomechanical Effects of an Injury Prevention Program in Preadolescent Female Soccer Athletes. American journal of sports medicine Thompson, J. A., Tran, A. A., Gatewood, C. T., Shultz, R., Silder, A., Delp, S. L., Dragoo, J. L. 2016

  Abstract

  Anterior cruciate ligament (ACL) injuries are common, and children as young as 10 years of age exhibit movement patterns associated with an ACL injury risk. Prevention programs have been shown to reduce injury rates, but the mechanisms behind these programs are largely unknown. Few studies have investigated biomechanical changes after injury prevention programs in children.To investigate the effects of the F-MARC 11+ injury prevention warm-up program on changes to biomechanical risk factors for an ACL injury in preadolescent female soccer players. We hypothesized that the primary ACL injury risk factor of peak knee valgus moment would improve after training. In addition, we explored other kinematic and kinetic variables associated with ACL injuries.Controlled laboratory study.A total of 51 female athletes aged 10 to 12 years were recruited from soccer clubs and were placed into an intervention group (n = 28; mean [±SD] age, 11.8 ± 0.8 years) and a control group (n = 23; mean age, 11.2 ± 0.6 years). The intervention group participated in 15 in-season sessions of the F-MARC 11+ program (2 times/wk). Pre- and postseason motion capture data were collected during preplanned cutting, unanticipated cutting, double-leg jump, and single-leg jump tasks. Lower extremity joint angles and moments were estimated using OpenSim, a biomechanical modeling system.Athletes in the intervention group reduced their peak knee valgus moment compared with the control group during the double-leg jump (mean [±standard error of the mean] pre- to posttest change, -0.57 ± 0.27 %BW×HT vs 0.25 ± 0.25 %BW×HT, respectively; P = .034). No significant differences in the change in peak knee valgus moment were found between the groups for any other activity; however, the intervention group displayed a significant pre- to posttest increase in peak knee valgus moment during unanticipated cutting (P = .044). Additional analyses revealed an improvement in peak ankle eversion moment after training during preplanned cutting (P = .015), unanticipated cutting (P = .004), and the double-leg jump (P = .016) compared with the control group. Other secondary risk factors did not significantly improve after training, although the peak knee valgus angle improved in the control group compared with the intervention group during unanticipated cutting (P = .018).The F-MARC 11+ program may be effective in improving some risk factors for an ACL injury during a double-leg jump in preadolescent athletes, most notably by reducing peak knee valgus moment.This study provides motivation for enhancing injury prevention programs to produce improvement in other ACL risk factors, particularly during cutting and single-leg tasks.

  View details for PubMedID 27793803

• Gait biomechanics in the era of data science. Journal of biomechanics Ferber, R., Osis, S. T., Hicks, J. L., Delp, S. L. 2016

  Abstract

  Data science has transformed fields such as computer vision and economics. The ability of modern data science methods to extract insights from large, complex, heterogeneous, and noisy datasets is beginning to provide a powerful complement to the traditional approaches of experimental motion capture and biomechanical modeling. The purpose of this article is to provide a perspective on how data science methods can be incorporated into our field to advance our understanding of gait biomechanics and improve treatment planning procedures. We provide examples of how data science approaches have been applied to biomechanical data. We then discuss the challenges that remain for effectively using data science approaches in clinical gait analysis and gait biomechanics research, including the need for new tools, better infrastructure and incentives for sharing data, and education across the disciplines of biomechanics and data science. By addressing these challenges, we can revolutionize treatment planning and biomechanics research by capitalizing on the wealth of knowledge gained by gait researchers over the past decades and the vast, but often siloed, data that are collected in clinical and research laboratories around the world.

  View details for DOI 10.1016/j.jbiomech.2016.10.033

  View details for PubMedID 27814971

• Full-Body Musculoskeletal Model for Muscle-Driven Simulation of Human Gait IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Rajagopal, A., Dembia, C. L., Demers, M. S., Delp, D. D., Hicks, J. L., Delp, S. L. 2016; 63 (10): 2068-2079

  Abstract

  Musculoskeletal models provide a non-invasive means to study human movement and predict the effects of interventions on gait. Our goal was to create an open-source 3-D musculoskeletal model with high-fidelity representations of the lower limb musculature of healthy young individuals that can be used to generate accurate simulations of gait.Our model includes bony geometry for the full body, 37 degrees of freedom to define joint kinematics, Hill-type models of 80 muscle-tendon units actuating the lower limbs, and 17 ideal torque actuators driving the upper body. The model's musculotendon parameters are derived from previous anatomical measurements of 21 cadaver specimens and magnetic resonance images of 24 young healthy subjects. We tested the model by evaluating its computational time and accuracy of simulations of healthy walking and running.Generating muscle-driven simulations of normal walking and running took approximately 10 minutes on a typical desktop computer. The differences between our muscle-generated and inverse dynamics joint moments were within 3% (RMSE) of the peak inverse dynamics joint moments in both walking and running, and our simulated muscle activity showed qualitative agreement with salient features from experimental electromyography data.These results suggest that our model is suitable for generating muscle-driven simulations of healthy gait. We encourage other researchers to further validate and apply the model to study other motions of the lower extremity.The model is implemented in the open-source software platform OpenSim. The model and data used to create and test the simulations are freely available at https://simtk.org/home/full_body/, allowing others to reproduce these results and create their own simulations.

  View details for DOI 10.1109/TBME.2016.2586891

  View details for Web of Science ID 000384570200010

  View details for PubMedID 27392337

• Simulating Ideal Assistive Devices to Reduce the Metabolic Cost of Running PLOS ONE Uchida, T. K., Seth, A., Pouya, S., Dembia, C. L., Hicks, J. L., Delp, S. L. 2016; 11 (9)

  Abstract

  Tools have been used for millions of years to augment the capabilities of the human body, allowing us to accomplish tasks that would otherwise be difficult or impossible. Powered exoskeletons and other assistive devices are sophisticated modern tools that have restored bipedal locomotion in individuals with paraplegia and have endowed unimpaired individuals with superhuman strength. Despite these successes, designing assistive devices that reduce energy consumption during running remains a substantial challenge, in part because these devices disrupt the dynamics of a complex, finely tuned biological system. Furthermore, designers have hitherto relied primarily on experiments, which cannot report muscle-level energy consumption and are fraught with practical challenges. In this study, we use OpenSim to generate muscle-driven simulations of 10 human subjects running at 2 and 5 m/s. We then add ideal, massless assistive devices to our simulations and examine the predicted changes in muscle recruitment patterns and metabolic power consumption. Our simulations suggest that an assistive device should not necessarily apply the net joint moment generated by muscles during unassisted running, and an assistive device can reduce the activity of muscles that do not cross the assisted joint. Our results corroborate and suggest biomechanical explanations for similar effects observed by experimentalists, and can be used to form hypotheses for future experimental studies. The models, simulations, and software used in this study are freely available at simtk.org and can provide insight into assistive device design that complements experimental approaches.

  View details for DOI 10.1371/journal.pone.0163417

  View details for Web of Science ID 000383893200115

  View details for PubMedID 27656901

  View details for PubMedCentralID PMC5033584

• Changes in sarcomere lengths of the human vastus lateralis muscle with knee flexion measured using in vivo microendoscopy JOURNAL OF BIOMECHANICS Chen, X., Sanchez, G. N., Schnitzer, M. J., Delp, S. L. 2016; 49 (13): 2989-2994

  Abstract

  Sarcomeres are the basic contractile units of muscle, and their lengths influence muscle force-generating capacity. Despite their importance, in vivo sarcomere lengths remain unknown for many human muscles. Second harmonic generation (SHG) microendoscopy is a minimally invasive technique for imaging sarcomeres in vivo and measuring their lengths. In this study, we used SHG microendoscopy to visualize sarcomeres of the human vastus lateralis, a large knee extensor muscle important for mobility, to examine how sarcomere lengths change with knee flexion and thus affect the muscle׳s force-generating capacity. We acquired in vivo sarcomere images of several muscle fibers of the resting vastus lateralis in six healthy individuals. Mean sarcomere lengths increased (p=0.031) from 2.84±0.16μm at 50° of knee flexion to 3.17±0.13μm at 110° of knee flexion. The standard deviation of sarcomere lengths among different fibers within a muscle was 0.21±0.09μm. Our results suggest that the sarcomeres of the resting vastus lateralis at 50° of knee flexion are near optimal length. At a knee flexion angle of 110° the resting sarcomeres of vastus lateralis are longer than optimal length. These results show a smaller sarcomere length change and greater conservation of force-generating capacity with knee flexion than estimated in previous studies.

  View details for DOI 10.1016/j.jbiomech.2016.07.013

  View details for Web of Science ID 000385472300057

  View details for PubMedID 27481293

• Beyond the brain: Optogenetic control in the spinal cord and peripheral nervous system SCIENCE TRANSLATIONAL MEDICINE Montgomery, K. L., Iyer, S. M., Christensen, A. J., Deisseroth, K., Delp, S. L. 2016; 8 (337)

  Abstract

  Optogenetics offers promise for dissecting the complex neural circuits of the spinal cord and peripheral nervous system and has therapeutic potential for addressing unmet clinical needs. Much progress has been made to enable optogenetic control in normal and disease states, both in proof-of-concept and mechanistic studies in rodent models. In this Review, we discuss challenges in using optogenetics to study the mammalian spinal cord and peripheral nervous system, synthesize common features that unite the work done thus far, and describe a route forward for the successful application of optogenetics to translational research beyond the brain.

  View details for DOI 10.1126/scitranslmed.aad7577

  View details for Web of Science ID 000375802500004

  View details for PubMedID 27147590

• Simulation-Based Design for Wearable Robotic Systems: An Optimization Framework for Enhancing a Standing Long Jump IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Ong, C. F., Hicks, J. L., Delp, S. L. 2016; 63 (5): 894-903

  Abstract

  Technologies that augment human performance are the focus of intensive research and development, driven by advances in wearable robotic systems. Success has been limited by the challenge of understanding human-robot interaction. To address this challenge, we developed an optimization framework to synthesize a realistic human standing long jump and used the framework to explore how simulated wearable robotic devices might enhance jump performance.A planar, five-segment, seven-degree-of-freedom model with physiological torque actuators, which have variable torque capacity depending on joint position and velocity, was used to represent human musculoskeletal dynamics. An active augmentation device was modeled as a torque actuator that could apply a single pulse of up to 100 Nm of extension torque. A passive design was modeled as rotational springs about each lower limb joint. Dynamic optimization searched for physiological and device actuation patterns to maximize jump distance.Optimization of the nominal case yielded a 2.27 m jump that captured salient kinematic and kinetic features of human jumps. When the active device was added to the ankle, knee, or hip, jump distance increased to between 2.49 and 2.52 m. Active augmentation of all three joints increased the jump distance to 3.10 m. The passive design increased jump distance to 3.32 m by adding torques of 135, 365, and 297 Nm to the ankle, knee, and hip, respectively.Dynamic optimization can be used to simulate a standing long jump and investigate human-robot interaction.Simulation can aid in the design of performance-enhancing technologies.

  View details for DOI 10.1109/TBME.2015.2463077

  View details for Web of Science ID 000375001600002

  View details for PubMedID 26258930

• Evaluation of an Algorithm to Detect the First Ventilatory Threshold from Heart Rate: 2450 June 3, 10: 30 AM - 10: 45 AM. Medicine and science in sports and exercise Silder, A., Gold, G. E., Bae, S., Ko, B., Jang, D., Delp, S. L. 2016; 48 (5): 672-673

  View details for DOI 10.1249/01.mss.0000487019.84069.12

  View details for PubMedID 27361086

• Optogenetic approaches addressing extracellular modulation of neural excitability SCIENTIFIC REPORTS Ferenczi, E. A., Vierock, J., Atsuta-Tsunoda, K., Tsunoda, S. P., Ramakrishnan, C., Gorini, C., Thompson, K., Lee, S. Y., Berndt, A., Perry, C., Minniberger, S., Vogt, A., Mattis, J., Prakash, R., Delp, S., Deisseroth, K., Hegemann, P. 2016; 6

  Abstract

  The extracellular ionic environment in neural tissue has the capacity to influence, and be influenced by, natural bouts of neural activity. We employed optogenetic approaches to control and investigate these interactions within and between cells, and across spatial scales. We began by developing a temporally precise means to study microdomain-scale interactions between extracellular protons and acid-sensing ion channels (ASICs). By coupling single-component proton-transporting optogenetic tools to ASICs to create two-component optogenetic constructs (TCOs), we found that acidification of the local extracellular membrane surface by a light-activated proton pump recruited a slow inward ASIC current, which required molecular proximity of the two components on the membrane. To elicit more global effects of activity modulation on 'bystander' neurons not under direct control, we used densely-expressed depolarizing (ChR2) or hyperpolarizing (eArch3.0, eNpHR3.0) tools to create a slow non-synaptic membrane current in bystander neurons, which matched the current direction seen in the directly modulated neurons. Extracellular protons played contributory role but were insufficient to explain the entire bystander effect, suggesting the recruitment of other mechanisms. Together, these findings present a new approach to the engineering of multicomponent optogenetic tools to manipulate ionic microdomains, and probe the complex neuronal-extracellular space interactions that regulate neural excitability.

  View details for DOI 10.1038/srep23947

  View details for Web of Science ID 000373399700001

  View details for PubMedCentralID PMC4820717

• A fast multi-obstacle muscle wrapping method using natural geodesic variations MULTIBODY SYSTEM DYNAMICS Scholz, A., Sherman, M., Stavness, I., Delp, S., Kecskemethy, A. 2016; 36 (2): 195-219

  View details for DOI 10.1007/s11044-015-9451-1

  View details for Web of Science ID 000371795500004

• Structural foundations of optogenetics: Determinants of channelrhodopsin ion selectivity PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Berndt, A., Lee, S. Y., Wietek, J., Ramakrishnan, C., Steinberg, E. E., Rashid, A. J., Kim, H., Park, S., Santoro, A., Frankland, P. W., Iyer, S. M., Pak, S., Ahrlund-Richter, S., Delp, S. L., Malenka, R. C., Josselyn, S. A., Carlen, M., Hegemann, P., Deisseroth, K. 2016; 113 (4): 822-829

  Abstract

  The structure-guided design of chloride-conducting channelrhodopsins has illuminated mechanisms underlying ion selectivity of this remarkable family of light-activated ion channels. The first generation of chloride-conducting channelrhodopsins, guided in part by development of a structure-informed electrostatic model for pore selectivity, included both the introduction of amino acids with positively charged side chains into the ion conduction pathway and the removal of residues hypothesized to support negatively charged binding sites for cations. Engineered channels indeed became chloride selective, reversing near -65 mV and enabling a new kind of optogenetic inhibition; however, these first-generation chloride-conducting channels displayed small photocurrents and were not tested for optogenetic inhibition of behavior. Here we report the validation and further development of the channelrhodopsin pore model via crystal structure-guided engineering of next-generation light-activated chloride channels (iC++) and a bistable variant (SwiChR++) with net photocurrents increased more than 15-fold under physiological conditions, reversal potential further decreased by another ∼ 15 mV, inhibition of spiking faithfully tracking chloride gradients and intrinsic cell properties, strong expression in vivo, and the initial microbial opsin channel-inhibitor-based control of freely moving behavior. We further show that inhibition by light-gated chloride channels is mediated mainly by shunting effects, which exert optogenetic control much more efficiently than the hyperpolarization induced by light-activated chloride pumps. The design and functional features of these next-generation chloride-conducting channelrhodopsins provide both chronic and acute timescale tools for reversible optogenetic inhibition, confirm fundamental predictions of the ion selectivity model, and further elucidate electrostatic and steric structure-function relationships of the light-gated pore.

  View details for DOI 10.1073/pnas.1523341113

  View details for Web of Science ID 000368617900023

  View details for PubMedCentralID PMC4743797

• Structural foundations of optogenetics: Determinants of channelrhodopsin ion selectivity. Proceedings of the National Academy of Sciences of the United States of America Berndt, A., Lee, S. Y., Wietek, J., Ramakrishnan, C., Steinberg, E. E., Rashid, A. J., Kim, H., Park, S., Santoro, A., Frankland, P. W., Iyer, S. M., Pak, S., Ährlund-Richter, S., Delp, S. L., Malenka, R. C., Josselyn, S. A., Carlén, M., Hegemann, P., Deisseroth, K. 2016; 113 (4): 822-9

  Abstract

  The structure-guided design of chloride-conducting channelrhodopsins has illuminated mechanisms underlying ion selectivity of this remarkable family of light-activated ion channels. The first generation of chloride-conducting channelrhodopsins, guided in part by development of a structure-informed electrostatic model for pore selectivity, included both the introduction of amino acids with positively charged side chains into the ion conduction pathway and the removal of residues hypothesized to support negatively charged binding sites for cations. Engineered channels indeed became chloride selective, reversing near -65 mV and enabling a new kind of optogenetic inhibition; however, these first-generation chloride-conducting channels displayed small photocurrents and were not tested for optogenetic inhibition of behavior. Here we report the validation and further development of the channelrhodopsin pore model via crystal structure-guided engineering of next-generation light-activated chloride channels (iC++) and a bistable variant (SwiChR++) with net photocurrents increased more than 15-fold under physiological conditions, reversal potential further decreased by another ∼ 15 mV, inhibition of spiking faithfully tracking chloride gradients and intrinsic cell properties, strong expression in vivo, and the initial microbial opsin channel-inhibitor-based control of freely moving behavior. We further show that inhibition by light-gated chloride channels is mediated mainly by shunting effects, which exert optogenetic control much more efficiently than the hyperpolarization induced by light-activated chloride pumps. The design and functional features of these next-generation chloride-conducting channelrhodopsins provide both chronic and acute timescale tools for reversible optogenetic inhibition, confirm fundamental predictions of the ion selectivity model, and further elucidate electrostatic and steric structure-function relationships of the light-gated pore.

  View details for DOI 10.1073/pnas.1523341113

  View details for PubMedID 26699459

  View details for PubMedCentralID PMC4743797

• A Biomechanical Model of the Scapulothoracic Joint to Accurately Capture Scapular Kinematics during Shoulder Movements PLOS ONE Seth, A., Matias, R., Veloso, A. P., Delp, S. L. 2016; 11 (1)

  View details for DOI 10.1371/journal.pone.0141028

  View details for Web of Science ID 000367805100001

• A Biomechanical Model of the Scapulothoracic Joint to Accurately Capture Scapular Kinematics during Shoulder Movements. PloS one Seth, A., Matias, R., Veloso, A. P., Delp, S. L. 2016; 11 (1): e0141028

  Abstract

  The complexity of shoulder mechanics combined with the movement of skin relative to the scapula makes it difficult to measure shoulder kinematics with sufficient accuracy to distinguish between symptomatic and asymptomatic individuals. Multibody skeletal models can improve motion capture accuracy by reducing the space of possible joint movements, and models are used widely to improve measurement of lower limb kinematics. In this study, we developed a rigid-body model of a scapulothoracic joint to describe the kinematics of the scapula relative to the thorax. This model describes scapular kinematics with four degrees of freedom: 1) elevation and 2) abduction of the scapula on an ellipsoidal thoracic surface, 3) upward rotation of the scapula normal to the thoracic surface, and 4) internal rotation of the scapula to lift the medial border of the scapula off the surface of the thorax. The surface dimensions and joint axes can be customized to match an individual's anthropometry. We compared the model to "gold standard" bone-pin kinematics collected during three shoulder tasks and found modeled scapular kinematics to be accurate to within 2 mm root-mean-squared error for individual bone-pin markers across all markers and movement tasks. As an additional test, we added random and systematic noise to the bone-pin marker data and found that the model reduced kinematic variability due to noise by 65% compared to Euler angles computed without the model. Our scapulothoracic joint model can be used for inverse and forward dynamics analyses and to compute joint reaction loads. The computational performance of the scapulothoracic joint model is well suited for real-time applications; it is freely available for use with OpenSim 3.2, and is customizable and usable with other OpenSim models.

  View details for DOI 10.1371/journal.pone.0141028

  View details for PubMedID 26734761

  View details for PubMedCentralID PMC4712143

• Optogenetic approaches addressing extracellular modulation of neural excitability. Scientific reports Ferenczi, E. A., Vierock, J., Atsuta-Tsunoda, K., Tsunoda, S. P., Ramakrishnan, C., Gorini, C., Thompson, K., Lee, S. Y., Berndt, A., Perry, C., Minniberger, S., Vogt, A., Mattis, J., Prakash, R., Delp, S., Deisseroth, K., Hegemann, P. 2016; 6: 23947-?

  Abstract

  The extracellular ionic environment in neural tissue has the capacity to influence, and be influenced by, natural bouts of neural activity. We employed optogenetic approaches to control and investigate these interactions within and between cells, and across spatial scales. We began by developing a temporally precise means to study microdomain-scale interactions between extracellular protons and acid-sensing ion channels (ASICs). By coupling single-component proton-transporting optogenetic tools to ASICs to create two-component optogenetic constructs (TCOs), we found that acidification of the local extracellular membrane surface by a light-activated proton pump recruited a slow inward ASIC current, which required molecular proximity of the two components on the membrane. To elicit more global effects of activity modulation on 'bystander' neurons not under direct control, we used densely-expressed depolarizing (ChR2) or hyperpolarizing (eArch3.0, eNpHR3.0) tools to create a slow non-synaptic membrane current in bystander neurons, which matched the current direction seen in the directly modulated neurons. Extracellular protons played contributory role but were insufficient to explain the entire bystander effect, suggesting the recruitment of other mechanisms. Together, these findings present a new approach to the engineering of multicomponent optogenetic tools to manipulate ionic microdomains, and probe the complex neuronal-extracellular space interactions that regulate neural excitability.

  View details for DOI 10.1038/srep23947

  View details for PubMedID 27045897

• Optogenetic and chemogenetic strategies for sustained inhibition of pain. Scientific reports Iyer, S. M., Vesuna, S., Ramakrishnan, C., Huynh, K., Young, S., Berndt, A., Lee, S. Y., Gorini, C. J., Deisseroth, K., Delp, S. L. 2016; 6: 30570-?

  Abstract

  Spatially targeted, genetically-specific strategies for sustained inhibition of nociceptors may help transform pain science and clinical management. Previous optogenetic strategies to inhibit pain have required constant illumination, and chemogenetic approaches in the periphery have not been shown to inhibit pain. Here, we show that the step-function inhibitory channelrhodopsin, SwiChR, can be used to persistently inhibit pain for long periods of time through infrequent transdermally delivered light pulses, reducing required light exposure by >98% and resolving a long-standing limitation in optogenetic inhibition. We demonstrate that the viral expression of the hM4D receptor in small-diameter primary afferent nociceptor enables chemogenetic inhibition of mechanical and thermal nociception thresholds. Finally, we develop optoPAIN, an optogenetic platform to non-invasively assess changes in pain sensitivity, and use this technique to examine pharmacological and chemogenetic inhibition of pain.

  View details for DOI 10.1038/srep30570

  View details for PubMedID 27484850

  View details for PubMedCentralID PMC4971509

• Stretching Your Energetic Budget: How Tendon Compliance Affects the Metabolic Cost of Running. PloS one Uchida, T. K., Hicks, J. L., Dembia, C. L., Delp, S. L. 2016; 11 (3)

  Abstract

  Muscles attach to bones via tendons that stretch and recoil, affecting muscle force generation and metabolic energy consumption. In this study, we investigated the effect of tendon compliance on the metabolic cost of running using a full-body musculoskeletal model with a detailed model of muscle energetics. We performed muscle-driven simulations of running at 2-5 m/s with tendon force-strain curves that produced between 1 and 10% strain when the muscles were developing maximum isometric force. We computed the average metabolic power consumed by each muscle when running at each speed and with each tendon compliance. Average whole-body metabolic power consumption increased as running speed increased, regardless of tendon compliance, and was lowest at each speed when tendon strain reached 2-3% as muscles were developing maximum isometric force. When running at 2 m/s, the soleus muscle consumed less metabolic power at high tendon compliance because the strain of the tendon allowed the muscle fibers to operate nearly isometrically during stance. In contrast, the medial and lateral gastrocnemii consumed less metabolic power at low tendon compliance because less compliant tendons allowed the muscle fibers to operate closer to their optimal lengths during stance. The software and simulations used in this study are freely available at simtk.org and enable examination of muscle energetics with unprecedented detail.

  View details for DOI 10.1371/journal.pone.0150378

  View details for PubMedID 26930416

  View details for PubMedCentralID PMC4773147

• In Vivo Imaging of Human Sarcomere Twitch Dynamics in Individual Motor Units. Neuron Sanchez, G. N., Sinha, S., Liske, H., Chen, X., Nguyen, V., Delp, S. L., Schnitzer, M. J. 2015; 88 (6): 1109-20

  Abstract

  Motor units comprise a pre-synaptic motor neuron and multiple post-synaptic muscle fibers. Many movement disorders disrupt motor unit contractile dynamics and the structure of sarcomeres, skeletal muscle's contractile units. Despite the motor unit's centrality to neuromuscular physiology, no extant technology can image sarcomere twitch dynamics in live humans. We created a wearable microscope equipped with a microendoscope for minimally invasive observation of sarcomere lengths and contractile dynamics in any major skeletal muscle. By electrically stimulating twitches via the microendoscope and visualizing the sarcomere displacements, we monitored single motor unit contractions in soleus and vastus lateralis muscles of healthy individuals. Control experiments verified that these evoked twitches involved neuromuscular transmission and faithfully reported muscle force generation. In post-stroke patients with spasticity of the biceps brachii, we found involuntary microscopic contractions and sarcomere length abnormalities. The wearable microscope facilitates exploration of many basic and disease-related neuromuscular phenomena never visualized before in live humans. VIDEO ABSTRACT.

  View details for DOI 10.1016/j.neuron.2015.11.022

  View details for PubMedID 26687220

• In Vivo Imaging of Human Sarcomere Twitch Dynamics in Individual Motor Units NEURON Sanchez, G. N., Sinha, S., Liske, H., Chen, X., Viet Nguyen, V., Delp, S. L., Schnitzer, M. J. 2015; 88 (6): 1109-1120

  Abstract

  Motor units comprise a pre-synaptic motor neuron and multiple post-synaptic muscle fibers. Many movement disorders disrupt motor unit contractile dynamics and the structure of sarcomeres, skeletal muscle's contractile units. Despite the motor unit's centrality to neuromuscular physiology, no extant technology can image sarcomere twitch dynamics in live humans. We created a wearable microscope equipped with a microendoscope for minimally invasive observation of sarcomere lengths and contractile dynamics in any major skeletal muscle. By electrically stimulating twitches via the microendoscope and visualizing the sarcomere displacements, we monitored single motor unit contractions in soleus and vastus lateralis muscles of healthy individuals. Control experiments verified that these evoked twitches involved neuromuscular transmission and faithfully reported muscle force generation. In post-stroke patients with spasticity of the biceps brachii, we found involuntary microscopic contractions and sarcomere length abnormalities. The wearable microscope facilitates exploration of many basic and disease-related neuromuscular phenomena never visualized before in live humans. VIDEO ABSTRACT.

  View details for DOI 10.1016/j.neuron.2015.11.022

  View details for Web of Science ID 000368443900008

• The Role of Cartilage Stress in Patellofemoral Pain MEDICINE AND SCIENCE IN SPORTS AND EXERCISE Besier, T. F., Pal, S., Draper, C. E., Fredericson, M., Gold, G. E., Delp, S. L., Beaupre, G. S. 2015; 47 (11): 2416-2422

  Abstract

  Elevated cartilage stress has been identified as a potential mechanism for retropatellar pain; however, there are limited data in the literature to support this mechanism. Females are more likely to develop patellofemoral pain than males, yet the causes of this dimorphism are unclear. We used experimental data and computational modeling to determine whether patients with patellofemoral pain had elevated cartilage stress compared with pain-free controls and test the hypothesis that females exhibit greater cartilage stress than males.We created finite element models of 24 patients with patellofemoral pain (11 males and 13 females) and 16 pain-free controls (8 males and 8 females) to estimate peak patellar cartilage stress (strain energy density) during a stair climb activity. Simulations took into account cartilage morphology from magnetic resonance imaging, joint posture from weight-bearing magnetic resonance imaging, and muscle forces from an EMG-driven model.We found no difference in peak patellar strain energy density between those with patellofemoral pain (1.9 ± 1.23 J·m(-3)) and control subjects (1.66 ± 0.75 J·m(-3), P = 0.52). Females exhibited greater cartilage stress compared with males (2.2 vs 1.3 J·m(-3), respectively; P = 0.0075), with large quadriceps muscle forces (3.7 body weight in females vs 3.3 body weight in males) and 23% smaller joint contact area (females, 467 ± 59 mm2, vs males, 608 ± 95 mm2).Patients with patellofemoral pain did not display significantly greater patellar cartilage stress compared with pain-free controls; however, there was a great deal of subject variation. Females exhibited greater peak cartilage stress compared with males, which might explain the greater prevalence of patellofemoral pain in females compared with that in males, but other mechanical and biological factors are clearly involved in this complex pathway to pain.

  View details for DOI 10.1249/MSS.0000000000000685

  View details for Web of Science ID 000362940900021

  View details for PubMedID 25899103

  View details for PubMedCentralID PMC4609225

• The mobilize center: an NIH big data to knowledge center to advance human movement research and improve mobility. Journal of the American Medical Informatics Association Ku, J. P., Hicks, J. L., Hastie, T., Leskovec, J., Ré, C., Delp, S. L. 2015; 22 (6): 1120-1125

  Abstract

  Regular physical activity helps prevent heart disease, stroke, diabetes, and other chronic diseases, yet a broad range of conditions impair mobility at great personal and societal cost. Vast amounts of data characterizing human movement are available from research labs, clinics, and millions of smartphones and wearable sensors, but integration and analysis of this large quantity of mobility data are extremely challenging. The authors have established the Mobilize Center (http://mobilize.stanford.edu) to harness these data to improve human mobility and help lay the foundation for using data science methods in biomedicine. The Center is organized around 4 data science research cores: biomechanical modeling, statistical learning, behavioral and social modeling, and integrative modeling. Important biomedical applications, such as osteoarthritis and weight management, will focus the development of new data science methods. By developing these new approaches, sharing data and validated software tools, and training thousands of researchers, the Mobilize Center will transform human movement research.

  View details for DOI 10.1093/jamia/ocv071

  View details for PubMedID 26272077

  View details for PubMedCentralID PMC4639715

• Wirelessly powered, fully internal optogenetics for brain, spinal and peripheral circuits in mice. Nature methods Montgomery, K. L., Yeh, A. J., Ho, J. S., Tsao, V., Mohan Iyer, S., Grosenick, L., Ferenczi, E. A., Tanabe, Y., Deisseroth, K., Delp, S. L., Poon, A. S. 2015; 12 (10): 969-974

  Abstract

  To enable sophisticated optogenetic manipulation of neural circuits throughout the nervous system with limited disruption of animal behavior, light-delivery systems beyond fiber optic tethering and large, head-mounted wireless receivers are desirable. We report the development of an easy-to-construct, implantable wireless optogenetic device. Our smallest version (20 mg, 10 mm(3)) is two orders of magnitude smaller than previously reported wireless optogenetic systems, allowing the entire device to be implanted subcutaneously. With a radio-frequency (RF) power source and controller, this implant produces sufficient light power for optogenetic stimulation with minimal tissue heating (<1 °C). We show how three adaptations of the implant allow for untethered optogenetic control throughout the nervous system (brain, spinal cord and peripheral nerve endings) of behaving mice. This technology opens the door for optogenetic experiments in which animals are able to behave naturally with optogenetic manipulation of both central and peripheral targets.

  View details for DOI 10.1038/nmeth.3536

  View details for PubMedID 26280330

• Self-Tracking Energy Transfer for Neural Stimulation in Untethered Mice PHYSICAL REVIEW APPLIED Ho, J. S., Tanabe, Y., Iyer, S. M., Christensen, A. J., Grosenick, L., Deisseroth, K., Delp, S. L., Poon, A. S. 2015; 4 (2)

  View details for DOI 10.1103/PhysRevApplied.4.024001

  View details for Web of Science ID 000358939400001

• Muscle velocity and inertial force from phase contrast MRI JOURNAL OF MAGNETIC RESONANCE IMAGING Wentland, A. L., McWalter, E. J., Pal, S., Delp, S. L., Gold, G. E. 2015; 42 (2): 526-532

  Abstract

  To evaluate velocity waveforms in muscle and to create a tool and algorithm for computing and analyzing muscle inertial forces derived from 2D phase contrast (PC) magnetic resonance imaging (MRI).PC MRI was performed in the forearm of four healthy volunteers during 1 Hz cycles of wrist flexion-extension as well as in the lower leg of six healthy volunteers during 1 Hz cycles of plantarflexion-dorsiflexion. Inertial forces (F) were derived via the equation F = ma. The mass, m, was derived by multiplying voxel volume by voxel-by-voxel estimates of density via fat-water separation techniques. Acceleration, a, was obtained via the derivative of the PC MRI velocity waveform.Mean velocities in the flexors of the forearm and lower leg were 1.94 ± 0.97 cm/s and 5.57 ± 2.72 cm/s, respectively, as averaged across all subjects; the inertial forces in the flexors of the forearm and lower leg were 1.9 × 10(-3)  ± 1.3 × 10(-3) N and 1.1 × 10(-2)  ± 6.1 × 10(-3) N, respectively, as averaged across all subjects.PC MRI provided a promising means of computing muscle velocities and inertial forces-providing the first method for quantifying inertial forces. J. Magn. Reson. Imaging 2015;42:526-532.

  View details for DOI 10.1002/jmri.24807

  View details for Web of Science ID 000358258600001

  View details for PubMedCentralID PMC4442766

• Muscle velocity and inertial force from phase contrast MRI. Journal of magnetic resonance imaging : JMRI Wentland, A. L., McWalter, E. J., Pal, S., Delp, S. L., Gold, G. E. 2015; 42 (2): 526-32

  Abstract

  To evaluate velocity waveforms in muscle and to create a tool and algorithm for computing and analyzing muscle inertial forces derived from 2D phase contrast (PC) magnetic resonance imaging (MRI).PC MRI was performed in the forearm of four healthy volunteers during 1 Hz cycles of wrist flexion-extension as well as in the lower leg of six healthy volunteers during 1 Hz cycles of plantarflexion-dorsiflexion. Inertial forces (F) were derived via the equation F = ma. The mass, m, was derived by multiplying voxel volume by voxel-by-voxel estimates of density via fat-water separation techniques. Acceleration, a, was obtained via the derivative of the PC MRI velocity waveform.Mean velocities in the flexors of the forearm and lower leg were 1.94 ± 0.97 cm/s and 5.57 ± 2.72 cm/s, respectively, as averaged across all subjects; the inertial forces in the flexors of the forearm and lower leg were 1.9 × 10(-3)  ± 1.3 × 10(-3) N and 1.1 × 10(-2)  ± 6.1 × 10(-3) N, respectively, as averaged across all subjects.PC MRI provided a promising means of computing muscle velocities and inertial forces-providing the first method for quantifying inertial forces. J. Magn. Reson. Imaging 2015;42:526-532.

  View details for DOI 10.1002/jmri.24807

  View details for PubMedID 25425185

  View details for PubMedCentralID PMC4442766

• Muscle velocity and inertial force from phase contrast MRI. Journal of magnetic resonance imaging Wentland, A. L., McWalter, E. J., Pal, S., Delp, S. L., Gold, G. E. 2015; 42 (2): spcone-?

  Abstract

  To evaluate velocity waveforms in muscle and to create a tool and algorithm for computing and analyzing muscle inertial forces derived from 2D phase contrast (PC) magnetic resonance imaging (MRI).PC MRI was performed in the forearm of four healthy volunteers during 1 Hz cycles of wrist flexion-extension as well as in the lower leg of six healthy volunteers during 1 Hz cycles of plantarflexion-dorsiflexion. Inertial forces (F) were derived via the equation F = ma. The mass, m, was derived by multiplying voxel volume by voxel-by-voxel estimates of density via fat-water separation techniques. Acceleration, a, was obtained via the derivative of the PC MRI velocity waveform.Mean velocities in the flexors of the forearm and lower leg were 1.94 ± 0.97 cm/s and 5.57 ± 2.72 cm/s, respectively, as averaged across all subjects; the inertial forces in the flexors of the forearm and lower leg were 1.9 × 10(-3)  ± 1.3 × 10(-3) N and 1.1 × 10(-2)  ± 6.1 × 10(-3) N, respectively, as averaged across all subjects.PC MRI provided a promising means of computing muscle velocities and inertial forces-providing the first method for quantifying inertial forces. J. Magn. Reson. Imaging 2015;42:526-532.

  View details for DOI 10.1002/jmri.25013

  View details for PubMedID 26192553

• Musculoskeletal modelling of an ostrich (Struthio camelus) pelvic limb: influence of limb orientation on muscular capacity during locomotion PEERJ Hutchinson, J. R., Rankin, J., Rubenson, J., Rosenbluth, K. H., Siston, R. A., Delp, S. L. 2015; 3

  Abstract

  We developed a three-dimensional, biomechanical computer model of the 36 major pelvic limb muscle groups in an ostrich (Struthio camelus) to investigate muscle function in this, the largest of extant birds and model organism for many studies of locomotor mechanics, body size, anatomy and evolution. Combined with experimental data, we use this model to test two main hypotheses. We first query whether ostriches use limb orientations (joint angles) that optimize the moment-generating capacities of their muscles during walking or running. Next, we test whether ostriches use limb orientations at mid-stance that keep their extensor muscles near maximal, and flexor muscles near minimal, moment arms. Our two hypotheses relate to the control priorities that a large bipedal animal might evolve under biomechanical constraints to achieve more effective static weight support. We find that ostriches do not use limb orientations to optimize the moment-generating capacities or moment arms of their muscles. We infer that dynamic properties of muscles or tendons might be better candidates for locomotor optimization. Regardless, general principles explaining why species choose particular joint orientations during locomotion are lacking, raising the question of whether such general principles exist or if clades evolve different patterns (e.g., weighting of muscle force-length or force-velocity properties in selecting postures). This leaves theoretical studies of muscle moment arms estimated for extinct animals at an impasse until studies of extant taxa answer these questions. Finally, we compare our model's results against those of two prior studies of ostrich limb muscle moment arms, finding general agreement for many muscles. Some flexor and extensor muscles exhibit self-stabilization patterns (posture-dependent switches between flexor/extensor action) that ostriches may use to coordinate their locomotion. However, some conspicuous areas of disagreement in our results illustrate some cautionary principles. Importantly, tendon-travel empirical measurements of muscle moment arms must be carefully designed to preserve 3D muscle geometry lest their accuracy suffer relative to that of anatomically realistic models. The dearth of accurate experimental measurements of 3D moment arms of muscles in birds leaves uncertainty regarding the relative accuracy of different modelling or experimental datasets such as in ostriches. Our model, however, provides a comprehensive set of 3D estimates of muscle actions in ostriches for the first time, emphasizing that avian limb mechanics are highly three-dimensional and complex, and how no muscles act purely in the sagittal plane. A comparative synthesis of experiments and models such as ours could provide powerful synthesis into how anatomy, mechanics and control interact during locomotion and how these interactions evolve. Such a framework could remove obstacles impeding the analysis of muscle function in extinct taxa.

  View details for DOI 10.7717/peerj.1001

  View details for Web of Science ID 000356351100005

  View details for PubMedID 26082859

  View details for PubMedCentralID PMC4465956

• Making a meaningful impact: modelling simultaneous frictional collisions in spatial multibody systems PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES Uchida, T. K., Sherman, M. A., Delp, S. L. 2015; 471 (2177)

  View details for DOI 10.1098/rspa.2014.0859

  View details for Web of Science ID 000353352400007

• Making a meaningful impact: modelling simultaneous frictional collisions in spatial multibody systems. Proceedings. Mathematical, physical, and engineering sciences Uchida, T. K., Sherman, M. A., Delp, S. L. 2015; 471 (2177): 20140859

  Abstract

  Impacts are instantaneous, computationally efficient approximations of collisions. Current impact models sacrifice important physical principles to achieve that efficiency, yielding qualitative and quantitative errors when applied to simultaneous impacts in spatial multibody systems. We present a new impact model that produces behaviour similar to that of a detailed compliant contact model, while retaining the efficiency of an instantaneous method. In our model, time and configuration are fixed, but the impact is resolved into distinct compression and expansion phases, themselves comprising sliding and rolling intervals. A constrained optimization problem is solved for each interval to compute incremental impulses while respecting physical laws and principles of contact mechanics. We present the mathematical model, algorithms for its practical implementation, and examples that demonstrate its effectiveness. In collisions involving materials of various stiffnesses, our model can be more than 20 times faster than integrating through the collision using a compliant contact model. This work extends the use of instantaneous impact models to scientific and engineering applications with strict accuracy requirements, where compliant contact models would otherwise be required. An open-source implementation is available in Simbody, a C++ multibody dynamics library widely used in biomechanical and robotic applications.

  View details for DOI 10.1098/rspa.2014.0859

  View details for PubMedID 27547093

  View details for PubMedCentralID PMC4984984

• Running with a load increases leg stiffness JOURNAL OF BIOMECHANICS Slider, A., Besier, T., Delp, S. L. 2015; 48 (6): 1003-1008

  Abstract

  Spring-mass models have been used to characterize running mechanics and leg stiffness in a variety of conditions, yet it remains unknown how running while carrying a load affects running mechanics and leg stiffness. The purpose of this study was to test the hypothesis that running with a load increases leg stiffness. Twenty-seven subjects ran at a constant speed on a force-measuring treadmill while carrying no load, and while wearing weight vests loaded with 10%, 20%, and 30% of body weight. We measured lower extremity motion and created a scaled musculoskeletal model of each subject, which we used to estimate lower extremity joint angles and leg length. We estimated dimensionless leg stiffness as the ratio of the peak vertical ground reaction force (normalized to body weight) and the change in stance phase leg length (normalized to leg length at initial foot contact). Leg length was calculated as the distance from the center of the pelvis to the center-of-pressure under the foot. We found that dimensionless leg stiffness increased when running with load (p=0.001); this resulted from an increase in the peak vertical ground reaction force (p<0.001) and a smaller change in stance phase leg length (p=0.025). When running with load, subjects had longer ground contact times (p<0.020), greater hip (p<0.001) and knee flexion (p=0.048) at the time of initial foot contact, and greater peak stance phase hip, knee, and ankle flexion (p<0.05). Our results reveal that subjects run in a more crouched posture and with higher leg stiffness to accommodate an added load.

  View details for DOI 10.1016/j.jbiomech.2015.01.051

  View details for Web of Science ID 000352672100017

  View details for PubMedID 25728581

• Use it or lose it: multiscale skeletal muscle adaptation to mechanical stimuli. Biomechanics and modeling in mechanobiology Wisdom, K. M., Delp, S. L., Kuhl, E. 2015; 14 (2): 195-215

  Abstract

  Skeletal muscle undergoes continuous turnover to adapt to changes in its mechanical environment. Overload increases muscle mass, whereas underload decreases muscle mass. These changes are correlated with, and enabled by, structural alterations across the molecular, subcellular, cellular, tissue, and organ scales. Despite extensive research on muscle adaptation at the individual scales, the interaction of the underlying mechanisms across the scales remains poorly understood. Here, we present a thorough review and a broad classification of multiscale muscle adaptation in response to a variety of mechanical stimuli. From this classification, we suggest that a mathematical model for skeletal muscle adaptation should include the four major stimuli, overstretch, understretch, overload, and underload, and the five key players in skeletal muscle adaptation, myosin heavy chain isoform, serial sarcomere number, parallel sarcomere number, pennation angle, and extracellular matrix composition. Including this information in multiscale computational models of muscle will shape our understanding of the interacting mechanisms of skeletal muscle adaptation across the scales. Ultimately, this will allow us to rationalize the design of exercise and rehabilitation programs, and improve the long-term success of interventional treatment in musculoskeletal disease.

  View details for DOI 10.1007/s10237-014-0607-3

  View details for PubMedID 25199941

  View details for PubMedCentralID PMC4352121

• T1 rho Dispersion in Articular Cartilage: Relationship to Material Properties and Macromolecular Content CARTILAGE Keenan, K. E., Besier, T. F., Pauly, J. M., Smith, R. L., Delp, S. L., Beaupre, G. S., Gold, G. E. 2015; 6 (2): 113-122

  Abstract

  This study assessed T1ρ relaxation dispersion, measured by magnetic resonance imaging (MRI), as a tool to noninvasively evaluate cartilage material and biochemical properties. The specific objective was to answer two questions: (1) does cartilage initial elastic modulus (E 0) correlate with T1ρ dispersion effects and (2) does collagen or proteoglycan content correlate with T1ρ dispersion effects?Cadaveric patellae with and without visible cartilage damage on conventional MR were included. T2 and T1ρ relaxation times at 500 and 1000 Hz spin-lock field amplitudes were measured. We estimated T1ρ dispersion effects by measuring T1ρ relaxation time at 500 and 1000 Hz and T2 relaxation time and using a new tool, the ratio T1ρ/T2. Cartilage initial elastic modulus, E 0, was measured from initial response of mechanical indentation creep tests. Collagen and proteoglycan contents were measured at the indentation test sites; proteoglycan content was measured by their covalently linked sulfated glycosaminoglycans (sGAG). Pearson correlation coefficients were determined, taking into account the clustering of multiple samples within a single patella specimen.Cartilage initial elastic modulus, E 0, increased with decreasing values of T1ρ/T2 measurements at both 500 Hz (P = 0.034) and 1000 Hz (P = 0.022). 1/T1ρ relaxation time (500 Hz) increased with increasing sGAG content (P = 0.041).T1ρ/T2 ratio, a new tool, and cartilage initial elastic modulus are both measures of water-protein interactions, are dependent on the cartilage structure, and were correlated in this study.

  View details for DOI 10.1177/1947603515569529

  View details for Web of Science ID 000356631400006

  View details for PubMedID 26069714

  View details for PubMedCentralID PMC4462251

• Use it or lose it: multiscale skeletal muscle adaptation to mechanical stimuli. Biomechanics and modeling in mechanobiology Wisdom, K. M., Delp, S. L., Kuhl, E. 2015; 14 (2): 195-215

  View details for DOI 10.1007/s10237-014-0607-3

  View details for PubMedID 25199941

• Predictive Simulation Generates Human Adaptations during Loaded and Inclined Walking PLOS ONE Dorn, T. W., Wang, J. M., Hicks, J. L., Delp, S. L. 2015; 10 (4)

  Abstract

  Predictive simulation is a powerful approach for analyzing human locomotion. Unlike techniques that track experimental data, predictive simulations synthesize gaits by minimizing a high-level objective such as metabolic energy expenditure while satisfying task requirements like achieving a target velocity. The fidelity of predictive gait simulations has only been systematically evaluated for locomotion data on flat ground. In this study, we construct a predictive simulation framework based on energy minimization and use it to generate normal walking, along with walking with a range of carried loads and up a range of inclines. The simulation is muscle-driven and includes controllers based on muscle force and stretch reflexes and contact state of the legs. We demonstrate how human-like locomotor strategies emerge from adapting the model to a range of environmental changes. Our simulation dynamics not only show good agreement with experimental data for normal walking on flat ground (92% of joint angle trajectories and 78% of joint torque trajectories lie within 1 standard deviation of experimental data), but also reproduce many of the salient changes in joint angles, joint moments, muscle coordination, and metabolic energy expenditure observed in experimental studies of loaded and inclined walking.

  View details for DOI 10.1371/journal.pone.0121407

  View details for Web of Science ID 000352135600071

  View details for PubMedID 25830913

  View details for PubMedCentralID PMC4382289

• How tibiofemoral alignment and contact locations affect predictions of medial and lateral tibiofemoral contact forces. Journal of biomechanics Lerner, Z. F., Demers, M. S., Delp, S. L., Browning, R. C. 2015; 48 (4): 644-650

  Abstract

  Understanding degeneration of biological and prosthetic knee joints requires knowledge of the in-vivo loading environment during activities of daily living. Musculoskeletal models can estimate medial/lateral tibiofemoral compartment contact forces, yet anthropometric differences between individuals make accurate predictions challenging. We developed a full-body OpenSim musculoskeletal model with a knee joint that incorporates subject-specific tibiofemoral alignment (i.e. knee varus-valgus) and geometry (i.e. contact locations). We tested the accuracy of our model and determined the importance of these subject-specific parameters by comparing estimated to measured medial and lateral contact forces during walking in an individual with an instrumented knee replacement and post-operative genu valgum (6°). The errors in the predictions of the first peak medial and lateral contact force were 12.4% and 11.9%, respectively, for a model with subject-specific tibiofemoral alignment and contact locations determined through radiographic analysis, vs. 63.1% and 42.0%, respectively, for a model with generic parameters. We found that each degree of tibiofemoral alignment deviation altered the first peak medial compartment contact force by 51N (r(2)=0.99), while each millimeter of medial-lateral translation of the compartment contact point locations altered the first peak medial compartment contact force by 41N (r(2)=0.99). The model, available at www.simtk.org/home/med-lat-knee/, enables the specification of subject-specific joint alignment and compartment contact locations to more accurately estimate medial and lateral tibiofemoral contact forces in individuals with non-neutral alignment.

  View details for DOI 10.1016/j.jbiomech.2014.12.049

  View details for PubMedID 25595425

  View details for PubMedCentralID PMC4330122

• Is My Model Good Enough? Best Practices for Verification and Validation of Musculoskeletal Models and Simulations of Movement JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME Hicks, J. L., Uchida, T. K., Seth, A., Rajagopal, A., Delp, S. L. 2015; 137 (2)

  Abstract

  Computational modeling and simulation of neuromusculoskeletal (NMS) systems enables researchers and clinicians to study the complex dynamics underlying human and animal movement. NMS models use equations derived from physical laws and biology to help solve challenging real-world problems, from designing prosthetics that maximize running speed to developing exoskeletal devices that enable walking after a stroke. NMS modeling and simulation has proliferated in the biomechanics research community over the past 25 years, but the lack of verification and validation standards remains a major barrier to wider adoption and impact. The goal of this paper is to establish practical guidelines for verification and validation of NMS models and simulations that researchers, clinicians, reviewers, and others can adopt to evaluate the accuracy and credibility of modeling studies. In particular, we review a general process for verification and validation applied to NMS models and simulations, including careful formulation of a research question and methods, traditional verification and validation steps, and documentation and sharing of results for use and testing by other researchers. Modeling the NMS system and simulating its motion involves methods to represent neural control, musculoskeletal geometry, muscle-tendon dynamics, contact forces, and multibody dynamics. For each of these components, we review modeling choices and software verification guidelines; discuss variability, errors, uncertainty, and sensitivity relationships; and provide recommendations for verification and validation by comparing experimental data and testing robustness. We present a series of case studies to illustrate key principles. In closing, we discuss challenges the community must overcome to ensure that modeling and simulation are successfully used to solve the broad spectrum of problems that limit human mobility.

  View details for DOI 10.1115/1.4029304

  View details for Web of Science ID 000350571400005

  View details for PubMedID 25474098

  View details for PubMedCentralID PMC4321112

• Differences in muscle activity between natural forefoot and rearfoot strikers during running JOURNAL OF BIOMECHANICS Yong, J. R., Silder, A., Delp, S. L. 2014; 47 (15): 3593-3597

  Abstract

  Running research has focused on reducing injuries by changing running technique. One proposed method is to change from rearfoot striking (RFS) to forefoot striking (FFS) because FFS is thought to be a more natural running pattern that may reduce loading and injury risk. Muscle activity affects loading and influences running patterns; however, the differences in muscle activity between natural FFS runners and natural RFS runners are unknown. The purpose of this study was to measure muscle activity in natural FFS runners and natural RFS runners. We tested the hypotheses that tibialis anterior activity would be significantly lower while activity of the plantarflexors would be significantly greater in FFS runners, compared to RFS runners, during late swing phase and early stance phase. Gait kinematics, ground reaction forces and electromyographic patterns of ten muscles were collected from twelve natural RFS runners and ten natural FFS runners. The root mean square (RMS) of each muscle׳s activity was calculated during terminal swing phase and early stance phase. We found significantly lower RMS activity in the tibialis anterior in FFS runners during terminal swing phase, compared to RFS runners. In contrast, the medial and lateral gastrocnemius showed significantly greater RMS activity in terminal swing phase in FFS runners. No significant differences were found during early stance phase for the tibialis anterior or the plantarflexors. Recognizing the differences in muscle activity between FFS and RFS runners is an important step toward understanding how foot strike patterns may contribute to different types of injury.

  View details for DOI 10.1016/j.jbiomech.2014.10.015

  View details for Web of Science ID 000345805500001

  View details for PubMedID 25458201

  View details for PubMedCentralID PMC4301610

• Are Subject-Specific Musculoskeletal Models Robust to the Uncertainties in Parameter Identification? PLOS ONE Valente, G., Pitto, L., Testi, D., Seth, A., Delp, S. L., Stagni, R., Viceconti, M., Taddei, F. 2014; 9 (11)

  Abstract

  Subject-specific musculoskeletal modeling can be applied to study musculoskeletal disorders, allowing inclusion of personalized anatomy and properties. Independent of the tools used for model creation, there are unavoidable uncertainties associated with parameter identification, whose effect on model predictions is still not fully understood. The aim of the present study was to analyze the sensitivity of subject-specific model predictions (i.e., joint angles, joint moments, muscle and joint contact forces) during walking to the uncertainties in the identification of body landmark positions, maximum muscle tension and musculotendon geometry. To this aim, we created an MRI-based musculoskeletal model of the lower limbs, defined as a 7-segment, 10-degree-of-freedom articulated linkage, actuated by 84 musculotendon units. We then performed a Monte-Carlo probabilistic analysis perturbing model parameters according to their uncertainty, and solving a typical inverse dynamics and static optimization problem using 500 models that included the different sets of perturbed variable values. Model creation and gait simulations were performed by using freely available software that we developed to standardize the process of model creation, integrate with OpenSim and create probabilistic simulations of movement. The uncertainties in input variables had a moderate effect on model predictions, as muscle and joint contact forces showed maximum standard deviation of 0.3 times body-weight and maximum range of 2.1 times body-weight. In addition, the output variables significantly correlated with few input variables (up to 7 out of 312) across the gait cycle, including the geometry definition of larger muscles and the maximum muscle tension in limited gait portions. Although we found subject-specific models not markedly sensitive to parameter identification, researchers should be aware of the model precision in relation to the intended application. In fact, force predictions could be affected by an uncertainty in the same order of magnitude of its value, although this condition has low probability to occur.

  View details for DOI 10.1371/journal.pone.0112625

  View details for Web of Science ID 000349144400111

  View details for PubMedID 25390896

  View details for PubMedCentralID PMC4229232

• Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping JOURNAL OF EXPERIMENTAL BIOLOGY Farris, D. J., Hicks, J. L., Delp, S. L., Sawicki, G. S. 2014; 217 (22): 4018-4028

  Abstract

  Experiments have shown that elastic ankle exoskeletons can be used to reduce ankle joint and plantar-flexor muscle loading when hopping in place and, in turn, reduce metabolic energy consumption. However, recent experimental work has shown that such exoskeletons cause less favourable soleus (SO) muscle-tendon mechanics than is observed during normal hopping, which might limit the capacity of the exoskeleton to reduce energy consumption. To directly link plantar-flexor mechanics and energy consumption when hopping in exoskeletons, we used a musculoskeletal model of the human leg and a model of muscle energetics in simulations of muscle-tendon dynamics during hopping with and without elastic ankle exoskeletons. Simulations were driven by experimental electromyograms, joint kinematics and exoskeleton torque taken from previously published data. The data were from seven males who hopped at 2.5 Hz with and without elastic ankle exoskeletons. The energetics model showed that the total rate of metabolic energy consumption by ankle muscles was not significantly reduced by an ankle exoskeleton. This was despite large reductions in plantar-flexor force production (40-50%). The lack of larger metabolic reductions with exoskeletons was attributed to increases in plantar-flexor muscle fibre velocities and a shift to less favourable muscle fibre lengths during active force production. This limited the capacity for plantar-flexors to reduce activation and energy consumption when hopping with exoskeleton assistance.

  View details for DOI 10.1242/jeb.107656

  View details for Web of Science ID 000344867000016

  View details for PubMedID 25278469

  View details for PubMedCentralID PMC4229366

• 3D finite element models of shoulder muscles for computing lines of actions and moment arms COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING Webb, J. D., Blemker, S. S., Delp, S. L. 2014; 17 (8): 829-837

  Abstract

  Accurate representation of musculoskeletal geometry is needed to characterise the function of shoulder muscles. Previous models of shoulder muscles have represented muscle geometry as a collection of line segments, making it difficult to account for the large attachment areas, muscle-muscle interactions and complex muscle fibre trajectories typical of shoulder muscles. To better represent shoulder muscle geometry, we developed 3D finite element models of the deltoid and rotator cuff muscles and used the models to examine muscle function. Muscle fibre paths within the muscles were approximated, and moment arms were calculated for two motions: thoracohumeral abduction and internal/external rotation. We found that muscle fibre moment arms varied substantially across each muscle. For example, supraspinatus is considered a weak external rotator, but the 3D model of supraspinatus showed that the anterior fibres provide substantial internal rotation while the posterior fibres act as external rotators. Including the effects of large attachment regions and 3D mechanical interactions of muscle fibres constrains muscle motion, generates more realistic muscle paths and allows deeper analysis of shoulder muscle function.

  View details for DOI 10.1080/10255842.2012.719605

  View details for Web of Science ID 000334075400002

  View details for PubMedID 22994141

• Quantified self and human movement: A review on the clinical impact of wearable sensing and feedback for gait analysis and intervention GAIT & POSTURE Shull, P. B., Jirattigalachote, W., Hunt, M. A., Cutkosky, M. R., Delp, S. L. 2014; 40 (1): 11-19

  View details for DOI 10.1016/j.gaitpost.2014.03.189

  View details for Web of Science ID 000336387300002

• Neuroscience. Optogenetic regeneration. Science Iyer, S. M., Delp, S. L. 2014; 344 (6179): 44-45

  View details for DOI 10.1126/science.1253088

  View details for PubMedID 24700845

• Changes in Tibiofemoral Forces due to Variations in Muscle Activity during Walking JOURNAL OF NEUROCHEMISTRY DeMers, M. S., Pal, S., Delp, S. L. 2014; 129 (2): 769-776

  View details for DOI 10.1002/jor.22601

  View details for Web of Science ID 000333716700006

• Virally mediated optogenetic excitation and inhibition of pain in freely moving nontransgenic mice NATURE BIOTECHNOLOGY Iyer, S. M., Montgomery, K. L., Towne, C., Lee, S. Y., Ramakrishnan, C., Deisseroth, K., Delp, S. L. 2014; 32 (3): 274-278

  Abstract

  Primary nociceptors are the first neurons involved in the complex processing system that regulates normal and pathological pain. Because of constraints on pharmacological and electrical stimulation, noninvasive excitation and inhibition of these neurons in freely moving nontransgenic animals has not been possible. Here we use an optogenetic strategy to bidirectionally control nociceptors of nontransgenic mice. Intrasciatic nerve injection of adeno-associated viruses encoding an excitatory opsin enabled light-inducible stimulation of acute pain, place aversion and optogenetically mediated reductions in withdrawal thresholds to mechanical and thermal stimuli. In contrast, viral delivery of an inhibitory opsin enabled light-inducible inhibition of acute pain perception, and reversed mechanical allodynia and thermal hyperalgesia in a model of neuropathic pain. Light was delivered transdermally, allowing these behaviors to be induced in freely moving animals. This approach may have utility in basic and translational pain research, and enable rapid drug screening and testing of newly engineered opsins.

  View details for DOI 10.1038/nbt.2834

  View details for Web of Science ID 000332819800026

  View details for PubMedID 24531797

• Rejuvenation of the muscle stem cell population restores strength to injured aged muscles. Nature medicine Cosgrove, B. D., Gilbert, P. M., Porpiglia, E., Mourkioti, F., Lee, S. P., Corbel, S. Y., Llewellyn, M. E., Delp, S. L., Blau, H. M. 2014; 20 (3): 255-264

  Abstract

  The elderly often suffer from progressive muscle weakness and regenerative failure. We demonstrate that muscle regeneration is impaired with aging owing in part to a cell-autonomous functional decline in skeletal muscle stem cells (MuSCs). Two-thirds of MuSCs from aged mice are intrinsically defective relative to MuSCs from young mice, with reduced capacity to repair myofibers and repopulate the stem cell reservoir in vivo following transplantation. This deficiency is correlated with a higher incidence of cells that express senescence markers and is due to elevated activity of the p38α and p38β mitogen-activated kinase pathway. We show that these limitations cannot be overcome by transplantation into the microenvironment of young recipient muscles. In contrast, subjecting the MuSC population from aged mice to transient inhibition of p38α and p38β in conjunction with culture on soft hydrogel substrates rapidly expands the residual functional MuSC population from aged mice, rejuvenating its potential for regeneration and serial transplantation as well as strengthening of damaged muscles of aged mice. These findings reveal a synergy between biophysical and biochemical cues that provides a paradigm for a localized autologous muscle stem cell therapy for the elderly.

  View details for DOI 10.1038/nm.3464

  View details for PubMedID 24531378

• Improved Muscle Wrapping Algorithms Using Explicit Path-Error Jacobians 6th International Workshop on Computational Kinematics (CK) Scholz, A., Stavness, I., Sherman, M., Delp, S., Kecskemethy, A. SPRINGER-VERLAG BERLIN. 2014: 395–403

  View details for DOI 10.1007/978-94-007-7214-4_44

  View details for Web of Science ID 000340420400044

• Quantified self and human movement: a review on the clinical impact of wearable sensing and feedback for gait analysis and intervention. Gait & posture Shull, P. B., Jirattigalachote, W., Hunt, M. A., Cutkosky, M. R., Delp, S. L. 2014; 40 (1): 11-19

  Abstract

  The proliferation of miniaturized electronics has fueled a shift toward wearable sensors and feedback devices for the mass population. Quantified self and other similar movements involving wearable systems have gained recent interest. However, it is unclear what the clinical impact of these enabling technologies is on human gait. The purpose of this review is to assess clinical applications of wearable sensing and feedback for human gait and to identify areas of future research. Four electronic databases were searched to find articles employing wearable sensing or feedback for movements of the foot, ankle, shank, thigh, hip, pelvis, and trunk during gait. We retrieved 76 articles that met the inclusion criteria and identified four common clinical applications: (1) identifying movement disorders, (2) assessing surgical outcomes, (3) improving walking stability, and (4) reducing joint loading. Characteristics of knee and trunk motion were the most frequent gait parameters for both wearable sensing and wearable feedback. Most articles performed testing on healthy subjects, and the most prevalent patient populations were osteoarthritis, vestibular loss, Parkinson's disease, and post-stroke hemiplegia. The most widely used wearable sensors were inertial measurement units (accelerometer and gyroscope packaged together) and goniometers. Haptic (touch) and auditory were the most common feedback sensations. This review highlights the current state of the literature and demonstrates substantial potential clinical benefits of wearable sensing and feedback. Future research should focus on wearable sensing and feedback in patient populations, in natural human environments outside the laboratory such as at home or work, and on continuous, long-term monitoring and intervention.

  View details for DOI 10.1016/j.gaitpost.2014.03.189

  View details for PubMedID 24768525

• Subject-specific knee joint geometry improves predictions of medial tibiofemoral contact forces JOURNAL OF BIOMECHANICS Gerus, P., Sartori, M., Besier, T. F., Fregly, B. J., Delp, S. L., Banks, S. A., Pandy, M. G., D'Lima, D. D., Lloyd, D. G. 2013; 46 (16): 2778-2786

  Abstract

  Estimating tibiofemoral joint contact forces is important for understanding the initiation and progression of knee osteoarthritis. However, tibiofemoral contact force predictions are influenced by many factors including muscle forces and anatomical representations of the knee joint. This study aimed to investigate the influence of subject-specific geometry and knee joint kinematics on the prediction of tibiofemoral contact forces using a calibrated EMG-driven neuromusculoskeletal model of the knee. One participant fitted with an instrumented total knee replacement walked at a self-selected speed while medial and lateral tibiofemoral contact forces, ground reaction forces, whole-body kinematics, and lower-limb muscle activity were simultaneously measured. The combination of generic and subject-specific knee joint geometry and kinematics resulted in four different OpenSim models used to estimate muscle-tendon lengths and moment arms. The subject-specific geometric model was created from CT scans and the subject-specific knee joint kinematics representing the translation of the tibia relative to the femur was obtained from fluoroscopy. The EMG-driven model was calibrated using one walking trial, but with three different cost functions that tracked the knee flexion/extension moments with and without constraint over the estimated joint contact forces. The calibrated models then predicted the medial and lateral tibiofemoral contact forces for five other different walking trials. The use of subject-specific models with minimization of the peak tibiofemoral contact forces improved the accuracy of medial contact forces by 47% and lateral contact forces by 7%, respectively compared with the use of generic musculoskeletal model.

  View details for DOI 10.1016/j.jbiomech.2013.09.005

  View details for Web of Science ID 000328093200004

  View details for PubMedID 24074941

• Men and women adopt similar walking mechanics and muscle activation patterns during load carriage. Journal of biomechanics Silder, A., Delp, S. L., Besier, T. 2013; 46 (14): 2522-2528

  Abstract

  Although numerous studies have investigated the effects of load carriage on gait mechanics, most have been conducted on active military men. It remains unknown whether men and women adapt differently to carrying load. The purpose of this study was to compare the effects of load carriage on gait mechanics, muscle activation patterns, and metabolic cost between men and women walking at their preferred, unloaded walking speed. We measured whole body motion, ground reaction forces, muscle activity, and metabolic cost from 17 men and 12 women. Subjects completed four walking trials on an instrumented treadmill, each five minutes in duration, while carrying no load or an additional 10%, 20%, or 30% of body weight. Women were shorter (p<0.01), had lower body mass (p=0.01), and had lower fat-free mass (p=0.02) compared to men. No significant differences between men and women were observed for any measured gait parameter or muscle activation pattern. As load increased, so did net metabolic cost, the duration of stance phase, peak stance phase hip, knee, and ankle flexion angles, and all peak joint extension moments. The increase in the peak vertical ground reaction force was less than the carried load (e.g. ground force increased approximately 6% with each 10% increase in load). Integrated muscle activity of the soleus, medial gastrocnemius, lateral hamstrings, vastus medialis, vastus lateralis, and rectus femoris increased with load. We conclude that, despite differences in anthropometry, men and women adopt similar gait adaptations when carrying load, adjusted as a percentage of body weight.

  View details for DOI 10.1016/j.jbiomech.2013.06.020

  View details for PubMedID 23968555

• Sarcomere lengths in human extensor carpi radialis brevis measured by microendoscopy MUSCLE & NERVE Cromie, M. J., Sanchez, G. N., Schnitzer, M. J., Delp, S. L. 2013; 48 (2): 286-292

  Abstract

  Second-harmonic generation microendoscopy is a minimally invasive technique to image sarcomeres and measure their lengths in humans, but motion artifact and low signal have limited the use of this novel technique.We discovered that an excitation wavelength of 960 nm maximized image signal; this enabled an image acquisition rate of 3 frames/s, which decreased motion artifact. We then used microendoscopy to measure sarcomere lengths in the human extensor carpi radialis brevis with the wrist at 45° extension and 45° flexion in 7 subjects. We also measured the variability in sarcomere lengths within single fibers.Average sarcomere lengths in 45° extension were 2.93±0.29 μm (±SD) and increased to 3.58±0.19 μm in 45° flexion. Within single fibers the standard deviation of sarcomere lengths in series was 0.20 μm.Microendoscopy can be used to measure sarcomere lengths at different body postures. Lengths of sarcomeres in series within a fiber vary substantially. Muscle Nerve, 48: 286-292, 2013.

  View details for DOI 10.1002/mus.23760

  View details for Web of Science ID 000322158500019

  View details for PubMedID 23813625

• WHAT IS A MOMENT ARM? CALCULATING MUSCLE EFFECTIVENESS IN BIOMECHANICAL MODELS USING GENERALIZED COORDINATES. Proceedings of the ... ASME Design Engineering Technical Conferences. ASME Design Engineering Technical Conferences Sherman, M. A., Seth, A., Delp, S. L. 2013; 2013

  Abstract

  Biomechanics researchers often use multibody models to represent biological systems. However, the mapping from biology to mechanics and back can be problematic. OpenSim is a popular open source tool used for this purpose, mapping between biological specifications and an underlying generalized coordinate multibody system called Simbody. One quantity of interest to biomechanical researchers and clinicians is "muscle moment arm," a measure of the effectiveness of a muscle at contributing to a particular motion over a range of configurations. OpenSim can automatically calculate these quantities for any muscle once a model has been built. For simple cases, this calculation is the same as the conventional moment arm calculation in mechanical engineering. But a muscle may span several joints (e.g., wrist, neck, back) and may follow a convoluted path over various curved surfaces. A biological joint may require several bodies or even a mechanism to accurately represent in the multibody model (e.g., knee, shoulder). In these situations we need a careful definition of muscle moment arm that is analogous to the mechanical engineering concept, yet generalized to be of use to biomedical researchers. Here we present some biomechanical modeling challenges and how they are resolved in OpenSim and Simbody to yield biologically meaningful muscle moment arms.

  View details for PubMedID 25905111

• Six-week gait retraining program reduces knee adduction moment, reduces pain, and improves function for individuals with medial compartment knee osteoarthritis. Journal of orthopaedic research Shull, P. B., Silder, A., Shultz, R., Dragoo, J. L., Besier, T. F., Delp, S. L., Cutkosky, M. R. 2013; 31 (7): 1020-1025

  Abstract

  This study examined the influence of a 6-week gait retraining program on the knee adduction moment (KAM) and knee pain and function. Ten subjects with medial compartment knee osteoarthritis and self-reported knee pain participated in weekly gait retraining sessions over 6 weeks. Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores and a 10-point visual-analog pain scale score were measured at baseline, post-training (end of 6 weeks), and 1 month after training ended. Gait retraining reduced the first peak KAM by 20% (p < 0.01) post-training as a result of a 7° decrease in foot progression angle (i.e., increased internal foot rotation), compared to baseline (p < 0.01). WOMAC pain and function scores were improved at post-training by 29% and 32%, respectively (p < 0.05) and visual-analog pain scale scores improved by two points (p < 0.05). Changes in WOMAC pain and function were approximately 75% larger than the expected placebo effect (p < 0.05). Changes in KAM, foot progression angle, WOMAC pain and function, and visual-analog pain score were retained 1 month after the end of the 6-week training period (p < 0.05). These results show that a 6-week gait retraining program can reduce the KAM and improve symptoms for individuals with medial compartment knee osteoarthritis and knee pain. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1020-1025, 2013.

  View details for DOI 10.1002/jor.22340

  View details for PubMedID 23494804

• A rolling constraint reproduces ground reaction forces and moments in dynamic simulations of walking, running, and crouch gait JOURNAL OF BIOMECHANICS Hamner, S. R., Seth, A., Steele, K. M., Delp, S. L. 2013; 46 (10): 1772-1776

  Abstract

  Recent advances in computational technology have dramatically increased the use of muscle-driven simulation to study accelerations produced by muscles during gait. Accelerations computed from muscle-driven simulations are sensitive to the model used to represent contact between the foot and ground. A foot-ground contact model must be able to calculate ground reaction forces and moments that are consistent with experimentally measured ground reaction forces and moments. We show here that a rolling constraint can model foot-ground contact and reproduce measured ground reaction forces and moments in an induced acceleration analysis of muscle-driven simulations of walking, running, and crouch gait. We also illustrate that a point constraint and a weld constraint used to model foot-ground contact in previous studies produce inaccurate reaction moments and lead to contradictory interpretations of muscle function. To enable others to use and test these different constraint types (i.e., rolling, point, and weld constraints) we have included them as part of an induced acceleration analysis in OpenSim, a freely-available biomechanics simulation package.

  View details for DOI 10.1016/j.jbiomech.2013.03.030

  View details for Web of Science ID 000321422400025

  View details for PubMedID 23702045

• Optical inhibition of motor nerve and muscle activity in vivo. Muscle & nerve Liske, H., Towne, C., Anikeeva, P., Zhao, S., Feng, G., Deisseroth, K., Delp, S. 2013; 47 (6): 916-921

  Abstract

  There is no therapeutic approach that provides precise and rapidly reversible inhibition of motor nerve and muscle activity for treatment of spastic hypertonia.We used optogenetics to demonstrate precise and rapidly reversible light-mediated inhibition of motor nerve and muscle activity in vivo in transgenic Thy1::eNpHR2.0 mice.We found optical inhibition of motor nerve and muscle activity to be effective at all muscle force amplitudes and determined that muscle activity can be modulated by changing light pulse duration and light power density.This demonstration of optical inhibition of motor nerves is an important advancement toward novel optogenetics-based therapies for spastic hypertonia.

  View details for DOI 10.1002/mus.23696

  View details for PubMedID 23629741

• How muscle fiber lengths and velocities affect muscle force generation as humans walk and run at different speeds. journal of experimental biology Arnold, E. M., Hamner, S. R., Seth, A., Millard, M., Delp, S. L. 2013; 216: 2150-2160

  Abstract

  The lengths and velocities of muscle fibers have a dramatic effect on muscle force generation. It is unknown, however, whether the lengths and velocities of lower limb muscle fibers substantially affect the ability of muscles to generate force during walking and running. We examined this issue by developing simulations of muscle-tendon dynamics to calculate the lengths and velocities of muscle fibers from electromyographic recordings of 11 lower limb muscles and kinematic measurements of the hip, knee and ankle made as five subjects walked at speeds of 1.0-1.75 m s(-1) and ran at speeds of 2.0-5.0 m s(-1). We analyzed the simulated fiber lengths, fiber velocities and forces to evaluate the influence of force-length and force-velocity properties on force generation at different walking and running speeds. The simulations revealed that force generation ability (i.e. the force generated per unit of activation) of eight of the 11 muscles was significantly affected by walking or running speed. Soleus force generation ability decreased with increasing walking speed, but the transition from walking to running increased the force generation ability by reducing fiber velocities. Our results demonstrate the influence of soleus muscle architecture on the walk-to-run transition and the effects of muscle-tendon compliance on the plantarflexors' ability to generate ankle moment and power. The study presents data that permit lower limb muscles to be studied in unprecedented detail by relating muscle fiber dynamics and force generation to the mechanical demands of walking and running.

  View details for DOI 10.1242/jeb.075697

  View details for PubMedID 23470656

• Muscle contributions to vertical and fore-aft accelerations are altered in subjects with crouch gait. Gait & posture Steele, K. M., Seth, A., Hicks, J. L., Schwartz, M. H., Delp, S. L. 2013; 38 (1): 86-91

  Abstract

  The goals of this study were to determine if the muscle contributions to vertical and fore-aft acceleration of the mass center differ between crouch gait and unimpaired gait and if these muscle contributions change with crouch severity. Examining muscle contributions to mass center acceleration provides insight into the roles of individual muscles during gait and can provide guidance for treatment planning. We calculated vertical and fore-aft accelerations using musculoskeletal simulations of typically developing children and children with cerebral palsy and crouch gait. Analysis of these simulations revealed that during unimpaired gait the quadriceps produce large upward and backward accelerations during early stance, whereas the ankle plantarflexors produce large upward and forward accelerations later in stance. In contrast, during crouch gait, the quadriceps and ankle plantarflexors produce large, opposing fore-aft accelerations throughout stance. The quadriceps force required to accelerate the mass center upward was significantly larger in crouch gait than in unimpaired gait and increased with crouch severity. The gluteus medius accelerated the mass center upward during midstance in unimpaired gait; however, during crouch gait the upward acceleration produced by the gluteus medius was significantly reduced. During unimpaired gait the quadriceps and ankle plantarflexors accelerate the mass center at different times, efficiently modulating fore-aft accelerations. However, during crouch gait, the quadriceps and ankle plantarflexors produce fore-aft accelerations at the same time and the opposing fore-aft accelerations generated by these muscles contribute to the inefficiency of crouch gait.

  View details for DOI 10.1016/j.gaitpost.2012.10.019

  View details for PubMedID 23200083

  View details for PubMedCentralID PMC3600387

• Stabilisation of walking by intrinsic muscle properties revealed in a three-dimensional muscle-driven simulation COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING John, C. T., Anderson, F. C., Higginson, J. S., Delp, S. L. 2013; 16 (4): 451-462

  Abstract

  A fundamental question in movement science is how humans perform stable movements in the presence of disturbances such as contact with objects. It remains unclear how the nervous system, with delayed responses to disturbances, maintains the stability of complex movements. We hypothesised that intrinsic muscle properties (i.e. the force-length-velocity properties of muscle fibres and tendon elasticity) may help stabilise human walking by responding instantaneously to a disturbance and providing forces that help maintain the movement trajectory. To investigate this issue, we generated a 3D muscle-driven simulation of walking and analysed the changes in the simulation's motion when a disturbance was applied to models with and without intrinsic muscle properties. Removing the intrinsic properties reduced the stability; this was true when the disturbing force was applied at a variety of times and in different directions. Thus, intrinsic muscle properties play a unique role in stabilising walking, complementing the delayed response of the central nervous system.

  View details for DOI 10.1080/10255842.2011.627560

  View details for PubMedID 22224406

• Patellar maltracking is prevalent among patellofemoral pain subjects with patella alta: An upright, weightbearing MRI study JOURNAL OF ORTHOPAEDIC RESEARCH Pal, S., Besier, T. F., Beaupre, G. S., Fredericson, M., Delp, S. L., Gold, G. E. 2013; 31 (3): 448-457

  Abstract

  The purpose of this study is to determine if patellar maltracking is more prevalent among patellofemoral (PF) pain subjects with patella alta compared to subjects with normal patella height. We imaged 37 PF pain and 15 pain free subjects in an open-configuration magnetic resonance imaging scanner while they stood in a weightbearing posture. We measured patella height using the Caton-Deschamps, Blackburne-Peel, Insall-Salvati, Modified Insall-Salvati, and Patellotrochlear indices, and classified the subjects into patella alta and normal patella height groups. We measured patella tilt and bisect offset from oblique-axial plane images, and classified the subjects into maltracking and normal tracking groups. Patellar maltracking was more prevalent among PF pain subjects with patella alta compared to PF pain subjects with normal patella height (two-tailed Fisher's exact test, p<0.050). Using the Caton-Deschamps index, 67% (8/12) of PF pain subjects with patella alta were maltrackers, whereas only 16% (4/25) of PF pain subjects with normal patella height were maltrackers. Patellofemoral pain subjects classified as maltrackers displayed a greater patella height compared to the pain free and PF pain subjects classified as normal trackers (two-tailed unpaired t-tests with Bonferroni correction, p<0.017). This study adds to our understanding of PF pain in two ways-(1) we demonstrate that patellar maltracking is more prevalent in PF pain subjects with patella alta compared to subjects with normal patella height; and (2) we show greater patella height in PF pain subjects compared to pain free subjects using four indices commonly used in clinics.

  View details for DOI 10.1002/jor.22256

  View details for Web of Science ID 000313980600016

  View details for PubMedID 23165335

  View details for PubMedCentralID PMC3562698

• Changes in in vivo knee contact forces through gait modification JOURNAL OF ORTHOPAEDIC RESEARCH Kinney, A. L., Besier, T. F., Silder, A., Delp, S. L., D'Lima, D. D., Fregly, B. J. 2013; 31 (3): 434-440

  Abstract

  Knee osteoarthritis (OA) commonly occurs in the medial compartment of the knee and has been linked to overloading of the medial articular cartilage. Gait modification represents a non-invasive treatment strategy for reducing medial compartment knee force. The purpose of this study was to evaluate the effectiveness of a variety of gait modifications that were expected to alter medial contact force. A single subject implanted with a force-measuring knee replacement walked using nine modified gait patterns, four of which involved different hiking pole configurations. Medial and lateral contact force at 25, 50, and 75% of stance phase, and the average value over all of stance phase (0-100%), were determined for each gait pattern. Changes in medial and lateral contact force values relative to the subject's normal gait pattern were determined by a Kruskal-Wallis test. Apart from early stance (25% of stance), medial contact force was most effectively reduced by walking with long hiking poles and wide pole placement, which significantly reduced medial and lateral contact force during stance phase by up to 34% (at 75% of stance) and 26% (at 50% of stance), respectively. Although this study is based on data from a single subject, the results provide important insight into changes in medial and lateral contact forces through gait modification. The results of this study suggest that an optimal configuration of bilateral hiking poles may significantly reduce both medial and lateral compartment knee forces in individuals with medial knee osteoarthritis.

  View details for DOI 10.1002/jor.22240

  View details for Web of Science ID 000313980600014

  View details for PubMedID 23027590

• Muscle contributions to fore-aft and vertical body mass center accelerations over a range of running speeds JOURNAL OF BIOMECHANICS Hamner, S. R., Delp, S. L. 2013; 46 (4): 780-787

  Abstract

  Running is a bouncing gait in which the body mass center slows and lowers during the first half of the stance phase; the mass center is then accelerated forward and upward into flight during the second half of the stance phase. Muscle-driven simulations can be analyzed to determine how muscle forces accelerate the body mass center. However, muscle-driven simulations of running at different speeds have not been previously developed, and it remains unclear how muscle forces modulate mass center accelerations at different running speeds. Thus, to examine how muscles generate accelerations of the body mass center, we created three-dimensional muscle-driven simulations of ten subjects running at 2.0, 3.0, 4.0, and 5.0m/s. An induced acceleration analysis determined the contribution of each muscle to mass center accelerations. Our simulations included arms, allowing us to investigate the contributions of arm motion to running dynamics. Analysis of the simulations revealed that soleus provides the greatest upward mass center acceleration at all running speeds; soleus generates a peak upward acceleration of 19.8m/s(2) (i.e., the equivalent of approximately 2.0 bodyweights of ground reaction force) at 5.0m/s. Soleus also provided the greatest contribution to forward mass center acceleration, which increased from 2.5m/s(2) at 2.0m/s to 4.0m/s(2) at 5.0m/s. At faster running speeds, greater velocity of the legs produced larger angular momentum about the vertical axis passing through the body mass center; angular momentum about this vertical axis from arm swing simultaneously increased to counterbalance the legs. We provide open-access to data and simulations from this study for further analysis in OpenSim at simtk.org/home/nmbl_running, enabling muscle actions during running to be studied in unprecedented detail.

  View details for DOI 10.1016/j.jbiomech.2012.11.024

  View details for Web of Science ID 000315973700022

  View details for PubMedID 23246045

• Flexing computational muscle: modeling and simulation of musculotendon dynamics. Journal of biomechanical engineering Millard, M., Uchida, T., Seth, A., Delp, S. L. 2013; 135 (2): 021005-?

  Abstract

  Muscle-driven simulations of human and animal motion are widely used to complement physical experiments for studying movement dynamics. Musculotendon models are an essential component of muscle-driven simulations, yet neither the computational speed nor the biological accuracy of the simulated forces has been adequately evaluated. Here we compare the speed and accuracy of three musculotendon models: two with an elastic tendon (an equilibrium model and a damped equilibrium model) and one with a rigid tendon. Our simulation benchmarks demonstrate that the equilibrium and damped equilibrium models produce similar force profiles but have different computational speeds. At low activation, the damped equilibrium model is 29 times faster than the equilibrium model when using an explicit integrator and 3 times faster when using an implicit integrator; at high activation, the two models have similar simulation speeds. In the special case of simulating a muscle with a short tendon, the rigid-tendon model produces forces that match those generated by the elastic-tendon models, but simulates 2-54 times faster when an explicit integrator is used and 6-31 times faster when an implicit integrator is used. The equilibrium, damped equilibrium, and rigid-tendon models reproduce forces generated by maximally-activated biological muscle with mean absolute errors less than 8.9%, 8.9%, and 20.9% of the maximum isometric muscle force, respectively. When compared to forces generated by submaximally-activated biological muscle, the forces produced by the equilibrium, damped equilibrium, and rigid-tendon models have mean absolute errors less than 16.2%, 16.4%, and 18.5%, respectively. To encourage further development of musculotendon models, we provide implementations of each of these models in OpenSim version 3.1 and benchmark data online, enabling others to reproduce our results and test their models of musculotendon dynamics.

  View details for DOI 10.1115/1.4023390

  View details for PubMedID 23445050

• Toe-in gait reduces the first peak knee adduction moment in patients with medial compartment knee osteoarthritis JOURNAL OF BIOMECHANICS Shull, P. B., Shultz, R., Slider, A., Dragoo, J. L., Besier, T. F., Cutkosky, M. R., Delp, S. L. 2013; 46 (1): 122-128

  Abstract

  The first peak of the knee adduction moment has been linked to the presence, severity, and progression of medial compartment knee osteoarthritis. The objective of this study was to evaluate toe-in gait (decreased foot progression angle from baseline through internal foot rotation) as a means to reduce the first peak of the knee adduction moment in subjects with medial compartment knee osteoarthritis. Additionally, we examined whether the first peak in the knee adduction moment would cause a concomitant increase in the peak external knee flexion moment, which can eliminate reductions in the medial compartment force that result from lowering the knee adduction moment. We tested the following hypotheses: (a) toe-in gait reduces the first peak of the knee adduction moment, and (b) toe-in gait does not increase the peak external knee flexion moment. Twelve patients with medial compartment knee osteoarthritis first performed baseline walking trials and then toe-in gait trials at their self-selected speed on an instrumented treadmill in a motion capture laboratory. Subjects altered their foot progression angle from baseline to toe-in gait by an average of 5° (p<0.01), which reduced the first peak of the knee adduction moment by an average of 13% (p<0.01). Toe-in gait did not increase the peak external knee flexion moment (p=0.85). The reduced knee adduction moment was accompanied by a medially-shifted knee joint center and a laterally-shifted center of pressure during early stance. These results suggest that toe-in gait may be a promising non-surgical treatment for patients with medial compartment knee osteoarthritis.

  View details for DOI 10.1016/j.jbiomech.2012.10.019

  View details for Web of Science ID 000314258000021

• Optogenetic control of targeted peripheral axons in freely moving animals. PloS one Towne, C., Montgomery, K. L., Iyer, S. M., Deisseroth, K., Delp, S. L. 2013; 8 (8): e72691

  Abstract

  Optogenetic control of the peripheral nervous system (PNS) would enable novel studies of motor control, somatosensory transduction, and pain processing. Such control requires the development of methods to deliver opsins and light to targeted sub-populations of neurons within peripheral nerves. We report here methods to deliver opsins and light to targeted peripheral neurons and robust optogenetic modulation of motor neuron activity in freely moving, non-transgenic mammals. We show that intramuscular injection of adeno-associated virus serotype 6 enables expression of channelrhodopsin (ChR2) in motor neurons innervating the injected muscle. Illumination of nerves containing mixed populations of axons from these targeted neurons and from neurons innervating other muscles produces ChR2-mediated optogenetic activation restricted to the injected muscle. We demonstrate that an implanted optical nerve cuff is well-tolerated, delivers light to the sciatic nerve, and optically stimulates muscle in freely moving rats. These methods can be broadly applied to study PNS disorders and lay the groundwork for future therapeutic application of optogenetics.

  View details for DOI 10.1371/journal.pone.0072691

  View details for PubMedID 23991144

  View details for PubMedCentralID PMC3749160

• Optogenetic control of targeted peripheral axons in freely moving animals. PloS one Towne, C., Montgomery, K. L., Iyer, S. M., Deisseroth, K., Delp, S. L. 2013; 8 (8)

  Abstract

  Optogenetic control of the peripheral nervous system (PNS) would enable novel studies of motor control, somatosensory transduction, and pain processing. Such control requires the development of methods to deliver opsins and light to targeted sub-populations of neurons within peripheral nerves. We report here methods to deliver opsins and light to targeted peripheral neurons and robust optogenetic modulation of motor neuron activity in freely moving, non-transgenic mammals. We show that intramuscular injection of adeno-associated virus serotype 6 enables expression of channelrhodopsin (ChR2) in motor neurons innervating the injected muscle. Illumination of nerves containing mixed populations of axons from these targeted neurons and from neurons innervating other muscles produces ChR2-mediated optogenetic activation restricted to the injected muscle. We demonstrate that an implanted optical nerve cuff is well-tolerated, delivers light to the sciatic nerve, and optically stimulates muscle in freely moving rats. These methods can be broadly applied to study PNS disorders and lay the groundwork for future therapeutic application of optogenetics.

  View details for DOI 10.1371/journal.pone.0072691

  View details for PubMedID 23991144

• Optical control of neuronal excitation and inhibition using a single opsin protein, ChR2. Scientific reports Liske, H., Qian, X., Anikeeva, P., Deisseroth, K., Delp, S. 2013; 3: 3110-?

  Abstract

  The effect of electrical stimulation on neuronal membrane potential is frequency dependent. Low frequency electrical stimulation can evoke action potentials, whereas high frequency stimulation can inhibit action potential transmission. Optical stimulation of channelrhodopsin-2 (ChR2) expressed in neuronal membranes can also excite action potentials. However, it is unknown whether optical stimulation of ChR2-expressing neurons produces a transition from excitation to inhibition with increasing light pulse frequencies. Here we report optical inhibition of motor neuron and muscle activity in vivo in the cooled sciatic nerves of Thy1-ChR2-EYFP mice. We also demonstrate all-optical single-wavelength control of neuronal excitation and inhibition without co-expression of inhibitory and excitatory opsins. This all-optical system is free from stimulation-induced electrical artifacts and thus provides a new approach to investigate mechanisms of high frequency inhibition in neuronal circuits in vivo and in vitro.

  View details for DOI 10.1038/srep03110

  View details for PubMedID 24173561

  View details for PubMedCentralID PMC3813941

• How much muscle strength is required to walk in a crouch gait? JOURNAL OF BIOMECHANICS Steele, K. M., van der Krogt, M. M., Schwartz, M. H., Delp, S. L. 2012; 45 (15): 2564-2569

  Abstract

  Muscle weakness is commonly cited as a cause of crouch gait in individuals with cerebral palsy; however, outcomes after strength training are variable and mechanisms by which muscle weakness may contribute to crouch gait are unclear. Understanding how much muscle strength is required to walk in a crouch gait compared to an unimpaired gait may provide insight into how muscle weakness contributes to crouch gait and assist in the design of strength training programs. The goal of this study was to examine how much muscle groups could be weakened before crouch gait becomes impossible. To investigate this question, we first created muscle-driven simulations of gait for three typically developing children and six children with cerebral palsy who walked with varying degrees of crouch severity. We then simulated muscle weakness by systematically reducing the maximum isometric force of each muscle group until the simulation could no longer reproduce each subject's gait. This analysis indicated that moderate crouch gait required significantly more knee extensor strength than unimpaired gait. In contrast, moderate crouch gait required significantly less hip abductor strength than unimpaired gait, and mild crouch gait required significantly less ankle plantarflexor strength than unimpaired gait. The reduced strength required from the hip abductors and ankle plantarflexors during crouch gait suggests that weakness of these muscle groups may contribute to crouch gait and that these muscle groups are potential targets for strength training.

  View details for DOI 10.1016/j.jbiomech.2012.07.028

  View details for Web of Science ID 000310107300012

  View details for PubMedID 22959837

  View details for PubMedCentralID PMC3524281

• Comparison of MRI and 18F-NaF PET/CT in patients with patellofemoral pain JOURNAL OF MAGNETIC RESONANCE IMAGING Draper, C. E., Quon, A., Fredericson, M., Besier, T. F., Delp, S. L., Beaupre, G. S., Gold, G. E. 2012; 36 (4): 928-932

  Abstract

  To determine whether bone metabolic activity corresponds to bone and cartilage damage in patients with patellofemoral pain.We acquired magnetic resonance imaging (MRI) and (18) F-NaF positron emission tomography (PET) / computed tomography (CT) scans of the knees of 22 subjects. We compared locations of increased tracer uptake on the (18) F-NaF PET images to bone marrow edema and cartilage damage visualized on MRI.We found that increased bone activity on (18) F-NaF PET does not always correspond to structural damage in the bone or cartilage as seen on MRI.Our results suggest that (18) F-NaF PET/CT may provide additional information in patellofemoral pain patients compared to MRI.

  View details for DOI 10.1002/jmri.23682

  View details for Web of Science ID 000308884300018

  View details for PubMedID 22549985

  View details for PubMedCentralID PMC3411864

• Contributions of muscles to mediolateral ground reaction force over a range of walking speeds JOURNAL OF BIOMECHANICS John, C. T., Seth, A., Schwartz, M. H., Delp, S. L. 2012; 45 (14): 2438-2443

  Abstract

  Impaired control of mediolateral body motion during walking is an important health concern. Developing treatments to improve mediolateral control is challenging, partly because the mechanisms by which muscles modulate mediolateral ground reaction force (and thereby modulate mediolateral acceleration of the body mass center) during unimpaired walking are poorly understood. To investigate this, we examined mediolateral ground reaction forces in eight unimpaired subjects walking at four speeds and determined the contributions of muscles, gravity, and velocity-related forces to the mediolateral ground reaction force by analyzing muscle-driven simulations of these subjects. During early stance (0-6% gait cycle), peak ground reaction force on the leading foot was directed laterally and increased significantly (p<0.05) with walking speed. During early single support (14-30% gait cycle), peak ground reaction force on the stance foot was directed medially and increased significantly (p<0.01) with speed. Muscles accounted for more than 92% of the mediolateral ground reaction force over all walking speeds, whereas gravity and velocity-related forces made relatively small contributions. Muscles coordinate mediolateral acceleration via an interplay between the medial ground reaction force contributed by the abductors and the lateral ground reaction forces contributed by the knee extensors, plantarflexors, and adductors. Our findings show how muscles that contribute to forward progression and body-weight support also modulate mediolateral acceleration of the body mass center while weight is transferred from one leg to another during double support.

  View details for DOI 10.1016/j.jbiomech.2012.06.037

  View details for PubMedID 22884038

• Optimizing Locomotion Controllers Using Biologically-Based Actuators and Objectives ACM TRANSACTIONS ON GRAPHICS Wang, J. M., Hamner, S. R., Delp, S. L., Koltun, V. 2012; 31 (4)

  View details for DOI 10.1145/2185520.2185521

  View details for Web of Science ID 000308250300001

• Predicting the metabolic cost of incline walking from muscle activity and walking mechanics JOURNAL OF BIOMECHANICS Slider, A., Besier, T., Delp, S. L. 2012; 45 (10): 1842-1849

  Abstract

  The goal of this study was to identify which muscle activation patterns and gait features best predict the metabolic cost of inclined walking. We measured muscle activation patterns, joint kinematics and kinetics, and metabolic cost in sixteen subjects during treadmill walking at inclines of 0%, 5%, and 10%. Multivariate regression models were developed to predict the net metabolic cost from selected groups of the measured variables. A linear regression model including incline and the squared integrated electromyographic signals of the soleus and vastus lateralis explained 96% of the variance in metabolic cost, suggesting that the activation patterns of these large muscles have a high predictive value for metabolic cost. A regression model including only the peak knee flexion angle during stance phase, peak knee extension moment, peak ankle plantarflexion moment, and peak hip flexion moment explained 89% of the variance in metabolic cost; this finding indicates that kinematics and kinetics alone can predict metabolic cost during incline walking. The ability of these models to predict metabolic cost from muscle activation patterns and gait features points the way toward future work aimed at predicting metabolic cost when gait is altered by changes in neuromuscular control or the use of an assistive technology.

  View details for DOI 10.1016/j.jbiomech.2012.03.032

  View details for Web of Science ID 000306451800014

  View details for PubMedID 22578744

• Patellar tilt correlates with vastus lateralis: Vastus medialis activation ratio in maltracking patellofemoral pain patients JOURNAL OF ORTHOPAEDIC RESEARCH Pal, S., Besier, T. F., Draper, C. E., Fredericson, M., Gold, G. E., Beaupre, G. S., Delp, S. L. 2012; 30 (6): 927-933

  Abstract

  Patellofemoral (PF) pain is a common ailment of the lower extremity. A theorized cause for pain is patellar maltracking due to vasti muscle activation imbalance, represented as large vastus lateralis:vastus medialis (VL:VM) activation ratios. However, evidence relating vasti muscle activation imbalance to patellar maltracking is limited. The purpose of this study was to investigate the relationship between VL:VM activation ratio and patellar tracking measures, patellar tilt and bisect offset, in PF pain subjects and pain-free controls. We evaluated VL:VM activation ratio and VM activation delay relative to VL activation in 39 PF pain subjects and 15 pain-free controls during walking. We classified the PF pain subjects into normal tracking and maltracking groups based on patellar tilt and bisect offset measured from weight-bearing magnetic resonance imaging. Patellar tilt correlated with VL:VM activation ratio only in PF pain subjects classified as maltrackers. This suggests that a clinical intervention targeting vasti muscle activation imbalance may be effective only in PF pain subjects classified as maltrackers.

  View details for DOI 10.1002/jor.22008

  View details for Web of Science ID 000302466700012

  View details for PubMedID 22086708

  View details for PubMedCentralID PMC3303943

• How robust is human gait to muscle weakness? GAIT & POSTURE van der Krogt, M. M., Delp, S. L., Schwartz, M. H. 2012; 36 (1): 113-119

  Abstract

  Humans have a remarkable capacity to perform complex movements requiring agility, timing, and strength. Disuse, aging, and disease can lead to a loss of muscle strength, which frequently limits the performance of motor tasks. It is unknown, however, how much weakness can be tolerated before normal daily activities become impaired. This study examines the extent to which lower limb muscles can be weakened before normal walking is affected. We developed muscle-driven simulations of normal walking and then progressively weakened all major muscle groups, one at the time and simultaneously, to evaluate how much weakness could be tolerated before execution of normal gait became impossible. We further examined the compensations that arose as a result of weakening muscles. Our simulations revealed that normal walking is remarkably robust to weakness of some muscles but sensitive to weakness of others. Gait appears most robust to weakness of hip and knee extensors, which can tolerate weakness well and without a substantial increase in muscle stress. In contrast, gait is most sensitive to weakness of plantarflexors, hip abductors, and hip flexors. Weakness of individual muscles results in increased activation of the weak muscle, and in compensatory activation of other muscles. These compensations are generally inefficient, and generate unbalanced joint moments that require compensatory activation in yet other muscles. As a result, total muscle activation increases with weakness as does the cost of walking. By clarifying which muscles are critical to maintaining normal gait, our results provide important insights for developing therapies to prevent or improve gait pathology.

  View details for DOI 10.1016/j.gaitpost.2012.01.017

  View details for Web of Science ID 000306449100021

  View details for PubMedID 22386624

• Compressive tibiofemoral force during crouch gait GAIT & POSTURE Steele, K. M., Demers, M. S., Schwartz, M. H., Delp, S. L. 2012; 35 (4): 556-560

  Abstract

  Crouch gait, a common walking pattern in individuals with cerebral palsy, is characterized by excessive flexion of the hip and knee. Many subjects with crouch gait experience knee pain, perhaps because of elevated muscle forces and joint loading. The goal of this study was to examine how muscle forces and compressive tibiofemoral force change with the increasing knee flexion associated with crouch gait. Muscle forces and tibiofemoral force were estimated for three unimpaired children and nine children with cerebral palsy who walked with varying degrees of knee flexion. We scaled a generic musculoskeletal model to each subject and used the model to estimate muscle forces and compressive tibiofemoral forces during walking. Mild crouch gait (minimum knee flexion 20-35°) produced a peak compressive tibiofemoral force similar to unimpaired walking; however, severe crouch gait (minimum knee flexion>50°) increased the peak force to greater than 6 times body-weight, more than double the load experienced during unimpaired gait. This increase in compressive tibiofemoral force was primarily due to increases in quadriceps force during crouch gait, which increased quadratically with average stance phase knee flexion (i.e., crouch severity). Increased quadriceps force contributes to larger tibiofemoral and patellofemoral loading which may contribute to knee pain in individuals with crouch gait.

  View details for DOI 10.1016/j.gaitpost.2011.11.023

  View details for Web of Science ID 000303229300006

  View details for PubMedID 22206783

  View details for PubMedCentralID PMC3319529

• Grand challenge competition to predict in vivo knee loads JOURNAL OF ORTHOPAEDIC RESEARCH Fregly, B. J., Besier, T. F., Lloyd, D. G., Delp, S. L., Banks, S. A., Pandy, M. G., D'Lima, D. D. 2012; 30 (4): 503-513

  Abstract

  Impairment of the human neuromusculoskeletal system can lead to significant mobility limitations and decreased quality of life. Computational models that accurately represent the musculoskeletal systems of individual patients could be used to explore different treatment options and optimize clinical outcome. The most significant barrier to model-based treatment design is validation of model-based estimates of in vivo contact and muscle forces. This paper introduces an annual "Grand Challenge Competition to Predict In Vivo Knee Loads" based on a series of comprehensive publicly available in vivo data sets for evaluating musculoskeletal model predictions of contact and muscle forces in the knee. The data sets come from patients implanted with force-measuring tibial prostheses. Following a historical review of musculoskeletal modeling methods used for estimating knee muscle and contact forces, we describe the first two data sets used for the first two competitions and summarize four subsequent data sets to be used for future competitions. These data sets include tibial contact force, video motion, ground reaction, muscle EMG, muscle strength, static and dynamic imaging, and implant geometry data. Competition participants create musculoskeletal models to predict tibial contact forces without having access to the corresponding in vivo measurements. These blinded predictions provide an unbiased evaluation of the capabilities and limitations of musculoskeletal modeling methods. The paper concludes with a discussion of how these unique data sets can be used by the musculoskeletal modeling research community to improve the estimation of in vivo muscle and contact forces and ultimately to help make musculoskeletal models clinically useful.

  View details for DOI 10.1002/jor.22023

  View details for Web of Science ID 000299935900001

  View details for PubMedID 22161745

• Simbios: an NIH national center for physics-based simulation of biological structures JOURNAL OF THE AMERICAN MEDICAL INFORMATICS ASSOCIATION Delp, S. L., Ku, J. P., Pande, V. S., Sherman, M. A., Altman, R. B. 2012; 19 (2): 186-189

  Abstract

  Physics-based simulation provides a powerful framework for understanding biological form and function. Simulations can be used by biologists to study macromolecular assemblies and by clinicians to design treatments for diseases. Simulations help biomedical researchers understand the physical constraints on biological systems as they engineer novel drugs, synthetic tissues, medical devices, and surgical interventions. Although individual biomedical investigators make outstanding contributions to physics-based simulation, the field has been fragmented. Applications are typically limited to a single physical scale, and individual investigators usually must create their own software. These conditions created a major barrier to advancing simulation capabilities. In 2004, we established a National Center for Physics-Based Simulation of Biological Structures (Simbios) to help integrate the field and accelerate biomedical research. In 6 years, Simbios has become a vibrant national center, with collaborators in 16 states and eight countries. Simbios focuses on problems at both the molecular scale and the organismal level, with a long-term goal of uniting these in accurate multiscale simulations.

  View details for DOI 10.1136/amiajnl-2011-000488

  View details for Web of Science ID 000300768100009

  View details for PubMedID 22081222

  View details for PubMedCentralID PMC3277621

• Patients with patellofemoral pain exhibit elevated bone metabolic activity at the patellofemoral joint JOURNAL OF ORTHOPAEDIC RESEARCH Draper, C. E., Fredericson, M., Gold, G. E., Besier, T. F., Delp, S. L., Beaupre, G. S., Quon, A. 2012; 30 (2): 209-213

  Abstract

  Patellofemoral pain is characterized by pain behind the kneecap and is often thought to be due to high stress at the patellofemoral joint. While we cannot measure bone stress in vivo, we can visualize bone metabolic activity using (18) F NaF PET/CT, which may be related to bone stress. Our goals were to use (18) F NaF PET/CT to evaluate whether subjects with patellofemoral pain exhibit elevated bone metabolic activity and to determine whether bone metabolic activity correlates with pain intensity. We examined 20 subjects diagnosed with patellofemoral pain. All subjects received an (18) F NaF PET/CT scan of their knees. Uptake of (18) F NaF in the patella and trochlea was quantified by computing the standardized uptake value and normalizing by the background tracer uptake in bone. We detected increased tracer uptake in 85% of the painful knees examined. We found that the painful knees exhibited increased tracer uptake compared to the pain-free knees of four subjects with unilateral pain (P = 0.0006). We also found a correlation between increasing tracer uptake and increasing pain intensity (r(2)  = 0.55; P = 0.0005). The implication of these results is that patellofemoral pain may be related to bone metabolic activity at the patellofemoral joint.

  View details for DOI 10.1002/jor.21523

  View details for Web of Science ID 000298581200007

  View details for PubMedID 21812024

  View details for PubMedCentralID PMC3219799

• EXPERIMENTAL EVALUATION OF COMPUTATIONALLY PREDICTED CHANGES IN KNEE LOADS RESULTING FROM MEDIAL THRUST GAIT ASME Summer Bioengineering Conference (SBC) Hall, A. L., Walter, J. P., Besier, T. F., Silder, A., Delp, S. L., D'Lima, D. D., Fregly, B. J. AMER SOC MECHANICAL ENGINEERS. 2012: 189–190

  View details for Web of Science ID 000325036600095

• Optimizing Locomotion Controllers Using Biologically-Based Actuators and Objectives. ACM transactions on graphics Wang, J. M., Hamner, S. R., Delp, S. L., Koltun, V. n. 2012; 31 (4)

  Abstract

  We present a technique for automatically synthesizing walking and running controllers for physically-simulated 3D humanoid characters. The sagittal hip, knee, and ankle degrees-of-freedom are actuated using a set of eight Hill-type musculotendon models in each leg, with biologically-motivated control laws. The parameters of these control laws are set by an optimization procedure that satisfies a number of locomotion task terms while minimizing a biological model of metabolic energy expenditure. We show that the use of biologically-based actuators and objectives measurably increases the realism of gaits generated by locomotion controllers that operate without the use of motion capture data, and that metabolic energy expenditure provides a simple and unifying measurement of effort that can be used for both walking and running control optimization.

  View details for PubMedID 26251560

• A COMPUTATIONALLY EFFICIENT MUSCLE MODEL ASME Summer Bioengineering Conference (SBC) Millard, M., Delp, S. AMER SOC MECHANICAL ENGINEERS. 2012: 1055–1056

  View details for Web of Science ID 000325036600527

• CHANGES IN MEDIAL KNEE CONTACT FORCE THROUGH GAIT MODIFICATION ASME Summer Bioengineering Conference (SBC) Hall, A. L., Besier, T. F., Silder, A., Delp, S. L., D'Lima, D. D., Fregly, B. J. AMER SOC MECHANICAL ENGINEERS. 2012: 239–240

  View details for Web of Science ID 000325036600119

• Characteristics associated with improved knee extension after strength training for individuals with cerebral palsy and crouch gait. Journal of pediatric rehabilitation medicine Steele, K. M., Damiano, D. L., Eek, M. N., Unger, M., Delp, S. L. 2012; 5 (2): 99-106

  Abstract

  Muscle weakness may contribute to crouch gait in individuals with cerebral palsy, and some individuals participate in strength training programs to improve crouch gait. Unfortunately, improvements in muscle strength and gait are inconsistent after completing strength training programs. The purpose of this study was to examine changes in knee extensor strength and knee extension angle during walking after strength training in individuals with cerebral palsy who walk in crouch gait and to determine subject characteristics associated with these changes. A literature review was performed of studies published since January 2000 that included strength training, three-dimensional motion analysis, and knee extensor strength measurements for individuals with cerebral palsy. Three studies met these criteria and individual subject data was obtained from the authors for thirty crouch gait subjects. Univariate regression analyses were performed to determine which of ten physical examination and motor performance variables were associated with changes in strength and knee extension during gait. Change in knee extensor strength ranged from a 25% decrease to a 215% increase, and change in minimum knee flexion angle during gait ranged from an improvement of 9° more knee extension to 15° more knee flexion. Individuals without hamstring spasticity had greater improvement in knee extension after strength training. Hamstring spasticity was associated with an undesired increase in knee flexion during walking. Subject-specific factors such as hamstring spasticity may be useful for predicting which subjects will benefit from strength training to improve crouch gait.

  View details for DOI 10.3233/PRM-2012-0201

  View details for PubMedID 22699100

  View details for PubMedCentralID PMC3579590

• Can biomechanical variables predict improvement in crouch gait? GAIT & POSTURE Hicks, J. L., Delp, S. L., Schwartz, M. H. 2011; 34 (2): 197-201

  Abstract

  Many patients respond positively to treatments for crouch gait, yet surgical outcomes are inconsistent and unpredictable. In this study, we developed a multivariable regression model to determine if biomechanical variables and other subject characteristics measured during a physical exam and gait analysis can predict which subjects with crouch gait will demonstrate improved knee kinematics on a follow-up gait analysis. We formulated the model and tested its performance by retrospectively analyzing 353 limbs of subjects who walked with crouch gait. The regression model was able to predict which subjects would demonstrate 'Improved' and 'Unimproved' knee kinematics with over 70% accuracy, and was able to explain approximately 49% of the variance in subjects' change in knee flexion between gait analyses. We found that improvement in stance phase knee flexion was positively associated with three variables that were drawn from knowledge about the biomechanical contributors to crouch gait: (i) adequate hamstrings lengths and velocities, possibly achieved via hamstrings lengthening surgery, (ii) normal tibial torsion, possibly achieved via tibial derotation osteotomy, and (iii) sufficient muscle strength.

  View details for DOI 10.1016/j.gaitpost.2011.04.009

  View details for Web of Science ID 000293429800010

  View details for PubMedID 21616666

  View details for PubMedCentralID PMC3130107

• Mechanics, modulation and modelling: how muscles actuate and control movement Introduction PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES Higham, T. E., Biewener, A. A., Delp, S. L. 2011; 366 (1570): 1463-1465

  Abstract

  Animal movement is often complex, unsteady and variable. The critical role of muscles in animal movement has captivated scientists for over 300 years. Despite this, emerging techniques and ideas are still shaping and advancing the field. For example, sonomicrometry and ultrasound techniques have enhanced our ability to quantify muscle length changes under in vivo conditions. Robotics and musculoskeletal models have benefited from improved computational tools and have enhanced our ability to understand muscle function in relation to movement by allowing one to simulate muscle-tendon dynamics under realistic conditions. The past decade, in particular, has seen a rapid advancement in technology and shifts in paradigms related to muscle function. In addition, there has been an increased focus on muscle function in relation to the complex locomotor behaviours, rather than relatively simple (and steady) behaviours. Thus, this Theme Issue will explore integrative aspects of muscle function in relation to diverse locomotor behaviours such as swimming, jumping, hopping, running, flying, moving over obstacles and transitioning between environments. Studies of walking and running have particular relevance to clinical aspects of human movement and sport. This Theme Issue includes contributions from scientists working on diverse taxa, ranging from humans to insects. In addition to contributions addressing locomotion in various taxa, several manuscripts will focus on recent advances in neuromuscular control and modulation during complex behaviours. Finally, some of the contributions address recent advances in biomechanical modelling and powered prostheses. We hope that our comprehensive and integrative Theme Issue will form the foundation for future work in the fields of neuromuscular mechanics and locomotion.

  View details for DOI 10.1098/rstb.2010.0354

  View details for Web of Science ID 000289621400001

  View details for PubMedID 21502117

• Fibre operating lengths of human lower limb muscles during walking PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES Arnold, E. M., Delp, S. L. 2011; 366 (1570): 1530-1539

  Abstract

  Muscles actuate movement by generating forces. The forces generated by muscles are highly dependent on their fibre lengths, yet it is difficult to measure the lengths over which muscle fibres operate during movement. We combined experimental measurements of joint angles and muscle activation patterns during walking with a musculoskeletal model that captures the relationships between muscle fibre lengths, joint angles and muscle activations for muscles of the lower limb. We used this musculoskeletal model to produce a simulation of muscle-tendon dynamics during walking and calculated fibre operating lengths (i.e. the length of muscle fibres relative to their optimal fibre length) for 17 lower limb muscles. Our results indicate that when musculotendon compliance is low, the muscle fibre operating length is determined predominantly by the joint angles and muscle moment arms. If musculotendon compliance is high, muscle fibre operating length is more dependent on activation level and force-length-velocity effects. We found that muscles operate on multiple limbs of the force-length curve (i.e. ascending, plateau and descending limbs) during the gait cycle, but are active within a smaller portion of their total operating range.

  View details for DOI 10.1098/rstb.2010.0345

  View details for Web of Science ID 000289621400008

  View details for PubMedID 21502124

  View details for PubMedCentralID PMC3130447

• New MR Imaging Methods for Metallic Implants in the Knee: Artifact Correction and Clinical Impact JOURNAL OF MAGNETIC RESONANCE IMAGING Chen, C. A., Chen, W., Goodman, S. B., Hargreaves, B. A., Koch, K. M., Lu, W., Brau, A. C., Draper, C. E., Delp, S. L., Gold, G. E. 2011; 33 (5): 1121-1127

  Abstract

  To evaluate two magnetic resonance imaging (MRI) techniques, slice encoding for metal artifact correction (SEMAC) and multiacquisition variable-resonance image combination (MAVRIC), for their ability to correct for artifacts in postoperative knees with metal.A total of 25 knees were imaged in this study. Fourteen total knee replacements (TKRs) in volunteers were scanned with SEMAC, MAVRIC, and 2D fast spin-echo (FSE) to measure artifact extent and implant rotation. The ability of the sequences to measure implant rotation and dimensions was compared in a TKR knee model. Eleven patients with a variety of metallic hardware were imaged with SEMAC and FSE to compare artifact extent and subsequent patient management was recorded.SEMAC and MAVRIC significantly reduced artifact extent compared to FSE (P < 0.0001) and were similar to each other (P = 0.58), allowing accurate measurement of implant dimensions and rotation. The TKRs were properly aligned in the volunteers. Clinical imaging with SEMAC in symptomatic knees significantly reduced artifact (P < 0.05) and showed findings that were on the majority confirmed by subsequent noninvasive or invasive patient studies.SEMAC and MAVRIC correct for metal artifact, noninvasively providing high-resolution images with superb bone and soft tissue contrast.

  View details for DOI 10.1002/jmri.22534

  View details for PubMedID 21509870

• Differences in Patellofemoral Kinematics between Weight-Bearing and Non-Weight-Bearing Conditions in Patients with Patellofemoral Pain JOURNAL OF ORTHOPAEDIC RESEARCH Draper, C. E., Besier, T. F., Fredericson, M., Santos, J. M., Beaupre, G. S., Delp, S. L., Gold, G. E. 2011; 29 (3): 312-317

  Abstract

  Patellar maltracking is thought to be one source of patellofemoral pain. Measurements of patellar tracking are frequently obtained during non-weight-bearing knee extension; however, pain typically arises during highly loaded activities, such as squatting, stair climbing, and running. It is unclear whether patellofemoral joint kinematics during lightly loaded tasks replicate patellofemoral joint motion during weight-bearing activities. The purpose of this study was to: evaluate differences between upright, weight-bearing and supine, non-weight-bearing joint kinematics in patients with patellofemoral pain; and evaluate whether the kinematics in subjects with maltracking respond differently to weight-bearing than those in nonmaltrackers. We used real-time magnetic resonance imaging to visualize the patellofemoral joint during dynamic knee extension from 30° to 0° of knee flexion during two conditions: upright, weight-bearing and supine, non-weight-bearing. We compared patellofemoral kinematics measured from the images. The patella translated more laterally during the supine task compared to the weight-bearing task for knee flexion angles between 0° and 5° (p = 0.001). The kinematics of the maltrackers responded differently to joint loading than those of the non-maltrackers. In subjects with excessive lateral patellar translation, the patella translated more laterally during upright, weight-bearing knee extension for knee flexion angles between 25° and 30° (p = 0.001). However, in subjects with normal patellar translation, the patella translated more laterally during supine, non-weight-bearing knee extension near full extension (p = 0.001). These results suggest that patellofemoral kinematics measured during supine, unloaded tasks do not accurately represent the joint motion during weight-bearing activities.

  View details for DOI 10.1002/jor.21253

  View details for Web of Science ID 000287173500002

  View details for PubMedID 20949442

• Patellar Maltracking Correlates With Vastus Medialis Activation Delay in Patellofemoral Pain Patients AMERICAN JOURNAL OF SPORTS MEDICINE Pal, S., Draper, C. E., Fredericson, M., Gold, G. E., Delp, S. L., Beaupre, G. S., Besier, T. F. 2011; 39 (3): 590-598

  Abstract

  Delayed onset of vastus medialis (VM) activity compared with vastus lateralis activity is a reported cause for patellofemoral pain. The delayed onset of VM activity in patellofemoral pain patients likely causes an imbalance in muscle forces and lateral maltracking of the patella; however, evidence relating VM activation delay to patellar maltracking is sparse. The aim of this study was to investigate the relationship between VM activation delay and patellar maltracking measures in pain-free controls and patellofemoral pain patients.Patellar tilt and bisect offset, measures of patellar tracking, correlate with VM activation delay in patellofemoral pain patients classified as maltrackers.Case control study; Level of evidence, 3.Vasti muscle activations were recorded in pain-free (n = 15) and patellofemoral pain (n = 40) participants during walking and jogging. All participants were scanned in an open-configuration magnetic resonance scanner in an upright weightbearing position to acquire the position of the patella with respect to the femur. Patellar tilt and bisect offset were measured, and patellofemoral pain participants were classified into normal tracking and maltracking groups.Correlations between VM activation delay and patellar maltracking measures were statistically significant in only the patellofemoral pain participants classified as maltrackers with both abnormal tilt and abnormal bisect offset (R(2) = .89, P < .001, with patellar tilt during walking; R(2) = .75, P = .012, with bisect offset during jogging). There were no differences between the means of activation delays in pain-free and all patellofemoral pain participants during walking (P = .516) or jogging (P = .731).There was a relationship between VM activation delay and patellar maltracking in the subgroup of patellofemoral pain participants classified as maltrackers with both abnormal tilt and abnormal bisect offset.A clinical intervention such as VM retraining may be effective in only a subset of patellofemoral pain participants-namely, those with excessive tilt and excessive bisect offset measures. The results highlight the importance of appropriate classification of patellofemoral pain patients before selection of a clinical intervention.

  View details for DOI 10.1177/0363546510384233

  View details for Web of Science ID 000288063900019

  View details for PubMedID 21076015

• Prediction of glycosaminoglycan content in human cartilage by age, T1 rho and T2 MRI OSTEOARTHRITIS AND CARTILAGE Keenan, K. E., Besier, T. F., Pauly, J. M., Han, E., Rosenberg, J., Smith, R. L., Delp, S. L., Beaupre, G. S., Gold, G. E. 2011; 19 (2): 171-179

  Abstract

  A relationship between T1ρ relaxation time and glycosaminoglycan (GAG) content has been demonstrated in chemically degraded bovine cartilage, but has not been demonstrated with quantitative biochemistry in human cartilage. A relationship has also been established between T2 relaxation time in cartilage and osteoarthritis (OA) severity. We hypothesized that T1ρ relaxation time would be associated with GAG content in human cartilage with normal T2 relaxation times.T2 relaxation time, T1ρ relaxation time, and glycosaminoglycan as a percentage of wet weight (sGAG) were measured for top and bottom regions at 7 anatomical locations in 21 human cadaver patellae. For our analysis, T2 relaxation time was classified as normal or elevated based on a threshold defined by the mean plus one standard deviation of the T2 relaxation time for all samples.In the normal T2 relaxation time subset, T1ρ relaxation time correlated with sGAG content in the full-thickness and bottom regions, but only marginally in the top region alone. sGAG content decreased significantly with age in all regions.In the subset of cartilage specimens with normal T2 relaxation time, T1ρ relaxation time was inversely associated with sGAG content, as hypothesized. A predictive model, which accounts for T2 relaxation time and the effects of age, might be able to determine longitudinal trends in GAG content in the same person based on T1ρ relaxation time maps.

  View details for DOI 10.1016/j.joca.2010.11.009

  View details for Web of Science ID 000287470600005

  View details for PubMedID 21112409

  View details for PubMedCentralID PMC3041640

• Simulation of human movement: applications using OpenSim IUTAM Symposium on Human Body Dynamics - From Multibody Systems to Biomechanics Reinbolt, J. A., Seth, A., Delp, S. L. ELSEVIER SCIENCE BV. 2011: 186–198

  View details for DOI 10.1016/j.piutam.2011.04.019

  View details for Web of Science ID 000314462800018

• Simbody: multibody dynamics for biomedical research. Procedia IUTAM Sherman, M. A., Seth, A., Delp, S. L. 2011; 2: 241-261

  Abstract

  Multibody software designed for mechanical engineering has been successfully employed in biomedical research for many years. For real time operation some biomedical researchers have also adapted game physics engines. However, these tools were built for other purposes and do not fully address the needs of biomedical researchers using them to analyze the dynamics of biological structures and make clinically meaningful recommendations. We are addressing this problem through the development of an open source, extensible, high performance toolkit including a multibody mechanics library aimed at the needs of biomedical researchers. The resulting code, Simbody, supports research in a variety of fields including neuromuscular, prosthetic, and biomolecular simulation, and related research such as biologically-inspired design and control of humanoid robots and avatars. Simbody is the dynamics engine behind OpenSim, a widely used biomechanics simulation application. This article reviews issues that arise uniquely in biomedical research, and reports on the architecture, theory, and computational methods Simbody uses to address them. By addressing these needs explicitly Simbody provides a better match to the needs of researchers than can be obtained by adaptation of mechanical engineering or gaming codes. Simbody is a community resource, free for any purpose. We encourage wide adoption and invite contributions to the code base at https://simtk.org/home/simbody.

  View details for PubMedID 25866705

  View details for PubMedCentralID PMC4390141

• OpenSim: a musculoskeletal modeling and simulation framework for in silico investigations and exchange. Procedia IUTAM Seth, A., Sherman, M., Reinbolt, J. A., Delp, S. L. 2011; 2: 212-232

  Abstract

  Movement science is driven by observation, but observation alone cannot elucidate principles of human and animal movement. Biomechanical modeling and computer simulation complement observations and inform experimental design. Biological models are complex and specialized software is required for building, validating, and studying them. Furthermore, common access is needed so that investigators can contribute models to a broader community and leverage past work. We are developing OpenSim, a freely available musculoskeletal modeling and simulation application and libraries specialized for these purposes, by providing: musculoskeletal modeling elements, such as biomechanical joints, muscle actuators, ligament forces, compliant contact, and controllers; and tools for fitting generic models to subject-specific data, performing inverse kinematics and forward dynamic simulations. OpenSim performs an array of physics-based analyses to delve into the behavior of musculoskeletal models by employing Simbody, an efficient and accurate multibody system dynamics code. Models are publicly available and are often reused for multiple investigations because they provide a rich set of behaviors that enables different lines of inquiry. This report will discuss one model developed to study walking and applied to gain deeper insights into muscle function in pathological gait and during running. We then illustrate how simulations can test fundamental hypotheses and focus the aims of in vivo experiments, with a postural stability platform and human model that provide a research environment for performing human posture experiments in silico. We encourage wide adoption of OpenSim for community exchange of biomechanical models and methods and welcome new contributors.

  View details for DOI 10.1016/j.piutam.2011.04.021

  View details for PubMedID 25893160

  View details for PubMedCentralID PMC4397580

• Simbody: multibody dynamics for biomedical research IUTAM Symposium on Human Body Dynamics - From Multibody Systems to Biomechanics Sherman, M. A., Seth, A., Delp, S. L. ELSEVIER SCIENCE BV. 2011: 241–261

  View details for DOI 10.1016/j.piutam.2011.04.023

  View details for Web of Science ID 000314462800022

• OpenSim: a musculoskeletal modeling and simulation framework for in silico investigations and exchange IUTAM Symposium on Human Body Dynamics - From Multibody Systems to Biomechanics Seth, A., Sherman, M., Reinbolt, J. A., Delp, S. L. ELSEVIER SCIENCE BV. 2011: 212–232

  View details for DOI 10.1016/j.piutam.2011.04.021

  View details for Web of Science ID 000314462800020

• Analysis of Vertical and Horizontal Circular C-Arm Trajectories Conference on Medical Imaging 2011 - Physics of Medical Imaging Maier, A., Choi, J., Keil, A., Niebler, C., Sarmiento, M., Fieselmann, A., Gold, G., Delp, S., Fahrig, R. SPIE-INT SOC OPTICAL ENGINEERING. 2011

  View details for DOI 10.1117/12.878502

  View details for Web of Science ID 000294178500070

• Short Telomeres and Stem Cell Exhaustion Model Duchenne Muscular Dystrophy in mdx/mTR Mice CELL Sacco, A., Mourkioti, F., Tran, R., Choi, J., Llewellyn, M., Kraft, P., Shkreli, M., Delp, S., Pomerantz, J. H., Artandi, S. E., Blau, H. M. 2010; 143 (7): 1059-1071

  Abstract

  In Duchenne muscular dystrophy (DMD), dystrophin mutation leads to progressive lethal skeletal muscle degeneration. For unknown reasons, dystrophin deficiency does not recapitulate DMD in mice (mdx), which have mild skeletal muscle defects and potent regenerative capacity. We postulated that human DMD progression is a consequence of loss of functional muscle stem cells (MuSC), and the mild mouse mdx phenotype results from greater MuSC reserve fueled by longer telomeres. We report that mdx mice lacking the RNA component of telomerase (mdx/mTR) have shortened telomeres in muscle cells and severe muscular dystrophy that progressively worsens with age. Muscle wasting severity parallels a decline in MuSC regenerative capacity and is ameliorated histologically by transplantation of wild-type MuSC. These data show that DMD progression results, in part, from a cell-autonomous failure of MuSC to maintain the damage-repair cycle initiated by dystrophin deficiency. The essential role of MuSC function has therapeutic implications for DMD.

  View details for DOI 10.1016/j.cell.2010.11.039

  View details for Web of Science ID 000285625400005

  View details for PubMedID 21145579

  View details for PubMedCentralID PMC3025608

• Muscle contributions to propulsion and support during running JOURNAL OF BIOMECHANICS Hamner, S. R., Seth, A., Delp, S. L. 2010; 43 (14): 2709-2716

  Abstract

  Muscles actuate running by developing forces that propel the body forward while supporting the body's weight. To understand how muscles contribute to propulsion (i.e., forward acceleration of the mass center) and support (i.e., upward acceleration of the mass center) during running we developed a three-dimensional muscle-actuated simulation of the running gait cycle. The simulation is driven by 92 musculotendon actuators of the lower extremities and torso and includes the dynamics of arm motion. We analyzed the simulation to determine how each muscle contributed to the acceleration of the body mass center. During the early part of the stance phase, the quadriceps muscle group was the largest contributor to braking (i.e., backward acceleration of the mass center) and support. During the second half of the stance phase, the soleus and gastrocnemius muscles were the greatest contributors to propulsion and support. The arms did not contribute substantially to either propulsion or support, generating less than 1% of the peak mass center acceleration. However, the arms effectively counterbalanced the vertical angular momentum of the lower extremities. Our analysis reveals that the quadriceps and plantarflexors are the major contributors to acceleration of the body mass center during running.

  View details for DOI 10.1016/j.jbiomech.2010.06.025

  View details for Web of Science ID 000284343700009

  View details for PubMedID 20691972

  View details for PubMedCentralID PMC2973845

• Orderly recruitment of motor units under optical control in vivo NATURE MEDICINE Llewellyn, M. E., Thompson, K. R., Deisseroth, K., Delp, S. L. 2010; 16 (10): 1161-U144

  Abstract

  A drawback of electrical stimulation for muscle control is that large, fatigable motor units are preferentially recruited before smaller motor units by the lowest-intensity electrical cuff stimulation. This phenomenon limits therapeutic applications because it is precisely the opposite of the normal physiological (orderly) recruitment pattern; therefore, a mechanism to achieve orderly recruitment has been a long-sought goal in physiology, medicine and engineering. Here we demonstrate a technology for reliable orderly recruitment in vivo. We find that under optical control with microbial opsins, recruitment of motor units proceeds in the physiological recruitment sequence, as indicated by multiple independent measures of motor unit recruitment including conduction latency, contraction and relaxation times, stimulation threshold and fatigue. As a result, we observed enhanced performance and reduced fatigue in vivo. These findings point to an unanticipated new modality of neural control with broad implications for nervous system and neuromuscular physiology, disease research and therapeutic innovation.

  View details for DOI 10.1038/nm.2228

  View details for Web of Science ID 000282644800049

  View details for PubMedID 20871612

• Minimal formulation of joint motion for biomechanisms NONLINEAR DYNAMICS Seth, A., Sherman, M., Eastman, P., Delp, S. 2010; 62 (1-2): 291-303

  View details for DOI 10.1007/s11071-010-9717-3

  View details for Web of Science ID 000281986000023

• Muscle contributions to support and progression during single-limb stance in crouch gait JOURNAL OF BIOMECHANICS Steele, K. M., Seth, A., Hicks, J. L., Schwartz, M. S., Delp, S. L. 2010; 43 (11): 2099-2105

  Abstract

  Pathological movement patterns like crouch gait are characterized by abnormal kinematics and muscle activations that alter how muscles support the body weight during walking. Individual muscles are often the target of interventions to improve crouch gait, yet the roles of individual muscles during crouch gait remain unknown. The goal of this study was to examine how muscles contribute to mass center accelerations and joint angular accelerations during single-limb stance in crouch gait, and compare these contributions to unimpaired gait. Subject-specific dynamic simulations were created for ten children who walked in a mild crouch gait and had no previous surgeries. The simulations were analyzed to determine the acceleration of the mass center and angular accelerations of the hip, knee, and ankle generated by individual muscles. The results of this analysis indicate that children walking in crouch gait have less passive skeletal support of body weight and utilize substantially higher muscle forces to walk than unimpaired individuals. Crouch gait relies on the same muscles as unimpaired gait to accelerate the mass center upward, including the soleus, vasti, gastrocnemius, gluteus medius, rectus femoris, and gluteus maximus. However, during crouch gait, these muscles are active throughout single-limb stance, in contrast to the modulation of muscle forces seen during single-limb stance in an unimpaired gait. Subjects walking in crouch gait rely more on proximal muscles, including the gluteus medius and hamstrings, to accelerate the mass center forward during single-limb stance than subjects with an unimpaired gait.

  View details for DOI 10.1016/j.jbiomech.2010.04.003

  View details for Web of Science ID 000281534000008

  View details for PubMedID 20493489

  View details for PubMedCentralID PMC2914221

• Contributions of muscles and passive dynamics to swing initiation over a range of walking speeds JOURNAL OF BIOMECHANICS Fox, M. D., Delp, S. L. 2010; 43 (8): 1450-1455

  Abstract

  Stiff-knee gait is a common walking problem in cerebral palsy characterized by insufficient knee flexion during swing. To identify factors that may limit knee flexion in swing, it is necessary to understand how unimpaired subjects successfully coordinate muscles and passive dynamics (gravity and velocity-related forces) to accelerate the knee into flexion during double support, a critical phase just prior to swing that establishes the conditions for achieving sufficient knee flexion during swing. It is also necessary to understand how contributions to swing initiation change with walking speed, since patients with stiff-knee gait often walk slowly. We analyzed muscle-driven dynamic simulations of eight unimpaired subjects walking at four speeds to quantify the contributions of muscles, gravity, and velocity-related forces (i.e. Coriolis and centrifugal forces) to preswing knee flexion acceleration during double support at each speed. Analysis of the simulations revealed contributions from muscles and passive dynamics varied systematically with walking speed. Preswing knee flexion acceleration was achieved primarily by hip flexor muscles on the preswing leg with assistance from biceps femoris short head. Hip flexors on the preswing leg were primarily responsible for the increase in preswing knee flexion acceleration during double support with faster walking speed. The hip extensors and abductors on the contralateral leg and velocity-related forces opposed preswing knee flexion acceleration during double support.

  View details for DOI 10.1016/j.jbiomech.2010.02.009

  View details for Web of Science ID 000278652000002

  View details for PubMedID 20236644

  View details for PubMedCentralID PMC2866832

• Variation of hamstrings lengths and velocities with walking speed JOURNAL OF BIOMECHANICS Agarwal-Harding, K. J., Schwartz, M. H., Delp, S. L. 2010; 43 (8): 1522-1526

  Abstract

  Crouch gait, one of the most prevalent movement abnormalities among children with cerebral palsy, is frequently treated with surgical lengthening of the hamstrings. To assist in surgical planning many clinical centers use musculoskeletal modeling to help determine if a patient's hamstrings are shorter or lengthen more slowly than during unimpaired gait. However, some subjects with crouch gait walk slowly, and gait speed may affect peak hamstring lengths and lengthening velocities. The purpose of this study was to evaluate the effects of walking speed on hamstrings lengths and velocities in a group of unimpaired subjects over a large range of speeds and to determine if evaluating subjects with crouch gait using speed matched controls alters subjects' characterization as having "short" or "slow" hamstrings. We examined 39 unimpaired subjects who walked at five different speeds. These subjects served as speed-matched controls for comparison to 74 subjects with cerebral palsy who walked in crouch gait. Our analysis revealed that peak hamstrings length and peak lengthening velocity in unimpaired subjects increased significantly with increasing walking speed. Fewer subjects with cerebral palsy were categorized as having hamstrings that were "short" (31/74) or "slow" (38/74) using a speed-matched control protocol compared to a non-speed-matched protocol (35/74 "short", 47/74 "slow"). Evaluation of patients with cerebral palsy using speed-matched controls alters and may improve selection of patients for hamstrings lengthening procedures.

  View details for DOI 10.1016/j.jbiomech.2010.01.008

  View details for Web of Science ID 000278652000013

  View details for PubMedID 20381047

  View details for PubMedCentralID PMC2918640

• A Model of the Lower Limb for Analysis of Human Movement ANNALS OF BIOMEDICAL ENGINEERING Arnold, E. M., Ward, S. R., Lieber, R. L., Delp, S. L. 2010; 38 (2): 269-279

  Abstract

  Computer models that estimate the force generation capacity of lower limb muscles have become widely used to simulate the effects of musculoskeletal surgeries and create dynamic simulations of movement. Previous lower limb models are based on severely limited data describing limb muscle architecture (i.e., muscle fiber lengths, pennation angles, and physiological cross-sectional areas). Here, we describe a new model of the lower limb based on data that quantifies the muscle architecture of 21 cadavers. The model includes geometric representations of the bones, kinematic descriptions of the joints, and Hill-type models of 44 muscle-tendon compartments. The model allows calculation of muscle-tendon lengths and moment arms over a wide range of body positions. The model also allows detailed examination of the force and moment generation capacities of muscles about the ankle, knee, and hip and is freely available at www.simtk.org .

  View details for DOI 10.1007/s10439-009-9852-5

  View details for Web of Science ID 000274237000005

  View details for PubMedID 19957039

  View details for PubMedCentralID PMC2903973

• Can Strength Training Predictably Improve Gait Kinematics? A Pilot Study on the Effects of Hip and Knee Extensor Strengthening on Lower-Extremity Alignment in Cerebral Palsy PHYSICAL THERAPY Damiano, D. L., Arnold, A. S., Steele, K. M., Delp, S. L. 2010; 90 (2): 269-279

  Abstract

  Computer simulations have demonstrated that excessive hip and knee flexion during gait, as frequently seen in ambulatory children with cerebral palsy (CP), can reduce the ability of muscles to provide antigravity support and increase the tendency of hip muscles to internally rotate the thigh. These findings suggest that therapies for improving upright posture during gait also may reduce excessive internal rotation.The goal of this study was to determine whether strength training can diminish the degree of crouched, internally rotated gait in children with spastic diplegic CP.This was a pilot prospective clinical trial.Eight children with CP participated in an 8-week progressive resistance exercise program, with 3-dimensional gait analysis and isokinetic testing performed before and after the program. Secondary measures included passive range of motion, the Ashworth Scale, and the PedsQL CP Module. To identify factors that may have influenced outcome, individual and subgroup data were examined for patterns of change within and across variables.Strength (force-generating capacity) increased significantly in the left hip extensors, with smaller, nonsignificant mean increases in the other 3 extensor muscle groups, yet kinematic and functional outcomes were inconsistent. The first reported subject-specific computer simulations of crouch gait were created for one child who showed substantial benefit to examine the factors that may have contributed to this outcome.The sample was small, with wide variability in outcomes.Strength training may improve walking function and alignment in some patients for whom weakness is a major contributor to their gait deficits. However, in other patients, it may produce no change or even undesired outcomes. Given the variability of outcomes in this and other strengthening studies in CP, analytical approaches to determine the sources of variability are needed to better identify those individuals who are most likely to benefit from strengthening.

  View details for DOI 10.2522/ptj.20090062

  View details for Web of Science ID 000274130000016

  View details for PubMedID 20022999

• Engineered Myosin VI Motors Reveal Minimal Structural Determinants of Directionality and Processivity JOURNAL OF MOLECULAR BIOLOGY Liao, J., Elting, M. W., Delp, S. L., Spudich, J. A., Bryant, Z. 2009; 392 (4): 862-867

  Abstract

  Myosins have diverse mechanical properties reflecting a range of cellular roles. A major challenge is to understand the structural basis for generating novel functions from a common motor core. Myosin VI (M6) is specialized for processive motion toward the (-) end of actin filaments. We have used engineered M6 motors to test and refine the "redirected power stroke" model for (-) end directionality and to explore poorly understood structural requirements for processive stepping. Guided by crystal structures and molecular modeling, we fused artificial lever arms to the catalytic head of M6 at several positions, retaining varying amounts of native structure. We found that an 18-residue alpha-helical insert is sufficient to reverse the directionality of the motor, with no requirement for any calmodulin light chains. Further, we observed robust processive stepping of motors with artificial lever arms, demonstrating that processivity can arise without optimizing lever arm composition or mechanics.

  View details for DOI 10.1016/j.jmb.2009.07.046

  View details for Web of Science ID 000270601200002

  View details for PubMedID 19631216

  View details for PubMedCentralID PMC3360974

• Coarse-Grained Structural Modeling of Molecular Motors Using Multibody Dynamics CELLULAR AND MOLECULAR BIOENGINEERING Parker, D., Bryant, Z., Delp, S. L. 2009; 2 (3): 366-374

  Abstract

  Experimental and computational approaches are needed to uncover the mechanisms by which molecular motors convert chemical energy into mechanical work. In this article, we describe methods and software to generate structurally realistic models of molecular motor conformations compatible with experimental data from different sources. Coarse-grained models of molecular structures are constructed by combining groups of atoms into a system of rigid bodies connected by joints. Contacts between rigid bodies enforce excluded volume constraints, and spring potentials model system elasticity. This simplified representation allows the conformations of complex molecular motors to be simulated interactively, providing a tool for hypothesis building and quantitative comparisons between models and experiments. In an example calculation, we have used the software to construct atomically detailed models of the myosin V molecular motor bound to its actin track. The software is available at www.simtk.org.

  View details for DOI 10.1007/s12195-009-0084-4

  View details for Web of Science ID 000270168900010

  View details for PubMedCentralID PMC2860290

• Coarse-Grained Structural Modeling of Molecular Motors Using Multibody Dynamics. Cellular and molecular bioengineering Parker, D., Bryant, Z., Delp, S. L. 2009; 2 (3): 366-374

  Abstract

  Experimental and computational approaches are needed to uncover the mechanisms by which molecular motors convert chemical energy into mechanical work. In this article, we describe methods and software to generate structurally realistic models of molecular motor conformations compatible with experimental data from different sources. Coarse-grained models of molecular structures are constructed by combining groups of atoms into a system of rigid bodies connected by joints. Contacts between rigid bodies enforce excluded volume constraints, and spring potentials model system elasticity. This simplified representation allows the conformations of complex molecular motors to be simulated interactively, providing a tool for hypothesis building and quantitative comparisons between models and experiments. In an example calculation, we have used the software to construct atomically detailed models of the myosin V molecular motor bound to its actin track. The software is available at www.simtk.org.

  View details for DOI 10.1007/s12195-009-0084-4

  View details for PubMedID 20428469

  View details for PubMedCentralID PMC2860290

• Multiecho IDEAL Gradient-Echo Water-Fat Separation for Rapid Assessment of Cartilage Volume at 1.5 T: Initial Experience RADIOLOGY Chen, C. A., Lu, W., John, C. T., Hargreaves, B. A., Reeder, S. B., Delp, S. L., Siston, R. A., Gold, G. E. 2009; 252 (2): 561-567

  Abstract

  Institutional review board approval and informed consent were obtained for this HIPAA-compliant study. The purpose was to prospectively compare multiecho iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) gradient-echo (GRE) magnetic resonance (MR) imaging with three-dimensional fat-suppressed (FS) spoiled GRE (SPGR) MR imaging to evaluate the articular cartilage of the knee. Six healthy volunteer and 10 cadaver knees were imaged at 1.5 T. Signal-to-noise ratio (SNR), SNR efficiency, and cartilage volume were measured. SNR and SNR efficiency were significantly higher with multiecho IDEAL GRE than with FS SPGR imaging (P < .031). Both methods produced equivalent cartilage volumes (overall concordance correlation coefficient, 0.998) with high precision and accuracy. The use of a cartilage phantom confirmed high accuracy in volume measurements and high reproducibility for both methods. Multiecho IDEAL GRE provides high signal intensity in cartilage and synovial fluid and is a promising technique for imaging articular cartilage of the knee.

  View details for DOI 10.1148/radiol.2522081424

  View details for PubMedID 19528355

• Predicting outcomes of rectus femoris transfer surgery GAIT & POSTURE Reinbolt, J. A., Fox, M. D., Schwartz, M. H., Delp, S. L. 2009; 30 (1): 100-105

  Abstract

  Rectus femoris transfer surgery is a common treatment for stiff knee gait in children with cerebral palsy. Unfortunately, the improvement in knee motion after surgery is inconsistent. There is great interest in understanding the causes of stiff knee gait and determining predictors of improved knee motion after surgery. This study demonstrates that it is possible to predict whether or not a patient's knee motion will improve following rectus femoris transfer surgery with greater than 80% accuracy. A predictive model was developed that requires only a few preoperative gait analysis measurements, already collected as a routine part of treatment planning. Our examination of 62 patients before and after rectus femoris transfer revealed that a combination of hip power, knee power, and knee flexion velocity at toe-off correctly predicted postoperative outcome for 80% of cases. With the addition of two more preoperative measurements, hip flexion and internal rotation, prediction accuracy increased to nearly 88%. Other combinations of preoperative gait analysis measurements also predicted outcomes with high accuracy. These results provide insight into factors related to positive outcomes and suggest that predictive models provide a valuable tool for determining indications for rectus femoris transfer.

  View details for DOI 10.1016/j.gaitpost.2009.03.008

  View details for Web of Science ID 000266905000019

  View details for PubMedID 19411175

  View details for PubMedCentralID PMC2747373

• Knee muscle forces during walking and running in patellofemoral pain patients and pain-free controls JOURNAL OF BIOMECHANICS Besier, T. F., Fredericson, M., Gold, G. E., Beaupre, G. S., Delp, S. L. 2009; 42 (7): 898-905

  Abstract

  One proposed mechanism of patellofemoral pain, increased stress in the joint, is dependent on forces generated by the quadriceps muscles. Describing causal relationships between muscle forces, tissue stresses, and pain is difficult due to the inability to directly measure these variables in vivo. The purpose of this study was to estimate quadriceps forces during walking and running in a group of male and female patients with patellofemoral pain (n = 27, 16 female; 11 male) and compare these to pain-free controls (n = 16, 8 female; 8 male). Subjects walked and ran at self-selected speeds in a gait laboratory. Lower limb kinematics and electromyography (EMG) data were input to an EMG-driven musculoskeletal model of the knee, which was scaled and calibrated to each individual to estimate forces in 10 muscles surrounding the joint. Compared to controls, the patellofemoral pain group had greater co-contraction of quadriceps and hamstrings (p = 0.025) and greater normalized muscle forces during walking, even though the net knee moment was similar between groups. Muscle forces during running were similar between groups, but the net knee extension moment was less in the patellofemoral pain group compared to controls. Females displayed 30-50% greater normalized hamstring and gastrocnemius muscle forces during both walking and running compared to males (p<0.05). These results suggest that some patellofemoral pain patients might experience greater joint contact forces and joint stresses than pain-free subjects. The muscle force data are available as supplementary material.

  View details for DOI 10.1016/j.jbiomech.2009.01.032

  View details for Web of Science ID 000266299300016

  View details for PubMedID 19268945

  View details for PubMedCentralID PMC2671570

• Using Real-Time MRI to Quantify Altered Joint Kinematics in Subjects with Patellofemoral Pain and to Evaluate the Effects of a Patellar Brace or Sleeve on Joint Motion JOURNAL OF ORTHOPAEDIC RESEARCH Draper, C. E., Besier, T. F., Santos, J. M., Jennings, F., Fredericson, M., Gold, G. E., Beaupre, G. S., Delp, S. L. 2009; 27 (5): 571-577

  Abstract

  Abnormal patellofemoral joint motion is a possible cause of patellofemoral pain, and patellar braces are thought to alleviate pain by restoring normal joint kinematics. We evaluated whether females with patellofemoral pain exhibit abnormal patellofemoral joint kinematics during dynamic, weight-bearing knee extension and assessed the effects of knee braces on patellofemoral motion. Real-time magnetic resonance (MR) images of the patellofemoral joints of 36 female volunteers (13 pain-free controls, 23 patellofemoral pain) were acquired during weight-bearing knee extension. Pain subjects were also imaged while wearing a patellar-stabilizing brace and a patellar sleeve. We measured axial-plane kinematics from the images. Females with patellofemoral pain exhibited increased lateral translation of the patella for knee flexion angles between 0 degrees and 50 degrees (p = 0.03), and increased lateral tilt for knee flexion angles between 0 degrees and 20 degrees (p = 0.04). The brace and sleeve reduced the lateral translation of the patella; however, the brace reduced lateral displacement more than the sleeve (p = 0.006). The brace reduced patellar tilt near full extension (p = 0.001), while the sleeve had no effect on patellar tilt. Our results indicate that some subjects with patellofemoral pain exhibit abnormal weight-bearing joint kinematics and that braces may be effective in reducing patellar maltracking in these subjects.

  View details for DOI 10.1002/jor.20790

  View details for Web of Science ID 000265009900002

  View details for PubMedID 18985690

  View details for PubMedCentralID PMC2891525

• Robotics-based synthesis of human motion JOURNAL OF PHYSIOLOGY-PARIS Khatib, O., Demircan, E., De Sapio, V., Sentis, L., Besier, T., Delp, S. 2009; 103 (3-5): 211-219

  Abstract

  The synthesis of human motion is a complex procedure that involves accurate reconstruction of movement sequences, modeling of musculoskeletal kinematics, dynamics and actuation, and characterization of reliable performance criteria. Many of these processes have much in common with the problems found in robotics research. Task-based methods used in robotics may be leveraged to provide novel musculoskeletal modeling methods and physiologically accurate performance predictions. In this paper, we present (i) a new method for the real-time reconstruction of human motion trajectories using direct marker tracking, (ii) a task-driven muscular effort minimization criterion and (iii) new human performance metrics for dynamic characterization of athletic skills. Dynamic motion reconstruction is achieved through the control of a simulated human model to follow the captured marker trajectories in real-time. The operational space control and real-time simulation provide human dynamics at any configuration of the performance. A new criteria of muscular effort minimization has been introduced to analyze human static postures. Extensive motion capture experiments were conducted to validate the new minimization criterion. Finally, new human performance metrics were introduced to study in details an athletic skill. These metrics include the effort expenditure and the feasible set of operational space accelerations during the performance of the skill. The dynamic characterization takes into account skeletal kinematics as well as muscle routing kinematics and force generating capacities. The developments draw upon an advanced musculoskeletal modeling platform and a task-oriented framework for the effective integration of biomechanics and robotics methods.

  View details for DOI 10.1016/j.jphysparis.2009.08.004

  View details for Web of Science ID 000271395900009

  View details for PubMedID 19665552

  View details for PubMedCentralID PMC2782476

• Mechanisms of improved knee flexion after rectus femoris transfer surgery JOURNAL OF BIOMECHANICS Fox, M. D., Reinbolt, J. A., Ounpuu, S., Delp, S. L. 2009; 42 (5): 614-619

  Abstract

  Rectus femoris transfer is frequently performed to treat stiff-knee gait in subjects with cerebral palsy. In this surgery, the distal tendon is released from the patella and re-attached to one of several sites, such as the sartorius or the iliotibial band. Surgical outcomes vary, and the mechanisms by which the surgery improves knee motion are unclear. The purpose of this study was to clarify the mechanism by which the transferred muscle improves knee flexion by examining three types of transfers. Muscle-actuated dynamic simulations were created of ten children diagnosed with cerebral palsy and stiff-knee gait. These simulations were altered to represent surgical transfers of the rectus femoris to the sartorius and the iliotibial band. Rectus femoris transfers in which the muscle remained attached to the underlying vasti through scar tissue were also simulated by reducing but not eliminating the muscle's knee extension moment. Simulated transfer to the sartorius, which converted the rectus femoris' knee extension moment to a flexion moment, produced 32+/-8 degrees improvement in peak knee flexion on average. Simulated transfer to the iliotibial band, which completely eliminated the muscle's knee extension moment, predicted only slightly less improvement in peak knee flexion (28+/-8 degrees ). Scarred transfer simulations, which reduced the muscle's knee extension moment, predicted significantly less (p<0.001) improvement in peak knee flexion (14+/-5 degrees ). Simulations revealed that improved knee flexion following rectus femoris transfer is achieved primarily by reduction of the muscle's knee extension moment. Reduction of scarring of the rectus femoris to underlying muscles has the potential to enhance knee flexion.

  View details for DOI 10.1016/j.jbiomech.2008.12.007

  View details for Web of Science ID 000264957900008

  View details for PubMedID 19217109

  View details for PubMedCentralID PMC2929172

• MUSCLE CONTRIBUTIONS TO MEDIAL-LATERAL ACCELERATION OF THE BODY DURING WALKING ASME Summer Bioengineering Conference John, C. T., Fox, M. D., Liu, M. Q., Schwartz, M. H., Delp, S. L. AMER SOC MECHANICAL ENGINEERS. 2009: 1127–1128

  View details for Web of Science ID 000280089000564

• CROUCH GAIT REPRESENTS A SIMPLIFIED MUSCULAR SUPPORT STRATEGY DURING SINGLE-LIMB STANCE COMPARED TO UNIMPAIRED GAIT ASME Summer Bioengineering Conference Steele, K. M., Seth, A., Hicks, J. L., Schwartz, M., Delp, S. L. AMER SOC MECHANICAL ENGINEERS. 2009: 1093–1094

  View details for Web of Science ID 000280089000547

• New resource for the computation of cartilage biphasic material properties with the interpolant response surface method COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING Keenan, K. E., Kourtis, L. C., Besier, T. F., Lindsey, D. P., Gold, G. E., Delp, S. L., Beaupre, G. S. 2009; 12 (4): 415-422

  Abstract

  Cartilage material properties are important for understanding joint function and diseases, but can be challenging to obtain. Three biphasic material properties (aggregate modulus, Poisson's ratio and permeability) can be determined using an analytical or finite element model combined with optimisation to find the material properties values that best reproduce an experimental creep curve. The purpose of this study was to develop an easy-to-use resource to determine biphasic cartilage material properties. A Cartilage Interpolant Response Surface was generated from interpolation of finite element simulations of creep indentation tests. Creep indentation tests were performed on five sites across a tibial plateau. A least-squares residual search of the Cartilage Interpolant Response Surface resulted in a best-fit curve for each experimental condition with corresponding material properties. These sites provided a representative range of aggregate moduli (0.48-1.58 MPa), Poisson's ratio (0.00-0.05) and permeability (1.7 x 10(- 15)-5.4 x 10(- 15) m(4)/N s) values found in human cartilage. The resource is freely available from https://simtk.org/home/va-squish.

  View details for DOI 10.1080/10255840802654319

  View details for Web of Science ID 000268912000005

  View details for PubMedID 19675978

  View details for PubMedCentralID PMC2858459

• Reconstruction and EMG-Informed Control, Simulation and Analysis of Human Movement for Athletics: Performance Improvement and Injury Prevention Annual International Conference of the IEEE-Engineering-in-Medicine-and-Biology-Society Demircan, E., Khatib, O., Wheeler, J., Delp, S. IEEE. 2009: 6534–6537

  Abstract

  In this paper we present methods to track and characterize human dynamic skills using motion capture and electromographic sensing. These methods are based on task-space control to obtain the joint kinematics and extract the key physiological parameters and on computed muscle control to solve the muscle force distribution problem. We also present a dynamic control and analysis framework that integrates these metrics for the purpose of reconstructing and analyzing sports motions in real-time.

  View details for Web of Science ID 000280543605089

  View details for PubMedID 19964175

• IMAGING SARCOMERES OF EXTENSOR CARPI RADIALIS BREVIS IN HUMANS USING MINIMALLY INVASIVE MICROENDOSCOPY ASME Summer Bioengineering Conference Cromie, M. J., Sanchez, G. N., Schnitzer, M. J., Delp, S. L. AMER SOC MECHANICAL ENGINEERS. 2009: 1009–1010

  View details for Web of Science ID 000280089000505

• Capacity to increase walking speed is limited by impaired hip and ankle power generation in lower functioning persons post-stroke GAIT & POSTURE Jonkers, I., Delp, S., Patten, C. 2009; 29 (1): 129-137

  Abstract

  It is well known that stroke patients walk with reduced speed, but their potential to increase walking speed can also be impaired and has not been thoroughly investigated. We hypothesized that failure to effectively recruit both hip flexor and ankle plantarflexor muscles of the paretic side limits the potential to increase walking speed in lower functioning hemiparetic subjects. To test this hypothesis, we measured gait kinematics and kinetics of 12 persons with hemiparesis following stroke at self-selected and fast walking conditions. Two groups were identified: (1) lower functioning subjects (n=6) who increased normalized walking speed from 0.52 leg lengths/s (ll/s, SEM: 0.04) to 0.72 ll/s (SEM: 0.03) and (2) higher functioning subjects (n=6) who increased walking speed from 0.88 ll/s (SEM: 0.04) to 1.4 ll/s (SEM 0.03). Changes in spatiotemporal parameters, joint kinematics and kinetics between self-selected and fast walking were compared to control subjects examined at matched walking speeds (0.35 ll/s (SEM: 0.03), 0.63 ll/s (SEM: 0.03), 0.92 ll/s (SEM: 0.04) and 1.4 ll/s (SEM: 0.04)). Similar to speed-matched controls, the higher functioning hemiparetic subjects increased paretic limb hip flexion power and ankle plantarflexion power to increase walking speed. The lower functioning hemiparetic subjects did not increase power generation at the hip or ankle to increase walking speed. This observation suggests that impaired ankle power generation combined with saturation of hip power generation limits the potential to increase walking speed in lower functioning hemiparetic subjects.

  View details for DOI 10.1016/j.gaitpost.2008.07.010

  View details for Web of Science ID 000262592200024

  View details for PubMedID 18789692

• The Influence of Femoral Internal and External Rotation on Cartilage Stresses within the Patellofemoral Joint JOURNAL OF ORTHOPAEDIC RESEARCH Besier, T. F., Gold, G. E., Delp, S. L., Fredericson, M., Beaupre, G. S. 2008; 26 (12): 1627-1635

  Abstract

  Internal and external rotation of the femur plays an important role in defining the orientation of the patellofemoral joint, influencing contact areas, pressures, and cartilage stress distributions. The purpose of this study was to determine the influence of femoral internal and external rotation on stresses in the patellofemoral cartilage. We constructed finite element models of the patellofemoral joint using magnetic resonance (MR) images from 16 volunteers (8 male and 8 female). Subjects performed an upright weight-bearing squat with the knee at 60 degrees of flexion inside an open-MR scanner and in a gait laboratory. Quadriceps muscle forces were estimated for each subject using an electromyographic-driven model and input to a finite element analysis. Hydrostatic and octahedral shear stresses within the cartilage were modeled with the tibiofemoral joint in a "neutral" position and also with the femur rotated internally or externally by 5 degrees increments to +/-15 degrees . Cartilage stresses were more sensitive to external rotation of the femur, compared with internal rotation, with large variation across subjects. Peak patellar shear stresses increased more than 10% with 15 degrees of external rotation in 75% of the subjects. Shear stresses were higher in the patellar cartilage compared to the femoral cartilage and patellar cartilage stresses were more sensitive to femoral rotation compared with femoral cartilage stress. Large variation in the cartilage stress response between individuals reflects the complex nature of the extensor mechanism and has clinical relevance when considering treatment strategies designed to reduce cartilage stresses by altering femoral internal and external rotation.

  View details for DOI 10.1002/jor.20663

  View details for Web of Science ID 000260934700012

  View details for PubMedID 18524000

• Muscle contributions to support and progression over a range of walking speeds JOURNAL OF BIOMECHANICS Liu, M. Q., Anderson, F. C., Schwartz, M. H., Delp, S. L. 2008; 41 (15): 3243-3252

  Abstract

  Muscles actuate walking by providing vertical support and forward progression of the mass center. To quantify muscle contributions to vertical support and forward progression (i.e., vertical and fore-aft accelerations of the mass center) over a range of walking speeds, three-dimensional muscle-actuated simulations of gait were generated and analyzed for eight subjects walking overground at very slow, slow, free, and fast speeds. We found that gluteus maximus, gluteus medius, vasti, hamstrings, gastrocnemius, and soleus were the primary contributors to support and progression at all speeds. With the exception of gluteus medius, contributions from these muscles generally increased with walking speed. During very slow and slow walking speeds, vertical support in early stance was primarily provided by a straighter limb, such that skeletal alignment, rather than muscles, provided resistance to gravity. When walking speed increased from slow to free, contributions to support from vasti and soleus increased dramatically. Greater stance-phase knee flexion during free and fast walking speeds caused increased vasti force, which provided support but also slowed progression, while contralateral soleus simultaneously provided increased propulsion. This study provides reference data for muscle contributions to support and progression over a wide range of walking speeds and highlights the importance of walking speed when evaluating muscle function.

  View details for DOI 10.1016/j.jbiomech.2008.07.031

  View details for Web of Science ID 000261657000022

  View details for PubMedID 18822415

• Posterior Cruciate Ligament Removal Contributes to Abnormal Knee Motion during Posterior Stabilized Total Knee Arthroplasty JOURNAL OF ORTHOPAEDIC RESEARCH Cromie, M. J., Siston, R. A., Giori, N. J., Delp, S. L. 2008; 26 (11): 1494-1499

  Abstract

  Abnormal anterior translation of the femur on the tibia has been observed in mid flexion (20-60 degrees ) following posterior stabilized total knee arthroplasty. The underlying biomechanical causes of this abnormal motion remain unknown. The purpose of this study was to isolate the effects of posterior cruciate ligament removal on knee motion after total knee arthroplasty. We posed two questions: Does removing the posterior cruciate ligament introduce abnormal anterior femoral translation? Does implanting a posterior stabilized prosthesis change the kinematics from the cruciate deficient case? Using a navigation system, we measured passive knee kinematics of ten male osteoarthritic patients during surgery after initial exposure, after removing the anterior cruciate ligament, after removing the posterior cruciate ligament, and after implanting the prosthesis. Passively flexing and extending the knee, we calculated anterior femoral translation and the flexion angle at which femoral rollback began. Removing the posterior cruciate ligament doubled anterior translation (from 5.1 +/- 4.3 mm to 10.4 +/- 5.1 mm) and increased the flexion angle at which femoral rollback began (from 31.2 +/- 9.6 degrees to 49.3 +/- 7.3 degrees). Implanting the prosthesis increased the amount of anterior translation (to 16.1 +/- 4.4 mm), and did not change the flexion angle at which femoral rollback began. Abnormal anterior translation was observed in low and mid flexion (0-60 degrees) after removing the posterior cruciate ligament, and normal motion was not restored by the posterior stabilized prosthesis.

  View details for DOI 10.1002/jor.20664

  View details for Web of Science ID 000260195800012

  View details for PubMedID 18464260

• Averaging Different Alignment Axes Improves Femoral Rotational Alignment in Computer-Navigated Total Knee Arthroplasty JOURNAL OF BONE AND JOINT SURGERY-AMERICAN VOLUME Siston, R. A., Cromie, M. J., Gold, G. E., Goodman, S. B., Delp, S. L., Maloney, W. J., Giori, N. J. 2008; 90A (10): 2098-2104

  Abstract

  Computer navigation systems generally establish the rotational alignment axis of the femoral component on the basis of user-defined anatomic landmarks. However, navigation systems can also record knee kinematics and average alignment axes established with multiple techniques. We hypothesized that establishing femoral rotational alignment with the use of kinematic techniques is more accurate and precise (repeatable) than the use of anatomic techniques and that establishing femoral rotational alignment by averaging the results of different alignment techniques is more accurate and precise than the use of a single technique.Twelve orthopaedic surgeons used three anatomic and two kinematic alignment techniques to establish femoral rotational alignment axes in a series of nine cadaver knees. The axes derived with the individual anatomic and kinematic techniques as well as the axes derived with six combination techniques--i.e., those involving averaging of the alignments established with two of the individual techniques--were compared against a reference axis established with computed tomography images of each femur.The kinematic methods were not more accurate (did not have smaller mean errors) or more precise (repeatable) than the anatomic techniques. The combination techniques were accurate (five of the six had a mean error of <5 degrees ) and significantly more precise than all but one of the single methods. The percentage of measurements with <5 degrees of error as compared with the reference epicondylar axis was 37% for the individual anatomic techniques, 30% for the individual kinematic techniques, and 58% for the combination techniques.Averaging the results of kinematic and anatomic techniques, which is possible with computer navigation systems, appears to improve the accuracy of rotational alignment of the femoral component. The number of rotational alignment outliers was reduced when combination techniques were used; however, they are still a problem and continued improvement in methods to accurately establish rotation of the femoral component in total knee arthroplasty is needed.

  View details for DOI 10.2106/JBJS.G.00996

  View details for Web of Science ID 000259873300006

• Importance of preswing rectus femoris activity in stiff-knee gait JOURNAL OF BIOMECHANICS Reinbolt, J. A., Fox, M. D., Arnold, A. S., Ounpuu, S., Delp, S. L. 2008; 41 (11): 2362-2369

  Abstract

  Stiff-knee gait is characterized by diminished and delayed knee flexion during swing. Rectus femoris transfer surgery, a common treatment for stiff-knee gait, is often recommended when a patient exhibits prolonged activity of the rectus femoris muscle during swing. Treatment outcomes are inconsistent, in part, due to limited understanding of the biomechanical factors contributing to stiff-knee gait. This study used a combination of gait analysis and dynamic simulation to examine how activity of the rectus femoris during swing, and prior to swing, contribute to knee flexion. A group of muscle-actuated dynamic simulations was created that accurately reproduced the gait dynamics of ten subjects with stiff-knee gait. These simulations were used to examine the effects of rectus femoris activity on knee motion by eliminating rectus femoris activity during preswing and separately during early swing. The increase in peak knee flexion by eliminating rectus femoris activity during preswing (7.5+/-3.1 degrees ) was significantly greater on average (paired t-test, p=0.035) than during early swing (4.7+/-3.6 degrees ). These results suggest that preswing rectus femoris activity is at least as influential as early swing activity in limiting the knee flexion of persons with stiff-knee gait. In evaluating rectus femoris activity for treatment of stiff-knee gait, preswing as well as early swing activity should be examined.

  View details for DOI 10.1016/j.jbiomech.2008.05.030

  View details for Web of Science ID 000259129000004

  View details for PubMedID 18617180

• Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans NATURE Llewellyn, M. E., Barretto, R. P., Delp, S. L., Schnitzer, M. J. 2008; 454 (7205): 784-788

  Abstract

  Sarcomeres are the basic contractile units of striated muscle. Our knowledge about sarcomere dynamics has primarily come from in vitro studies of muscle fibres and analysis of optical diffraction patterns obtained from living muscles. Both approaches involve highly invasive procedures and neither allows examination of individual sarcomeres in live subjects. Here we report direct visualization of individual sarcomeres and their dynamical length variations using minimally invasive optical microendoscopy to observe second-harmonic frequencies of light generated in the muscle fibres of live mice and humans. Using microendoscopes as small as 350 microm in diameter, we imaged individual sarcomeres in both passive and activated muscle. Our measurements permit in vivo characterization of sarcomere length changes that occur with alterations in body posture and visualization of local variations in sarcomere length not apparent in aggregate length determinations. High-speed data acquisition enabled observation of sarcomere contractile dynamics with millisecond-scale resolution. These experiments point the way to in vivo imaging studies demonstrating how sarcomere performance varies with physical conditioning and physiological state, as well as imaging diagnostics revealing how neuromuscular diseases affect contractile dynamics.

  View details for DOI 10.1038/nature07104

  View details for Web of Science ID 000258228000049

  View details for PubMedID 18600262

  View details for PubMedCentralID PMC2826360

• The Simbios National Center: Systems biology in motion PROCEEDINGS OF THE IEEE Schmidt, J. P., Delp, S. L., Sherman, M. A., Taylor, C. A., Pande, V. S., Altman, R. B. 2008; 96 (8): 1266-1280

  Abstract

  Physics-based simulation is needed to understand the function of biological structures and can be applied across a wide range of scales, from molecules to organisms. Simbios (the National Center for Physics-Based Simulation of Biological Structures, http://www.simbios.stanford.edu/) is one of seven NIH-supported National Centers for Biomedical Computation. This article provides an overview of the mission and achievements of Simbios, and describes its place within systems biology. Understanding the interactions between various parts of a biological system and integrating this information to understand how biological systems function is the goal of systems biology. Many important biological systems comprise complex structural systems whose components interact through the exchange of physical forces, and whose movement and function is dictated by those forces. In particular, systems that are made of multiple identifiable components that move relative to one another in a constrained manner are multibody systems. Simbios' focus is creating methods for their simulation. Simbios is also investigating the biomechanical forces that govern fluid flow through deformable vessels, a central problem in cardiovascular dynamics. In this application, the system is governed by the interplay of classical forces, but the motion is distributed smoothly through the materials and fluids, requiring the use of continuum methods. In addition to the research aims, Simbios is working to disseminate information, software and other resources relevant to biological systems in motion.

  View details for DOI 10.1109/JPROC.2008.925454

  View details for Web of Science ID 000257860800004

  View details for PubMedCentralID PMC2811325

• Feasibility of using real-time MRI to measure joint kinematics in 1.5T and open-bore 0.5T systems JOURNAL OF MAGNETIC RESONANCE IMAGING Draper, C. E., Santos, J. M., Kourtis, L. C., Besier, T. F., Fredericson, M., Beaupre, G. S., Gold, G. E., Delp, S. L. 2008; 28 (1): 158-166

  Abstract

  To test the feasibility and accuracy of measuring joint motion with real-time MRI in a 1.5T scanner and in a 0.5T open-bore scanner and to assess the dependence of measurement accuracy on movement speed.We developed an MRI-compatible motion phantom to evaluate the accuracy of tracking bone positions with real-time MRI for varying movement speeds. The measurement error was determined by comparing phantom positions estimated from real-time MRI to those measured using optical motion capture techniques. To assess the feasibility of measuring in vivo joint motion, we calculated 2D knee joint kinematics during knee extension in six subjects and compared them to previously reported measurements.Measurement accuracy decreased as the phantom's movement speed increased. The measurement accuracy was within 2 mm for velocities up to 217 mm/s in the 1.5T scanner and 38 mm/s in the 0.5T scanner. We measured knee joint kinematics with small intraobserver variation (variance of 0.8 degrees for rotation and 3.6% of patellar width for translation).Our results suggest that real-time MRI can be used to measure joint kinematics when 2 mm accuracy is sufficient. They can also be used to prescribe the speed of joint motion necessary to achieve certain measurement accuracy.

  View details for DOI 10.1002/jmri.21413

  View details for Web of Science ID 000257865800021

  View details for PubMedID 18581329

• iTools: A Framework for Classification, Categorization and Integration of Computational Biology Resources PLOS ONE Dinov, I. D., Rubin, D., Lorensen, W., Dugan, J., Ma, J., Murphy, S., Kirschner, B., Bug, W., Sherman, M., Floratos, A., Kennedy, D., Jagadish, H. V., Schmidt, J., Athey, B., Califano, A., Musen, M., Altman, R., Kikinis, R., Kohane, I., Delp, S., Parker, D. S., Toga, A. W. 2008; 3 (5)

  Abstract

  The advancement of the computational biology field hinges on progress in three fundamental directions--the development of new computational algorithms, the availability of informatics resource management infrastructures and the capability of tools to interoperate and synergize. There is an explosion in algorithms and tools for computational biology, which makes it difficult for biologists to find, compare and integrate such resources. We describe a new infrastructure, iTools, for managing the query, traversal and comparison of diverse computational biology resources. Specifically, iTools stores information about three types of resources--data, software tools and web-services. The iTools design, implementation and resource meta-data content reflect the broad research, computational, applied and scientific expertise available at the seven National Centers for Biomedical Computing. iTools provides a system for classification, categorization and integration of different computational biology resources across space-and-time scales, biomedical problems, computational infrastructures and mathematical foundations. A large number of resources are already iTools-accessible to the community and this infrastructure is rapidly growing. iTools includes human and machine interfaces to its resource meta-data repository. Investigators or computer programs may utilize these interfaces to search, compare, expand, revise and mine meta-data descriptions of existent computational biology resources. We propose two ways to browse and display the iTools dynamic collection of resources. The first one is based on an ontology of computational biology resources, and the second one is derived from hyperbolic projections of manifolds or complex structures onto planar discs. iTools is an open source project both in terms of the source code development as well as its meta-data content. iTools employs a decentralized, portable, scalable and lightweight framework for long-term resource management. We demonstrate several applications of iTools as a framework for integrated bioinformatics. iTools and the complete details about its specifications, usage and interfaces are available at the iTools web page http://iTools.ccb.ucla.edu.

  View details for DOI 10.1371/journal.pone.0002265

  View details for Web of Science ID 000262268500012

  View details for PubMedID 18509477

  View details for PubMedCentralID PMC2386255

• Least action principles and their application to constrained and task-level problems in robotics and biomechanics MULTIBODY SYSTEM DYNAMICS De Sapio, V., Khatib, O., Delp, S. 2008; 19 (3): 303-322

  View details for DOI 10.1007/s11044-007-9097-8

  View details for Web of Science ID 000253528900006

• Crouched postures reduce the capacity of muscles to extend the hip and knee during the single-limb stance phase of gait JOURNAL OF BIOMECHANICS Hicks, J. L., Schwartz, M. H., Arnold, A. S., Delp, S. L. 2008; 41 (5): 960-967

  Abstract

  Many children with cerebral palsy walk in a crouch gait that progressively worsens over time, decreasing walking efficiency and leading to joint degeneration. This study examined the effect of crouched postures on the capacity of muscles to extend the hip and knee joints and the joint flexions induced by gravity during the single-limb stance phase of gait. We first characterized representative mild, moderate, and severe crouch gait kinematics based on a large group of subjects with cerebral palsy (N=316). We then used a three-dimensional model of the musculoskeletal system and its associated equations of motion to determine the effect of these crouched gait postures on (1) the capacity of individual muscles to extend the hip and knee joints, which we defined as the angular accelerations of the joints, towards extension, that resulted from applying a 1N muscle force to the model, and (2) the angular acceleration of the joints induced by gravity. Our analysis showed that the capacities of almost all the major hip and knee extensors were markedly reduced in a crouched gait posture, with the exception of the hamstrings muscle group, whose extension capacity was maintained in a crouched posture. Crouch gait also increased the flexion accelerations induced by gravity at the hip and knee throughout single-limb stance. These findings help explain the increased energy requirements and progressive nature of crouch gait in patients with cerebral palsy.

  View details for DOI 10.1016/j.jbiomech.2008.01.002

  View details for Web of Science ID 000254943800005

  View details for PubMedID 18291404

  View details for PubMedCentralID PMC2443703

• OpenSim: open-source software to create and analyze dynamic Simulations of movement IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Delp, S. L., Anderson, F. C., Arnold, A. S., Loan, P., Habib, A., John, C. T., Guendelman, E., Thelen, D. G. 2007; 54 (11): 1940-1950

  Abstract

  Dynamic simulations of movement allow one to study neuromuscular coordination, analyze athletic performance, and estimate internal loading of the musculoskeletal system. Simulations can also be used to identify the sources of pathological movement and establish a scientific basis for treatment planning. We have developed a freely available, open-source software system (OpenSim) that lets users develop models of musculoskeletal structures and create dynamic simulations of a wide variety of movements. We are using this system to simulate the dynamics of individuals with pathological gait and to explore the biomechanical effects of treatments. OpenSim provides a platform on which the biomechanics community can build a library of simulations that can be exchanged, tested, analyzed, and improved through a multi-institutional collaboration. Developing software that enables a concerted effort from many investigators poses technical and sociological challenges. Meeting those challenges will accelerate the discovery of principles that govern movement control and improve treatments for individuals with movement pathologies.

  View details for DOI 10.1109/TBME.2007.901024

  View details for PubMedID 18018689

• Coronal plane stability before and after total knee arthroplasty CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Siston, R. A., Goodman, S. B., Delp, S. L., Giori, N. J. 2007: 43-49

  Abstract

  The success of total knee arthroplasty depends in part on proper soft tissue management to achieve a stable joint. It is unknown to what degree total knee arthroplasty changes joint stability. We used a surgical navigation system to intraoperatively measure joint stability in 24 patients under going primary total knee arthroplasty to address two questions: (1) Is the total arc of varus-valgus motion after total knee arthroplasty different from the arc of varus-valgus motion in an osteoarthritic knee? (2) Does total knee arthroplasty produce equal amounts of varus/valgus motion (ie, is the knee "balanced")? We observed no difference between the total arc of varus-valgus motion before and after total knee arthroplasty; the total amount of motion was unchanged. On average, osteoarthritic knees were "unbalanced" but were "balanced" after prosthesis implantation. We found a negative correlation between the relative amount of varus/valgus motion in extension before and after prosthesis implantation in extension and a positive correlation between how well the knees were balanced after prosthesis implantation in extension and in flexion. Our data suggest immediately after implantation knees retain a greater than normal amount of varus-valgus motion, but this motion is more evenly distributed.

  View details for DOI 10.1097/BLO.0b013e318137a182

  View details for PubMedID 17621236

• The effect of excessive tibial torsion on the capacity of muscles to extend the hip and knee during single-limb stance GAIT & POSTURE Hicks, J., Arnold, A., Anderson, F., Schwartz, M., Delp, S. 2007; 26 (4): 546-552

  Abstract

  Excessive tibial torsion, a rotational deformity about the long axis of the tibia, is common in patients with cerebral palsy who walk with a crouch gait. Previous research suggests that this deformity may contribute to crouch gait by reducing the capacity of soleus to extend the knee; however, the effects of excess external torsion on the capacity of other muscles to extend the stance limb during walking are unknown. A computer model of the musculoskeletal system was developed to simulate a range of tibial torsion deformities. A dynamic analysis was then performed to determine the effect of these deformities on the capacity of lower limb muscles to extend the hip and knee at body positions corresponding to the single-limb stance phase of a normal gait cycle. Analysis of the model confirmed that excessive external torsion reduces the extension capacity of soleus. In addition, our analysis revealed that several important muscles crossing the hip and knee are also adversely affected by excessive tibial torsion. With a tibial torsion deformity of 30 degrees , the capacities of soleus, posterior gluteus medius, and gluteus maximus to extend both the hip and knee were all reduced by over 10%. Since a tibial torsion deformity reduces the capacity of muscles to extend the hip and knee, it may be a significant contributor to crouch gait, especially when greater than 30 degrees from normal, and thus should be considered by clinicians when making treatment decisions.

  View details for DOI 10.1016/j.gaitpost.2006.12.003

  View details for Web of Science ID 000250291100011

  View details for PubMedID 17229573

  View details for PubMedCentralID PMC2443695

• Extending the absorbing boundary method to fit dwell-time distributions of molecular motors with complex kinetic pathways PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Liao, J., Spudich, J. A., Parker, D., Delp, S. L. 2007; 104 (9): 3171-3176

  Abstract

  Dwell-time distributions, waiting-time distributions, and distributions of pause durations are widely reported for molecular motors based on single-molecule biophysical experiments. These distributions provide important information concerning the functional mechanisms of enzymes and their underlying kinetic and mechanical processes. We have extended the absorbing boundary method to simulate dwell-time distributions of complex kinetic schemes, which include cyclic, branching, and reverse transitions typically observed in molecular motors. This extended absorbing boundary method allows global fitting of dwell-time distributions for enzymes subject to different experimental conditions. We applied the extended absorbing boundary method to experimental dwell-time distributions of single-headed myosin V, and were able to use a single kinetic scheme to fit dwell-time distributions observed under different ligand concentrations and different directions of optical trap forces. The ability to use a single kinetic scheme to fit dwell-time distributions arising from a variety of experimental conditions is important for identifying a mechanochemical model of a molecular motor. This efficient method can be used to study dwell-time distributions for a broad class of molecular motors, including kinesin, RNA polymerase, helicase, F(1) ATPase, and to examine conformational dynamics of other enzymes such as ion channels.

  View details for DOI 10.1073/pnas.0611519104

  View details for Web of Science ID 000244661400029

  View details for PubMedID 17360624

  View details for PubMedCentralID PMC1805548

• Image-based musculoskeletal modeling: Applications, advances, and future opportunities JOURNAL OF MAGNETIC RESONANCE IMAGING Blemker, S. S., Asakawa, D. S., Gold, G. E., Delp, S. L. 2007; 25 (2): 441-451

  Abstract

  Computer models of the musculoskeletal system are broadly used to study the mechanisms of musculoskeletal disorders and to simulate surgical treatments. Musculoskeletal models have historically been created based on data derived in anatomical and biomechanical studies of cadaveric specimens. MRI offers an abundance of novel methods for acquisition of data from living subjects and is revolutionizing the field of musculoskeletal modeling. The need to create accurate, individualized models of the musculoskeletal system is driving advances in MRI techniques including static imaging, dynamic imaging, diffusion imaging, body imaging, pulse-sequence design, and coil design. These techniques apply to imaging musculoskeletal anatomy, muscle architecture, joint motions, muscle moment arms, and muscle tissue deformations. Further advancements in image-based musculoskeletal modeling will expand the accuracy and utility of models used to study musculoskeletal and neuromuscular impairments.

  View details for DOI 10.1002/jmri.20805

  View details for Web of Science ID 000244133000020

  View details for PubMedID 17260405

• Muscular coordination of knee motion during the terminal-swing phase of normal gait JOURNAL OF BIOMECHANICS Arnold, A. S., Thelen, D. G., Schwartz, M. H., Anderson, F. C., Delp, S. L. 2007; 40 (15): 3314-3324

  Abstract

  Children with cerebral palsy often walk with diminished knee extension during the terminal-swing phase, resulting in a troublesome "crouched" posture at initial contact and a shortened stride. Treatment of this gait abnormality is challenging because the factors that extend the knee during normal walking are not well understood, and because the potential of individual muscles to limit terminal-swing knee extension is unknown. This study analyzed a series of three-dimensional, muscle-driven dynamic simulations to quantify the angular accelerations of the knee induced by muscles and other factors during swing. Simulations were generated that reproduced the measured gait dynamics and muscle excitation patterns of six typically developing children walking at self-selected speeds. The knee was accelerated toward extension in the simulations by velocity-related forces (i.e., Coriolis and centrifugal forces) and by a number of muscles, notably the vasti in mid-swing (passive), the hip extensors in terminal swing, and the stance-limb hip abductors, which accelerated the pelvis upward. Knee extension was slowed in terminal swing by the stance-limb hip flexors, which accelerated the pelvis backward. The hamstrings decelerated the forward motion of the swing-limb shank, but did not contribute substantially to angular motions of the knee. Based on these data, we hypothesize that the diminished knee extension in terminal swing exhibited by children with cerebral palsy may, in part, be caused by weak hip extensors or by impaired hip muscles on the stance limb that result in abnormal accelerations of the pelvis.

  View details for DOI 10.1016/j.jbiomech.2007.05.006

  View details for Web of Science ID 000251342800003

  View details for PubMedID 17572431

  View details for PubMedCentralID PMC2795578

• The Gait E-Book - Development of Effective Participatory Learning using Simulation and Active Electronic Books 11TH MEDITERRANEAN CONFERENCE ON MEDICAL AND BIOLOGICAL ENGINEERING AND COMPUTING 2007, VOLS 1 AND 2 Sandholm, A., Fritzson, P., Arora, V., Delp, S., Petersson, G., Rose, J. 2007; 16 (1-2): 685-?

  View details for Web of Science ID 000261088900178

• Upper limb muscle volumes in adult subjects JOURNAL OF BIOMECHANICS Holzbaur, K. R., Murray, W. M., Gold, G. E., Delp, S. L. 2007; 40 (4): 742-749

  Abstract

  Muscle force-generating properties are often derived from cadaveric studies of muscle architecture. While the relative sizes of muscles at a single upper limb joint have been established in cadaveric specimens, the relative sizes of muscles across upper limb joints in living subjects remain unclear. We used magnetic resonance imaging to measure the volumes of the 32 upper limb muscles crossing the glenohumeral joint, elbow, forearm, and wrist in 10 young, healthy subjects, ranging from a 20th percentile female to a 97th percentile male, based on height. We measured the volume and volume fraction of these muscles. Muscles crossing the shoulder, elbow, and wrist comprised 52.5, 31.4, and 16.0% of the total muscle volume, respectively. The deltoid had the largest volume fraction (15.2%+/-1%) and the extensor indicis propius had the smallest (0.2%+/-0.05%). We determined that the distribution of muscle volume in the upper limb is highly conserved across these subjects with a three-fold variation in total muscle volumes (1427-4426cm(3)). When we predicted the volume of an individual muscle from the mean volume fraction, on average 85% of the variation among subjects was accounted for (average p=0.0008). This study provides normative data that forms the basis for investigating muscle volumes in other populations, and for scaling computer models to more accurately represent the muscle volume of a specific individual.

  View details for Web of Science ID 000245111200005

  View details for PubMedID 17241636

• Moment-generating capacity of upper limb muscles in healthy adults JOURNAL OF BIOMECHANICS Holzbaur, K. R., Delp, S. L., Gold, G. E., Murray, W. M. 2007; 40 (11): 2442-2449

  Abstract

  Muscle strength and volume vary greatly among individuals. Maximum isometric joint moment, a standard measurement of strength, has typically been assessed in young, healthy subjects, whereas muscle volumes have generally been measured in cadavers. This has made it difficult to characterize the relationship between isometric strength and muscle size in humans. We measured maximum isometric moments about the shoulder, elbow, and wrist in 10 young, healthy subjects, ranging in size from a 20th percentile female to a 97th percentile male. The volumes of 32 upper limb muscles were determined from magnetic resonance images of these same subjects, and grouped according to their primary function. The maximum moments produced using the shoulder adductors (67.9+/-28.4 Nm) were largest, and were approximately 6.5(+/-1.2) times greater than those produced using the wrist extensors (10.2+/-4.6 Nm), which were smallest. While there were substantial differences in moment-generating capacity among these 10 subjects, moment significantly covaried with muscle volume of the appropriate functional group, explaining between 95% (p<0.0001; shoulder adductors) and 68% (p=0.004; wrist flexors) of the variation in the maximum isometric joint moments among subjects. While other factors, such as muscle moment arms or neural activation and coordination, can contribute to variation in strength among subjects, they either were relatively constant across these subjects compared to large differences in muscle volumes or they covaried with muscle volume. We conclude that differences in strength among healthy young adults are primarily a consequence of variation in muscle volume, as opposed to other factors.

  View details for DOI 10.1016/j.jbiomech.2006.11.013

  View details for Web of Science ID 000248990600011

  View details for PubMedID 17250841

• Contributions of muscles to terminal-swing knee motions vary with walking speed JOURNAL OF BIOMECHANICS Arnold, A. S., Schwartz, M. H., Thelen, D. G., Delp, S. L. 2007; 40 (16): 3660-3671

  Abstract

  Many children with cerebral palsy walk with diminished knee extension during terminal swing, at speeds much slower than unimpaired children. Treatment of these gait abnormalities is challenging because the factors that extend the knee during normal walking, over a range of speeds, are not well understood. This study analyzed a series of three-dimensional, muscle-driven dynamic simulations to determine whether the relative contributions of individual muscles and other factors to angular motions of the swing-limb knee vary with walking speed. Simulations were developed that reproduced the measured gait dynamics of seven unimpaired children walking at self-selected, fast, slow, and very slow speeds (7 subjects x 4 speeds=28 simulations). In mid-swing, muscles on the stance limb made the largest net contribution to extension of the swing-limb knee at all speeds examined. The stance-limb hip abductors, in particular, accelerated the pelvis upward, inducing reaction forces at the swing-limb hip that powerfully extended the knee. Velocity-related forces (i.e., Coriolis and centrifugal forces) also contributed to knee extension in mid-swing, though these contributions were diminished at slower speeds. In terminal swing, the hip flexors and other muscles on the swing-limb decelerated knee extension at the subjects' self-selected, slow, and very slow speeds, but had only a minimal net effect on knee motions at the fastest speeds. Muscles on the stance limb helped brake knee extension at the subjects' fastest speeds, but induced a net knee extension acceleration at the slowest speeds. These data--which show that the contributions of muscular and velocity-related forces to terminal-swing knee motions vary systematically with walking speed--emphasize the need for speed-matched control subjects when attempting to determine the causes of a patient's abnormal gait.

  View details for DOI 10.1016/j.jbiomech.2007.06.006

  View details for Web of Science ID 000251845100015

  View details for PubMedID 17659289

  View details for PubMedCentralID PMC2795577

• Surgical navigation for total knee arthroplasty: A perspective JOURNAL OF BIOMECHANICS Siston, R. A., Giori, N. J., Goodman, S. B., Delp, S. L. 2007; 40 (4): 728-735

  Abstract

  A new generation of surgical tools, known as surgical navigation systems, has been developed to help surgeons install implants more accurately and reproducibly. Navigation systems also record quantitative information such as joint range of motion, laxity, and kinematics intra-operatively. This article reviews the history of surgical navigation for total knee arthroplasty, the biomechanical principles associated with this technology, and the related clinical research studies. We describe how navigation has the potential to address three main challenges for total knee arthroplasty: ensuring excellent and consistent outcomes, treating younger and more physically active patients, and enabling less invasive surgery.

  View details for DOI 10.1016/j.jbiomech.2007.01.006

  View details for PubMedID 17317419

• Dynamic magnetic resonance imaging of muscle function after surgery SKELETAL RADIOLOGY Asakawa, D. S., Blemker, S. S., Gold, G. E., Delp, S. L. 2006; 35 (12): 885-886

  View details for DOI 10.1007/s00256-006-0163-8

  View details for Web of Science ID 000241797800001

  View details for PubMedID 16810541

• The high variability of tibial rotational alignment in total knee arthroplasty Open Scientific Meeting of the Knee-Society Siston, R. A., Goodman, S. B., Patel, J. J., Delp, S. L., Giori, N. J. SPRINGER. 2006: 65–69

  Abstract

  Although various techniques are advocated to establish tibial rotational alignment during total knee arthroplasty, it is unknown which is most repeatable. We evaluated the precision and accuracy of five tibial rotational alignment techniques to determine whether computer-assisted navigation systems can reduce variability of tibial component rotational alignment when compared to traditional instrumentation. Eleven orthopaedic surgeons used four computer-assisted techniques that required identification of anatomical landmarks and one that used traditional extramedullary instrumentation to establish tibial rotational alignment axes on 10 cadaver legs. Two computer-assisted techniques (axes between the most medial and lateral border of the tibial plateau, and between the posterior cruciate ligament [PCL] and the anterior tibial crest) and the traditional technique were least variable, with standard deviations of 9.9 degrees, 10.8 degrees, and 12.1 degrees, respectively. Computer-assisted techniques referencing the tibial tubercle (axes between the PCL and the medial border or medial 1/3 of the tubercle) were most variable, with standard deviations of 27.4 degrees and 28.1 degrees. The axis between the medial border of the tibial tubercle and the PCL was internally rotated compared to the other techniques. None of the techniques consistently established tibial rotational alignment, and navigation systems that establish rotational alignment by identifying anatomic landmarks were not more reliable than traditional instrumentation.

  View details for DOI 10.1097/01.blo.0000229335.36900.a0

  View details for PubMedID 16906095

• Is cartilage thickness different in young subjects with and without patellofemoral pain? OSTEOARTHRITIS AND CARTILAGE Draper, C. E., Besier, T. F., Gold, G. E., Fredericson, M., Fiene, A., Beaupre, G. S., Delp, S. L. 2006; 14 (9): 931-937

  Abstract

  To determine the differences in load-bearing patellofemoral joint cartilage thickness between genders. To determine the differences in load-bearing cartilage thickness between pain-free controls and individuals with patellofemoral pain.The articular cartilage thickness of the patella and anterior femur was estimated from magnetic resonance images in 16 young, pain-free control subjects (eight males, eight females) and 34 young individuals with patellofemoral pain (12 males, 22 females). The average age of all subjects was 28+/-4 years. The cartilage surfaces were divided into regions approximating the location of patellofemoral joint contact during knee flexion. The mean and peak cartilage thicknesses of each region were computed and compared using a repeated-measures Analysis of Variance.On average, males had 22% and 23% thicker cartilage than females in the patella (P < 0.01) and femur (P < 0.05), respectively. Male control subjects had 18% greater peak patellar cartilage thickness than males with patellofemoral pain (P < 0.05); however, we did not detect differences in patellar cartilage thickness between female control subjects and females with patellofemoral pain (P = 0.45). We detected no significant differences in femoral cartilage thickness between the control and pain groups.Thin cartilage at the patella may be one mechanism of patellofemoral pain in male subjects, but is unlikely to be a dominant factor in the development of pain in the female population.

  View details for DOI 10.1016/j.joca.2006.03.006

  View details for Web of Science ID 000239898500012

  View details for PubMedID 16647278

• Task-level approaches for the control of constrained multibody systems MULTIBODY SYSTEM DYNAMICS De Sapio, V., Khatib, O., Delp, S. 2006; 16 (1): 73-102

  View details for DOI 10.1007/s11044-006-9017-3

  View details for Web of Science ID 000239300200005

• Intraoperative passive kinematics of osteoarthritic knees before and after total knee arthroplasty JOURNAL OF ORTHOPAEDIC RESEARCH Siston, R. A., Giori, N. J., Goodman, S. B., Delp, S. L. 2006; 24 (8): 1607-1614

  Abstract

  Total knee arthroplasty is a successful procedure to treat pain and functional disability due to osteoarthritis. However, precisely how a total knee arthroplasty changes the kinematics of an osteoarthritic knee is unknown. We used a surgical navigation system to measure normal passive kinematics from 7 embalmed cadaver lower extremities and in vivo intraoperative passive kinematics on 17 patients undergoing primary total knee arthroplasty to address two questions: How do the kinematics of knees with advanced osteoarthritis differ from normal knees?; and, Does posterior substituting total knee arthroplasty restore kinematics towards normal? Osteoarthritic knees displayed a decreased screw-home motion and abnormal varus/valgus rotations between 10 degrees and 90 degrees of knee flexion when compared to normal knees. The anterior-posterior motion of the femur in osteoarthritic knees was not different than in normal knees. Following total knee arthroplasty, we found abnormal varus/valgus rotations in early flexion, a reduced screw-home motion when compared to the osteoarthritic knees, and an abnormal anterior translation of the femur during the first 60 degrees of flexion. Posterior substituting total knee arthroplasty does not appear to restore normal passive varus/valgus rotations or the screw motion and introduces an abnormal anterior translation of the femur during intraoperative evaluation.

  View details for DOI 10.1002/jor.20163

  View details for PubMedID 16770795

• The role of estimating muscle-tendon lengths and velocities of the hamstrings in the evaluation and treatment of crouch gait GAIT & POSTURE Arnold, A. S., Liu, M. Q., Schwartz, M. H., Ounpuu, S., Delp, S. L. 2006; 23 (3): 273-281

  Abstract

  Persons with cerebral palsy frequently walk with excessive knee flexion during terminal swing and stance. This gait abnormality is often attributed to "short" or "spastic" hamstrings that restrict knee extension, and is often treated by hamstrings lengthening surgery. At present, the outcomes of these procedures are inconsistent. This study examined whether analyses of the muscle-tendon lengths and lengthening velocities of patients' hamstrings during walking may be helpful when deciding whether a candidate is likely to benefit from hamstrings surgery. One hundred and fifty-two subjects were cross-classified in a series of multi-way contingency tables based on their pre- and postoperative gait kinematics, muscle-tendon lengths, muscle-tendon velocities, and hamstrings surgeries. The lengths and velocities of the subjects' semimembranosus muscles were estimated by combining kinematic data from gait analysis with a three-dimensional computer model of the lower extremity. Log-linear analysis revealed that the subjects who walked with abnormally "short" or "slow" hamstrings preoperatively, and whose hamstrings did not operate at longer lengths or faster velocities postoperatively, were unlikely to walk with improved knee extension after treatment (p < 0.05). Subjects who did not walk with abnormally short or slow hamstrings preoperatively, and whose hamstrings did operate at longer lengths or faster velocities postoperatively, tended to exhibit unimproved or worsened anterior pelvic tilt after treatment (p < 0.05). Examination of the muscle-tendon lengths and velocities allows individuals who walk with abnormally short or slow hamstrings to be distinguished from those who do not, and thus may help to identify patients who are at risk for unsatisfactory postsurgical changes in knee extension or anterior pelvic tilt.

  View details for DOI 10.1016/j.gaitpost.2005.03.003

  View details for Web of Science ID 000236337700003

  View details for PubMedID 15964759

• Muscle contributions to support during gait in an individual with post-stroke hemiparesis JOURNAL OF BIOMECHANICS Higginson, J. S., Zajac, F. E., Neptune, R. R., Kautz, S. A., Delp, S. L. 2006; 39 (10): 1769-1777

  Abstract

  Walking requires coordination of muscles to support the body during single stance. Impaired ability to coordinate muscles following stroke frequently compromises walking performance and results in extremely low walking speeds. Slow gait in post-stroke hemiparesis is further complicated by asymmetries in lower limb muscle excitations. The objectives of the current study were: (1) to compare the muscle coordination patterns of an individual with flexed stance limb posture secondary to post-stroke hemiparesis with that of healthy adults walking very slowly, and (2) to identify how paretic and non-paretic muscles provide support of the body center of mass in this individual. Simulations were generated based on the kinematics and kinetics of a stroke survivor walking at his self-selected speed (0.3 m/s) and of three speed-matched, healthy older individuals. For each simulation, muscle forces were perturbed to determine the muscles contributing most to body weight support (i.e., height of the center of mass during midstance). Differences in muscle excitations and midstance body configuration caused paretic and non-paretic ankle plantarflexors to contribute less to midstance support than in healthy slow gait. Excitation of paretic ankle dorsiflexors and knee flexors during stance opposed support and necessitated compensation by knee and hip extensors. During gait for an individual with post-stroke hemiparesis, adequate body weight support is provided via reorganized muscle coordination patterns of the paretic and non-paretic lower limbs relative to healthy slow gait.

  View details for DOI 10.1016/j.jbiomech.2005.05.032

  View details for Web of Science ID 000239417900002

  View details for PubMedID 16046223

• Physics-based simulation of biological sturctures 3rd IEEE International Symposium on Biomedical Imaging Delp, S. L., Anderson, F. C., Altman, R. B. IEEE. 2006: 802–803

  View details for Web of Science ID 000244446000202

• Kinematic and kinetic factors that correlate with improved knee flexion following treatment for stiff-knee gait JOURNAL OF BIOMECHANICS Goldberg, S. R., Ounpuu, S., Arnold, A. S., Gage, J. R., Delp, S. L. 2006; 39 (4): 689-698

  Abstract

  Stiff-knee gait is a movement abnormality in which knee flexion during swing phase is significantly diminished. This study investigates the relationships between knee flexion velocity at toe-off, joint moments during swing phase and double support, and improvements in stiff-knee gait following rectus femoris transfer surgery in subjects with cerebral palsy. Forty subjects who underwent a rectus femoris transfer were categorized as "stiff" or "not-stiff" preoperatively based on kinematic measures of knee motion during walking. Subjects classified as stiff were further categorized as having "good" or "poor" outcomes based on whether their swing-phase knee flexion improved substantially after surgery. We hypothesized that subjects with stiff-knee gait would exhibit abnormal joint moments in swing phase and/or diminished knee flexion velocity at toe-off, and that subjects with diminished knee flexion velocity at toe-off would exhibit abnormal joint moments during double support. We further hypothesized that subjects classified as having a good outcome would exhibit postoperative improvements in these factors. Subjects classified as stiff tended to exhibit abnormally low knee flexion velocities at toe-off (p<0.001) and excessive knee extension moments during double support (p=0.001). Subjects in the good outcome group on average showed substantial improvement in these factors postoperatively. All eight subjects in this group walked with normal knee flexion velocity at toe-off postoperatively and only two walked with excessive knee extension moments in double support. By contrast, all 10 of the poor outcome subjects walked with low knee flexion velocity at toe-off postoperatively and seven walked with excessive knee extension moments in double support. Our analyses suggest that improvements in stiff-knee gait are associated with sufficient increases in knee flexion velocity at toe-off and corresponding decreases in excessive knee extension moments during double support. Therefore, while stiff-knee gait manifests during the swing phase of the gait cycle, it may be caused by abnormal muscle activity during stance.

  View details for DOI 10.1016/j.jbiomech.2005.01.015

  View details for Web of Science ID 000235461300011

  View details for PubMedID 16439238

• Evaluation of a new algorithm to determine the hip joint center JOURNAL OF BIOMECHANICS Siston, R. A., Delp, S. L. 2006; 39 (1): 125-130

  Abstract

  Accurately locating the hip joint center is a challenging and important step in many biomechanical investigations. The purpose of this study was to test the accuracy and robustness of a "pivoting" algorithm used to locate the hip center. We tested the performance of this algorithm with data acquired by manipulating a ball and socket model of the hip through several motion patterns. The smallest mean errors of 2.2+/-0.2 mm occurred with a circumduction motion pattern, while the largest errors of 4.2+/-1.3 mm occurred with single-plane motion (e.g., flexion/extension). Introducing random noise with an amplitude of 30 mm increased the errors by only 1.3+/-0.5 mm with a circumduction motion pattern. The pivoting algorithm performs well in the laboratory, and further work is warranted to evaluate its performance in a clinical setting.

  View details for DOI 10.1016/j.jbiomech.2004.10.032

  View details for Web of Science ID 000233702500014

  View details for PubMedID 16271596

• Do the hamstrings operate at increased muscle-tendon lengths and velocities after surgical lengthening? JOURNAL OF BIOMECHANICS Arnold, A. S., Liu, M. Q., Schwartz, M. H., Ounpuu, S., Dias, L. S., Delp, S. L. 2006; 39 (8): 1498-1506

  Abstract

  Children with crouch gait frequently walk with improved knee extension during the terminal swing and stance phases following hamstrings lengthening surgery; however, the mechanisms responsible for these improvements are unclear. This study tested the hypothesis that surgical lengthening enables the hamstrings of persons with cerebral palsy to operate at longer muscle-tendon lengths or lengthen at faster muscle-tendon velocities during walking. Sixty-nine subjects who had improved knee extension after surgery were retrospectively examined. The muscle-tendon lengths and velocities of the subjects' semimembranosus muscles were estimated by combining kinematic data from gait analysis with a three-dimensional computer model of the lower extremity. Log-linear analyses confirmed that the subjects who walked with abnormally short muscle-tendon lengths and/or slow muscle-tendon velocities preoperatively tended to walk with longer lengths (21 of 29 subjects, p<0.01) or faster velocities (30 of 40 subjects, p<0.01) postoperatively. In these cases, surgical lengthening may have slackened the subjects' tight hamstrings and/or diminished the hamstrings' spastic response to stretch. Other subjects walked with muscle-tendon lengths and velocities that were neither shorter nor slower than normal preoperatively (22 of 69 subjects), and the semimembranosus muscles of most of these subjects did not operate at increased lengths or velocities after surgery; in these cases, the subjects' postsurgical improvements in knee extension may have been unrelated to the hamstrings surgery. Analyses of muscle-tendon lengths and velocities may help to distinguish individuals who have "short" or "spastic" hamstrings from those who do not, and thus may augment conventional methods used to describe patients' neuromusculoskeletal impairments and gait abnormalities.

  View details for DOI 10.1016/j.jbiomech.2005.03.026

  View details for Web of Science ID 000238104600016

  View details for PubMedID 15923009

• Optimal control simulations reveal mechanisms by which arm movement improves standing long jump performance JOURNAL OF BIOMECHANICS Ashby, B. M., Delp, S. L. 2006; 39 (9): 1726-1734

  Abstract

  Optimal control simulations of the standing long jump were developed to gain insight into the mechanisms of enhanced performance due to arm motion. The activations that maximize standing long jump distance of a joint torque actuated model were determined for jumps with free and restricted arm movement. The simulated jump distance was 40 cm greater when arm movement was free (2.00 m) than when it was restricted (1.60 m). The majority of the performance improvement in the free arm jump was due to the 15% increase (3.30 vs. 2.86 m/s) in the take-off velocity of the center of gravity. Some of the performance improvement in the free arm jump was attributable to the ability of the jumper to swing the arms backwards during the flight phase to alleviate excessive forward rotation and position the body segments properly for landing. In restricted arm jumps, the excessive forward rotation was avoided by "holding back" during the propulsive phase and reducing the activation levels of the ankle, knee, and hip joint torque actuators. In addition, swinging the arm segments allowed the lower body joint torque actuators to perform 26 J more work in the free arm jump. However, the most significant contribution to developing greater take-off velocity came from the additional 80 J work done by the shoulder actuator in the jump with free arm movement.

  View details for DOI 10.1016/j.jbiomech.2005.04.017

  View details for Web of Science ID 000238777500017

  View details for PubMedID 15992805

• Muscles that support the body also modulate forward progression during walking JOURNAL OF BIOMECHANICS Liu, M. Q., Anderson, F. C., Pandy, M. G., Delp, S. L. 2006; 39 (14): 2623-2630

  Abstract

  The purpose of this study was to characterize the contributions of individual muscles to forward progression and vertical support during walking. We systematically perturbed the forces in 54 muscles during a three-dimensional simulation of walking, and computed the changes in fore-aft and vertical accelerations of the body mass center due to the altered muscle forces during the stance phase. Our results indicate that muscles that provided most of the vertical acceleration (i.e., support) also decreased the forward speed of the mass center during the first half of stance (vasti and gluteus maximus). Similarly, muscles that supported the body also propelled it forward during the second half of stance (soleus and gastrocnemius). The gluteus medius was important for generating both forward progression and support, especially during single-limb stance. These findings suggest that a relatively small group of muscles provides most of the forward progression and support needed for normal walking. The results also suggest that walking dynamics are influenced by non-sagittal muscles, such as the gluteus medius, even though walking is primarily a sagittal-plane task.

  View details for DOI 10.1016/j.jbiomech.2005.08.017

  View details for Web of Science ID 000241850100009

  View details for PubMedID 16216251

• Effect of equinus foot placement and intrinsic muscle response on knee extension during stance GAIT & POSTURE Higginson, J. S., Zajac, F. E., Neptune, R. R., Kautz, S. A., Burgar, C. G., Delp, S. L. 2006; 23 (1): 32-36

  Abstract

  Equinus gait, a common movement abnormality among individuals with stroke and cerebral palsy, is often associated with knee hyperextension during stance. Whether there exists a causal mechanism linking equinus foot placement with knee hyperextension remains unknown. To investigate the response of the musculoskeletal system to equinus foot placement, a forward dynamic simulation of normal walking was perturbed by augmenting ankle plantarflexion by 10 degrees at initial contact. The subsequent effect on knee extension was assessed when the muscle forces were allowed, or not allowed, to change in response to altered kinematics and intrinsic force-length-velocity properties. We found that an increase in ankle plantarflexion at initial contact without concomitant changes in muscle forces caused the knee to hyperextend. The intrinsic force-length-velocity properties of muscle, particularly in gastrocnemius and vastus, diminished the effect of equinus posture alone, causing the abnormal knee extension to be less pronounced. We conclude that the effect of ankle position at initial contact on knee motion should be considered in the analysis of equinus gait.

  View details for DOI 10.1016/j.gaitpost.2004.11.011

  View details for Web of Science ID 000234166500005

  View details for PubMedID 16311192

• Rectus femoris and vastus intermedius fiber excursions predicted by three-dimensional muscle models JOURNAL OF BIOMECHANICS Blemker, S. S., Delp, S. L. 2006; 39 (8): 1383-1391

  Abstract

  Computer models of the musculoskeletal system frequently represent the force-length behavior of muscle with a lumped-parameter model. Lumped-parameter models use simple geometric shapes to characterize the arrangement of muscle fibers and tendon; this may inaccurately represent changes in fiber length and the resulting force-length behavior, especially for muscles with complex architecture. The purpose of this study was to determine the extent to which the complex features of the rectus femoris and vastus intermedius architectures affect the fiber changes in length ("fiber excursions"). We created three-dimensional finite-element models of the rectus femoris and vastus intermedius muscles based on magnetic resonance (MR) images, and compared the fiber excursions predicted by the finite-element models with fiber excursions predicted by lumped-parameter models of these muscles. The finite-element models predicted rectus femoris fiber excursions (over a 100 degrees range of knee flexion) that varied from 55% to 70% of the excursion of the muscle-tendon unit and vastus intermedius fiber excursions that varied from 55% to 98% of the excursion muscle-tendon unit. In contrast, the lumped-parameter model of the rectus femoris predicted fiber excursions that were 86% of the excursion of the muscle-tendon unit and vastus intermedius fiber excursions that were 97% of the excursion of the muscle-tendon unit. These results suggest that fiber excursions of many fibers are overestimated in lumped-parameter models of these muscles. These new representations of muscle architecture can improve the accuracy of computer simulations of movement and provide insight into muscle design.

  View details for DOI 10.1016/j.jbiomech.2005.04.012

  View details for Web of Science ID 000238104600003

  View details for PubMedID 15972213

• Three-dimensional representation of complex muscle architectures and geometries (vol 33, pg 661, 2005) ANNALS OF BIOMEDICAL ENGINEERING Blemker, S. S., Delp, S. L. 2005; 33 (8): 1134-1134

  View details for DOI 10.1007/s10439-005-7385-0

  View details for Web of Science ID 000231245600017

• Muscular contributions to hip and knee extension during the single limb stance phase of normal gait: a framework for investigating the causes of crouch gait JOURNAL OF BIOMECHANICS Arnold, A. S., Anderson, F. C., Pandy, M. G., Delp, S. L. 2005; 38 (11): 2181-2189

  Abstract

  Crouch gait, a troublesome movement abnormality among persons with cerebral palsy, is characterized by excessive flexion of the hips and knees during stance. Treatment of crouch gait is challenging, at present, because the factors that contribute to hip and knee extension during normal gait are not well understood, and because the potential of individual muscles to produce flexion or extension of the joints during stance is unknown. This study analyzed a three-dimensional, muscle-actuated dynamic simulation of walking to quantify the angular accelerations of the hip and knee induced by muscles during normal gait, and to rank the potential of the muscles to alter motions of these joints. Examination of the muscle actions during single limb stance showed that the gluteus maximus, vasti, and soleus make substantial contributions to hip and knee extension during normal gait. Per unit force, the gluteus maximus had greater potential than the vasti to accelerate the knee toward extension. These data suggest that weak hip extensors, knee extensors, or ankle plantar flexors may contribute to crouch gait, and strengthening these muscles--particularly gluteus maximus--may improve hip and knee extension. Abnormal forces generated by the iliopsoas or adductors may also contribute to crouch gait, as our analysis showed that these muscles have the potential to accelerate the hip and knee toward flexion. This work emphasizes the need to consider how muscular forces contribute to multijoint movements when attempting to identify the causes of abnormal gait.

  View details for DOI 10.1016/j.jbiomech.2004.09.036

  View details for Web of Science ID 000232456400006

  View details for PubMedID 16154404

• A modeling framework to estimate patellofemoral joint cartilage stress in vivo MEDICINE AND SCIENCE IN SPORTS AND EXERCISE Besier, T. F., Gold, G. E., Beaupre, G. S., Delp, S. L. 2005; 37 (11): 1924-1930

  Abstract

  Patellofemoral (PF) pain is common among athletes and may be caused by increased subchondral bone stress as a result of increased stress in the cartilage of the femur or patella. This article presents a modeling pipeline to estimate in vivo cartilage stress in the PF joint.The modeling pipeline uses the finite element method to calculate stresses and strains in the PF joint cartilage. Model inputs include an accurate geometrical representation of the bones and cartilage from magnetic resonance imaging (MRI), cartilage material properties, and an estimate of muscle forces from an EMG-driven musculoskeletal model. Validation is performed using PF joint contact area and patellar orientation measured from upright, weight-bearing MRI. Preliminary data from an active, pain-free subject illustrate the modeling pipeline to calculate cartilage stress during a static squat.The quasistatic finite element simulation reproduced the orientation of the patella to within 2.1 mm and predicted the PF joint contact area to within 2.3%. Octahedral shear stresses were highest in the central, lateral aspect of the patella cartilage with a peak of 2.5 MPa. The corresponding stresses in the femoral cartilage reached only 2.0 MPa. However, peak hydrostatic pressures were higher within the femoral cartilage (3.5 MPa) than the patellar cartilage (2.3 MPa).The methods presented in this article offer a novel approach to calculate PF joint cartilage stress in vivo. Future efforts will use this modeling pipeline to further our knowledge of PF pain and potential rehabilitation strategies.

  View details for DOI 10.1249/01.mss.0000176686.18683.64

  View details for Web of Science ID 000233451000015

  View details for PubMedID 16286863

• Evaluation of methods that locate the center of the ankle for computer-assisted total knee arthroplasty CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Siston, R. A., Daub, A. C., Giori, N. J., Goodman, S. B., Delp, S. L. 2005: 129-135

  Abstract

  Accurate alignment of the mechanical axis of the limb is important to the success of a total knee arthroplasty. Although computer-assisted navigation systems can align implants more accurately than traditional mechanical guides, the ideal technique to determine the distal end point of the mechanical axis, the center of the ankle, is unknown. In this study, we evaluated the accuracy, precision, objectivity, and speed of five anatomic methods and two kinematic methods for estimating the ankle center in 11 healthy subjects. Magnetic resonance images were used to characterize the shape of the ankle and establish the true ankle center. The most accurate and precise anatomic method was establishing the midpoint of the most medial and most lateral aspects of the malleoli (4.5 +/- 4.1 mm lateral error; 2.7 +/- 4.5 mm posterior error). A biaxial model of the ankle (2.0 +/- 6.4 mm medial error; 0.3 +/- 7.6 mm anterior error) was the most accurate kinematic method. Establishing the midpoint of the most medial and most lateral aspects of the malleoli was an accurate, precise, objective, and fast method for establishing the center of the ankle.

  View details for DOI 10.1097/01.blo.0000170873.88306.56

  View details for PubMedID 16205151

• The variability of femoral rotational alignment in total knee arthroplasty JOURNAL OF BONE AND JOINT SURGERY-AMERICAN VOLUME Siston, R. A., Patel, J. J., Goodman, S. B., Delp, S. L., Giori, N. J. 2005; 87A (10): 2276-2280

  Abstract

  Several reference axes are used to establish femoral rotational alignment during total knee arthroplasty, but debate continues with regard to which axis is most accurately and easily identified during surgery. Computer-assisted navigation systems have been developed in an attempt to more accurately and consistently align implants during total knee arthroplasty, but it is unknown if navigation systems can improve the accuracy of femoral rotational alignment as compared with that achieved with more traditional techniques involving mechanical guides. The purposes of the present study were to characterize the variability associated with femoral rotational alignment techniques and to determine whether the use of a computer-assisted surgical navigation system reduced this variability.Eleven orthopaedic surgeons used five alignment techniques (including one computer-assisted technique and four traditional techniques) to establish femoral rotational alignment axes on ten cadaveric specimens, and the orientation of these axes was recorded with use of a navigation system. These derived axes were compared against a reference transepicondylar axis on each femur that was established after complete dissection of all soft tissues.There was no difference between the mean errors of all five techniques (p > 0.11). Only 17% of the knees were rotated <5 degrees from the reference transepicondylar axis, with alignment errors ranging from 13 degrees of internal rotation to 16 degrees of external rotation. There were significant differences among the surgeons with regard to their ability to accurately establish femoral rotational alignment axes (p < 0.001).All techniques resulted in highly variable rotational alignment, with no technique being superior. This variability was primarily due to the particular surgeon who was performing the alignment procedure. A navigation system that relies on directly digitizing the femoral epicondyles to establish an alignment axis did not provide a more reliable means of establishing femoral rotational alignment than traditional techniques did.

  View details for DOI 10.2106/JBJS.D.02945

  View details for Web of Science ID 000232421500018

• Analysis of hindlimb muscle moment arms in Tyrannosaurus rex using a three-dimensional musculoskeletal computer model: implications for stance, gait, and speed PALEOBIOLOGY Hutchinson, J. R., Anderson, F. C., Blemker, S. S., Delp, S. L. 2005; 31 (4): 676-701

  View details for Web of Science ID 000232524500009

• Simulating the task-level control of human motion: a methodology and framework for implementation VISUAL COMPUTER De Sapio, V., Warren, J., Khatib, O., Delp, S. 2005; 21 (5): 289-302

  View details for DOI 10.1007/s00371-005-0284-4

  View details for Web of Science ID 000229935100002

• A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control ANNALS OF BIOMEDICAL ENGINEERING Holzbaur, K. R., Murray, W. M., Delp, S. L. 2005; 33 (6): 829-840

  Abstract

  Biomechanical models of the musculoskeletal system are frequently used to study neuromuscular control and simulate surgical procedures. To be broadly applicable, a model must be accessible to users, provide accurate representations of muscles and joints, and capture important interactions between joints. We have developed a model of the upper extremity that includes 15 degrees of freedom representing the shoulder, elbow, forearm, wrist, thumb, and index finger, and 50 muscle compartments crossing these joints. The kinematics of each joint and the force-generating parameters for each muscle were derived from experimental data. The model estimates the muscle-tendon lengths and moment arms for each of the muscles over a wide range of postures. Given a pattern of muscle activations, the model also estimates muscle forces and joint moments. The moment arms and maximum moment-generating capacity of each muscle group (e.g., elbow flexors) were compared to experimental data to assess the accuracy of the model. These comparisons showed that moment arms and joint moments estimated using the model captured important features of upper extremity geometry and mechanics. The model also revealed coupling between joints, such as increased passive finger flexion moment with wrist extension. The computer model is available to researchers at http://nmbl.stanford.edu.

  View details for DOI 10.1007/s10439-005-3320-7

  View details for Web of Science ID 000230075100011

  View details for PubMedID 16078622

• Three-dimensional representation of complex muscle architectures and geometries ANNALS OF BIOMEDICAL ENGINEERING Blemker, S. S., Delp, S. L. 2005; 33 (5): 661-673

  Abstract

  Almost all computer models of the musculoskeletal system represent muscle geometry using a series of line segments. This simplification (i) limits the ability of models to accurately represent the paths of muscles with complex geometry and (ii) assumes that moment arms are equivalent for all fibers within a muscle (or muscle compartment). The goal of this work was to develop and evaluate a new method for creating three-dimensional (3D) finite-element models that represent complex muscle geometry and the variation in moment arms across fibers within a muscle. We created 3D models of the psoas, iliacus, gluteus maximus, and gluteus medius muscles from magnetic resonance (MR) images. Peak fiber moment arms varied substantially among fibers within each muscle (e.g., for the psoas the peak fiber hip flexion moment arms varied from 2 to 3 cm, and for the gluteus maximus the peak fiber hip extension moment arms varied from 1 to 7 cm). Moment arms from the literature were generally within the range of fiber moment arms predicted by the 3D models. The models accurately predicted changes in muscle surface geometry over a 55 degrees range of hip flexion, as compared to changes in shape predicted from MR images (average errors between the model and measured surfaces were between 1.7 and 5.2 mm). This new framework for representing muscle will enhance the accuracy of computer models of the musculoskeletal system.

  View details for DOI 10.1007/s10439-005-1433-7

  View details for Web of Science ID 000229740900010

  View details for PubMedID 15981866

• A 3D model of muscle reveals the causes of nonuniform strains in the biceps brachii JOURNAL OF BIOMECHANICS Blemker, S. S., PINSKY, P. M., Delp, S. L. 2005; 38 (4): 657-665

  Abstract

  Biomechanical models generally assume that muscle fascicles shorten uniformly. However, dynamic magnetic resonance (MR) images of the biceps brachii have recently shown nonuniform shortening along some muscle fascicles during low-load elbow flexion (J. Appl. Physiol. 92 (2002) 2381). The purpose of this study was to uncover the features of the biceps brachii architecture and material properties that could lead to nonuniform shortening. We created a three-dimensional finite-element model of the biceps brachii and compared the tissue strains predicted by the model with experimentally measured tissue strains. The finite-element model predicted strains that were within one standard deviation of the experimentally measured strains. Analysis of the model revealed that the variation in fascicle lengths within the muscle and the curvature of the fascicles were the primary factors contributing to nonuniform strains. Continuum representations of muscle, combined with in vivo image data, are needed to deepen our understanding of how complex geometric arrangements of muscle fibers affect muscle contraction mechanics.

  View details for DOI 10.1016/j.jbiomech.2004.04.009

  View details for Web of Science ID 000227590400002

  View details for PubMedID 15713285

• Patellofemoral joint contact area increases with knee flexion and weight-bearing JOURNAL OF ORTHOPAEDIC RESEARCH Besier, T. F., Draper, C. E., Gold, G. E., Beaupre, G. S., Delp, S. L. 2005; 23 (2): 345-350

  Abstract

  Patellofemoral pain is a common and debilitating disorder. Elevated cartilage stress of the patellofemoral joint is hypothesized to play a role in the onset of pain. Estimating cartilage stress requires accurate measurements of contact area. The purpose of this study was to estimate patellofemoral joint contact areas in a group of healthy, pain-free subjects during upright, weight-bearing conditions. Sixteen subjects (8 female, 8 male) were scanned in a GE Signa SP open configuration MRI scanner, which allowed subjects to stand or squat while reclining 25 degrees from vertical with the knee positioned at 0 degrees , 30 degrees , or 60 degrees of flexion. A custom-built backrest enabled subjects to be scanned without motion artifact in both weight-bearing (0.45 body weight per leg) and reduced loading conditions ('unloaded' at 0.15 body weight) at each knee flexion posture. Male subjects displayed mean unloaded patellofemoral joint contact areas of 210, 414, and 520 mm(2) at 0 degrees , 30 degrees and 60 degrees of knee flexion, respectively. Female subjects' unloaded contact areas were similar at full extension (0 degrees ), but significantly smaller at 30 degrees and 60 degrees (p<0.01), with mean values of 269 and 396 mm(2), respectively. When normalized by patellar dimensions (heightxwidth), contact areas were not different between genders. Under weight-bearing conditions, contact areas increased by an average of 24% (p<0.05). This study highlights the differences in patellofemoral joint contact area between gender, knee flexion postures, and physiologic loading conditions.

  View details for DOI 10.1016/j.orthres.2004.08.003

  View details for Web of Science ID 000227567100017

  View details for PubMedID 15734247

• Computer modeling of gait abnormalities in cerebral palsy: application to treatment planning THEORETICAL ISSUES IN ERGONOMICS SCIENCE Arnold, A. S., Delp, S. L. 2005; 6 (3-4): 305–12

  View details for DOI 10.1080/14639220412331329636

  View details for Web of Science ID 000212886800013

• Weight-bearing MRI of patellofemoral joint cartilage contact area JOURNAL OF MAGNETIC RESONANCE IMAGING Gold, G. E., Besier, T. F., Draper, C. E., Asakawa, D. S., Delp, S. L., Beaupre, G. S. 2004; 20 (3): 526-530

  Abstract

  To measure contact area of cartilage in the patellofemoral joint during weight bearing using an open MRI scanner.We developed an MR-compatible back support that allows three-dimensional imaging of the patellofemoral cartilage under physiologic weight-bearing conditions with negligible motion artifact in an open MRI scanner. To measure contact areas, we trained observers using a phantom of known area and tested intra- and interobserver variability. We measured in vivo contact areas between the patella and femoral cartilage with the knee in 30 degrees of flexion, loaded and unloaded, in six volunteers.We were able to measure the contact area of the patellofemoral cartilage with small interobserver (CV 7.0%) and intraobserver (CV 3.0%) variation. At 30 degrees of knee flexion, mean contact area increased from 400 mm2 (unloaded) to 522 mm2(loaded to 0.45 times body weight per leg).Using an open magnet and specially designed apparatus, it is possible to image the patellar cartilage during physiologic loading. Knowledge of patellar cartilage contact area is needed to assess patellofemoral stress, which may be increased in patients with patellofemoral pain syndrome.

  View details for DOI 10.1002/jmri.20146

  View details for Web of Science ID 000223522200024

  View details for PubMedID 15332263

• Muscles that influence knee flexion velocity in double support: implications for stiff-knee gait JOURNAL OF BIOMECHANICS Goldberg, S. R., Anderson, F. C., Pandy, M. G., Delp, S. L. 2004; 37 (8): 1189-1196

  Abstract

  Adequate knee flexion velocity at toe-off is important for achieving normal swing-phase knee flexion during gait. Consequently, insufficient knee flexion velocity at toe-off can contribute to stiff-knee gait, a movement abnormality in which swing-phase knee flexion is diminished. This work aims to identify the muscles that contribute to knee flexion velocity during double support in normal gait and the muscles that have the most potential to alter this velocity. This objective was achieved by perturbing the forces generated by individual muscles during double support in a forward dynamic simulation of normal gait and observing the effects of the perturbations on peak knee flexion velocity. Iliopsoas and gastrocnemius were identified as the muscles that contribute most to increasing knee flexion velocity during double support. Increased forces in vasti, rectus femoris, and soleus were found to decrease knee flexion velocity. Vasti, rectus femoris, gastrocnemius, and iliopsoas were all found to have large potentials to influence peak knee flexion velocity during double support. The results of this work indicate which muscles likely contribute to the diminished knee flexion velocity at toe-off observed in stiff-knee gait, and identify the treatment strategies that have the most potential to increase this velocity in persons with stiff-knee gait.

  View details for DOI 10.1016/j.jbiomech.2003.12.005

  View details for Web of Science ID 000222713100008

  View details for PubMedID 15212924

• Contributions of muscle forces and toe-off kinematics to peak knee flexion during the swing phase of normal gait: an induced position analysis JOURNAL OF BIOMECHANICS Anderson, F. C., Goldberg, S. R., Pandy, M. G., Delp, S. L. 2004; 37 (5): 731-737

  Abstract

  A three-dimensional dynamic simulation of walking was used together with induced position analysis to determine how kinematic conditions at toe-off and muscle forces following toe-off affect peak knee flexion during the swing phase of normal gait. The flexion velocity of the swing-limb knee at toe-off contributed 30 degrees to the peak knee flexion angle; this was larger than any contribution from an individual muscle or joint moment. Swing-limb muscles individually made large contributions to knee angle (i.e., as large as 22 degrees), but their actions tended to balance one another, so that the combined contribution from all swing-limb muscles was small (i.e., less than 3 degrees of flexion). The uniarticular muscles of the swing limb made contributions to knee flexion that were an order of magnitude larger than the biarticular muscles of the swing limb. The results of the induced position analysis make clear the importance of knee flexion velocity at toe-off relative to the effects of muscle forces exerted after toe-off in generating peak knee flexion angle. In addition to improving our understanding of normal gait, this study provides a basis for analyzing stiff-knee gait, a movement abnormality in which knee flexion in swing is diminished.

  View details for DOI 10.1016/j.jbiomech.2003.09.018

  View details for Web of Science ID 000220781600015

  View details for PubMedID 15047002

• Three-dimensional muscle-tendon geometry after rectus femoris tendon transfer JOURNAL OF BONE AND JOINT SURGERY-AMERICAN VOLUME Asakawa, D. S., Blemker, S. S., Rab, G. T., Bagley, A., Delp, S. L. 2004; 86A (2): 348-354

  View details for Web of Science ID 000188605700019

• Magnetic resonance imaging findings after rectus femoris transfer surgery 3rd Special Scientific Session of the International-Skeletal-Society Gold, G. E., Asakawa, D. S., Blemker, S. S., Delp, S. L. SPRINGER. 2004: 34–40

  Abstract

  We describe the magnetic resonance (MR) imaging appearance of the knee flexor and extensor tendons after bilateral rectus femoris transfer and hamstring lengthening surgery in five patients (10 limbs) with cerebral palsy. Three-dimensional models of the path of the transferred tendon were constructed in all cases. MR images of the transferred and lengthened tendons were examined and compared with images from ten non-surgical subjects. The models showed that the path of the transferred rectus femoris tendon had a marked angular deviation near the transfer site in all cases. MR imaging demonstrated irregular areas of low signal intensity near the transferred rectus femoris and around the hamstrings in all subjects. Eight of the ten post-surgical limbs showed evidence of fluid near or around the transferred or lengthened tendons. This was not observed in the non-surgical subjects. Thus, MR imaging of patients with cerebral palsy after rectus femoris transfer and hamstring-lengthening surgery shows evidence of signal intensity and contour changes, even several years after surgery.

  View details for DOI 10.1007/s00256-003-0702-5

  View details for Web of Science ID 000187505400005

  View details for PubMedID 14605768

• Real-time imaging of skeletal muscle velocity 9th Annual Meeting of the ISMRM Asakawa, D. S., Nayak, K. S., Blemker, S. S., Delp, S. L., Pauly, J. M., Nishimura, D. G., Gold, G. E. JOHN WILEY & SONS INC. 2003: 734–39

  Abstract

  To test the feasibility of using real-time phase contrast (PC) magnetic resonance imaging (MRI) to track velocities (1-20 cm/second) of skeletal muscle motion.To do this we modified a fast real-time spiral PC pulse sequence to accommodate through-plane velocity encoding in the range of -20 to +20 cm/second. We successfully imaged motion of the biceps brachii and triceps brachii muscles during elbow flexion and extension in seven unimpaired adult subjects using real-time PC MRI.The velocity data demonstrate that the biceps brachii and the triceps brachii, antagonistic muscles, move in opposite directions during elbow flexion and extension with velocity values in the muscle tissue ranging from -10 to +10 cm/second.With further development, real-time PC MRI may provide a means to analyze muscle function in individuals with neurologic or movement disorders who cannot actively complete the repeated motions required for dynamic MRI techniques, such as cine PC MRI, that are more commonly used in musculoskeletal biomechanics applications.

  View details for DOI 10.1002/jmri.10422

  View details for Web of Science ID 000186844200013

  View details for PubMedID 14635159

• Cine phase-contrast magnetic resonance imaging as a tool for quantification of skeletal muscle motion. Seminars in musculoskeletal radiology Asakawa, D. S., Pappas, G. P., Blemker, S. S., Drace, J. E., Delp, S. L. 2003; 7 (4): 287-295

  Abstract

  In recent years, biomechanics researchers have increasingly used dynamic magnetic resonance imaging techniques, such as cine phase contrast (cine PC), to study muscle and bone motion in vivo. Magnetic resonance imaging provides a non-invasive tool to visualize the anatomy and measure musculoskeletal tissue velocities during joint motion. Current application of cine PC magnetic resonance imaging in biomechanics includes study of knee joint kinematics, tendon strain, and skeletal muscle displacement and shortening. This paper article reviews the use of cine PC magnetic resonance imaging for quantification of skeletal muscle motion. The imaging studies presented examine the relative motion of the knee flexor and extensor muscles after orthopedic surgery and examine the uniformity of shortening within the biceps brachii muscle. The current challenges and limitations of using cine PC magnetic resonance imaging in biomechanics research are addressed as well as opportunities for future studies of skeletal muscle motion using dynamic magnetic resonance imaging.

  View details for PubMedID 14735427

• Biomechanics of the Steindler flexorplasty surgery: A computer simulation study JOURNAL OF HAND SURGERY-AMERICAN VOLUME Saul, K. R., Murray, W. M., Hentz, V. R., Delp, S. L. 2003; 28A (6): 979-986

  Abstract

  Our goal was to investigate the capacity of a Steindler flexorplasty to restore elbow flexion to persons with C5-C6 brachial plexus palsy. In this procedure the origin of the flexor-pronator mass is moved proximally onto the humeral shaft. We examined how the choice of the proximal attachment site for the flexor-pronator mass affects elbow flexion restoration, especially considering possible side effects including limited wrist and forearm motion owing to passive restraint from stretched muscles.A computer model of the upper extremity was used to simulate the biomechanical consequences of various surgical alterations. Unimpaired, preoperative, and postoperative conditions were simulated. Seven possible transfer locations were used to investigate the effects of choice of transfer location.Each transfer site produced a large increase in elbow flexion strength. Transfer to more proximal attachment sites also produced large increases in passive resistance to wrist extension and forearm supination.To reduce detrimental side effects while achieving clinical goals our theoretical analysis suggests a transfer to the distal limit of the traditional transfer region.

  View details for DOI 10.1016/S0363-5023(03)00484-2

  View details for Web of Science ID 000186710500014

• The importance of swing-phase initial conditions in stiff-knee gait JOURNAL OF BIOMECHANICS Goldberg, S. R., Ounpuu, S., Delp, S. L. 2003; 36 (8): 1111-1116

  Abstract

  The diminished knee flexion associated with stiff-knee gait, a movement abnormality commonly observed in persons with cerebral palsy, is thought to be caused by an over-active rectus femoris muscle producing an excessive knee extension moment during the swing phase of gait. As a result, treatment for stiff-knee gait is aimed at altering swing-phase muscle function. Unfortunately, this treatment strategy does not consistently result in improved knee flexion. We believe this is because multiple factors contribute to stiff-knee gait. Specifically, we hypothesize that many individuals with stiff-knee gait exhibit diminished knee flexion not because they have an excessive knee extension moment during swing, but because they walk with insufficient knee flexion velocity at toe-off. We measured the knee flexion velocity at toe-off and computed the average knee extension moment from toe-off to peak flexion in 17 subjects (18 limbs) with stiff-knee gait and 15 subjects (15 limbs) without movement abnormalities. We used forward dynamic simulation to determine how adjusting each stiff-knee subject's knee flexion velocity at toe-off to normal levels would affect knee flexion during swing. We found that only one of the 18 stiff-knee limbs exhibited an average knee extension moment from toe-off to peak flexion that was larger than normal. However, 15 of the 18 limbs exhibited a knee flexion velocity at toe-off that was below normal. Simulating an increase in the knee flexion velocity at toe-off to normal levels resulted in a normal or greater than normal range of knee flexion for each of these limbs. These results suggest that the diminished knee flexion of many persons with stiff-knee gait may be caused by abnormally low knee flexion velocity at toe-off as opposed to excessive knee extension moments during swing.

  View details for DOI 10.1016/S0021-9290(03)00106-4

  View details for Web of Science ID 000184212900006

  View details for PubMedID 12831736

• Abnormal coupling of knee and hip moments during maximal exertions in persons with cerebral palsy Annual Meeting of the American-Society-of-Biomechanics Thelen, D. G., Riewald, S. A., Asakawa, D. S., Sanger, T. D., Delp, S. L. JOHN WILEY & SONS INC. 2003: 486–93

  Abstract

  The motions of lower-limb extension, adduction, and internal rotation are frequently coupled in persons with cerebral palsy (CP) and are commonly referred to as an extension synergy. However, the underlying joint moments that give rise to these coupled motions are not well understood. We hypothesized that maximal voluntary exertions in a direction of one component of a synergy (e.g., hip extension) would result in the concurrent presence of other components of the synergy in subjects with CP but not in control subjects. To test this hypothesis, we measured three-dimensional moments about the hip and knee as nine subjects with spastic CP and six control subjects performed maximal isometric exertions of the hip and knee flexors and extensors. During maximal hip extension exertions, control subjects simultaneously generated a knee flexion moment, whereas CP subjects generated a knee extension moment (P < 0.05) and a larger hip internal rotation moment than did controls (P < 0.05). During maximal knee extension exertions, control subjects generated a hip flexion moment, whereas CP subjects generated a hip extension moment (P < 0.05). The patterns of joint moments generated by CP subjects are consistent with an extension synergy and may underlie the coupled motion patterns of the lower extremity in such persons.

  View details for DOI 10.1002/mus.10357

  View details for Web of Science ID 000181946000012

  View details for PubMedID 12661051

• Generating dynamic simulations of movement using computed muscle control JOURNAL OF BIOMECHANICS Thelen, D. G., Anderson, F. C., Delp, S. L. 2003; 36 (3): 321-328

  Abstract

  Computation of muscle excitation patterns that produce coordinated movements of muscle-actuated dynamic models is an important and challenging problem. Using dynamic optimization to compute excitation patterns comes at a large computational cost, which has limited the use of muscle-actuated simulations. This paper introduces a new algorithm, which we call computed muscle control, that uses static optimization along with feedforward and feedback controls to drive the kinematic trajectory of a musculoskeletal model toward a set of desired kinematics. We illustrate the algorithm by computing a set of muscle excitations that drive a 30-muscle, 3-degree-of-freedom model of pedaling to track measured pedaling kinematics and forces. Only 10 min of computer time were required to compute muscle excitations that reproduced the measured pedaling dynamics, which is over two orders of magnitude faster than conventional dynamic optimization techniques. Simulated kinematics were within 1 degrees of experimental values, simulated pedal forces were within one standard deviation of measured pedal forces for nearly all of the crank cycle, and computed muscle excitations were similar in timing to measured electromyographic patterns. The speed and accuracy of this new algorithm improves the feasibility of using detailed musculoskeletal models to simulate and analyze movement.

  View details for DOI 10.1016/S0021-9290(02)00432-3

  View details for Web of Science ID 000181247600002

  View details for PubMedID 12594980

• What causes increased muscle stiffness in cerebral palsy? MUSCLE & NERVE Delp, S. L. 2003; 27 (2): 131-132

  View details for DOI 10.1002/10284

  View details for Web of Science ID 000180792600001

  View details for PubMedID 12548519

• Three-dimensional spatial tuning of neck muscle activation in humans EXPERIMENTAL BRAIN RESEARCH Vasavada, A. N., Peterson, B. W., Delp, S. L. 2002; 147 (4): 437-448

  Abstract

  The complex structure of the neck musculoskeletal system poses challenges to understanding central nervous system (CNS) control strategies. Examining muscle activation patterns in relation to musculoskeletal geometry and three-dimensional mechanics may reveal organizing principles. We analyzed the spatial tuning of neck muscle electromyographic (EMG) activity while subjects generated moments in three dimensions. EMG tuning curves were characterized by their orientation (mean direction) and focus (spread of activity). For the four muscles that were studied (sternocleidomastoid, splenius capitis, semispinalis capitis and trapezius), EMG tuning curves exhibited directional preference, with consistent orientation and focus among 12 subjects. However, the directional preference (orientation) of three of the four neck muscles did not correspond to the muscle's moment arm, indicating that maximizing a muscle's mechanical advantage is not the only factor in determining muscle activation. The focus of muscle tuning did not change with moment magnitude, demonstrating that co-contraction did not increase with load. Axial rotation was found to have a strong influence on neck muscle spatial tuning. The uniform results among subjects indicate that the CNS has consistent strategies for selecting neck muscle activations to generate moments in specific directions; however, these strategies depend on three-dimensional mechanics in a complex manner.

  View details for DOI 10.1007/s00221-002-1275-6

  View details for Web of Science ID 000179924800003

  View details for PubMedID 12444475

• APONEUROSIS LENGTH AND FASCICLE INSERTION ANGLES OF THE BICEPS BRACHII JOURNAL OF MECHANICS IN MEDICINE AND BIOLOGY Asakawa, D. S., Pappas, G. P., Drace, J. E., Delp, S. L. 2002; 2 (3-4): 449-455

  View details for DOI 10.1142/S0219519402000484

  View details for Web of Science ID 000208564400016

• In vivo motion of the rectus femoris muscle after tendon transfer surgery JOURNAL OF BIOMECHANICS Asakawa, D. S., Blemker, S. S., Gold, G. E., Delp, S. L. 2002; 35 (8): 1029-1037

  Abstract

  Rectus femoris transfer surgery is performed to convert the rectus femoris muscle from a knee extensor to a knee flexor. In this surgery, the distal tendon of the rectus femoris is detached from the patella and reattached to one of the knee flexor tendons. The outcomes of this procedure are variable, and it is not known if the surgery successfully converts the muscle to a knee flexor. We measured the motion of muscle tissue within the rectus femoris and vastus intermedius during knee extension in 10 unimpaired control subjects (10 limbs) and 6 subjects (10 limbs) after rectus femoris transfer using cine phase-contrast magnetic resonance imaging. Displacements of the vastus intermedius during knee extension were similar between control and tendon transfer subjects. In the control subjects, the rectus femoris muscle consistently moved in the direction of the knee extensors and displaced more than the vastus intermedius. The rectus femoris also moved in the direction of the knee extensors in the tendon transfer subjects; however, the transferred rectus femoris displaced less than the vastus intermedius. These results suggest that the rectus femoris is not converted to a knee flexor after its distal tendon is transferred to the posterior side of the knee, but its capacity for knee extension is diminished by the surgery.

  View details for Web of Science ID 000177318000003

  View details for PubMedID 12126662

• Nonuniform shortening in the biceps brachii during elbow flexion JOURNAL OF APPLIED PHYSIOLOGY Pappas, G. P., Asakawa, D. S., Delp, S. L., Zajac, F. E., Drace, J. E. 2002; 92 (6): 2381-2389

  Abstract

  This study tested the common assumption that skeletal muscle shortens uniformly in the direction of its fascicles during low-load contraction. Cine phase contrast magnetic resonance imaging was used to characterize shortening of the biceps brachii muscle in 12 subjects during repeated elbow flexion against 5 and 15% maximum voluntary contraction (MVC) loads. Mean shortening was relatively constant along the anterior boundary of the muscle and averaged 21% for both loading conditions. In contrast, mean shortening was nonuniform along the centerline of the muscle during active elbow flexion. Centerline shortening in the distal region of the biceps brachii (7.3% for 5% MVC and 3.7% for 15% MVC) was significantly less (P < 0.001) than shortening in the muscle midportion (26.3% for 5% MVC and 28.2% for 15% MVC). Nonuniform shortening along the centerline was likely due to the presence of an internal aponeurosis that spanned the distal third of the longitudinal axis of the biceps brachii. However, muscle shortening was also nonuniform proximal to the centerline aponeurosis. Because muscle fascicles follow the anterior contour and centerline of the biceps brachii, our results suggest that shortening is uniform along anterior muscle fascicles and nonuniform along centerline fascicles.

  View details for DOI 10.1152/japplphysiol.00843.2001

  View details for Web of Science ID 000175739200021

  View details for PubMedID 12015351

• Scaling of peak moment arms of elbow muscles with upper extremity bone dimensions JOURNAL OF BIOMECHANICS Murray, W. M., Buchanan, T. S., Delp, S. L. 2002; 35 (1): 19-26

  Abstract

  It is often assumed that moment arms scale with size and can be normalized by body segment lengths or limb circumferences. However, quantitative scaling relationships between moment arms and anthropometric dimensions are generally not available. We hypothesized that peak moment arms of the elbow flexor and extensor muscles scale with the shorter distance (D(s)) between the elbow flexion axis and a muscle's origin and insertion. To test this hypothesis, we estimated moment arms of six muscles that cross the elbow, digitized muscle attachment sites and bone surface geometry, and estimated the location of the elbow flexion axis in 10 upper extremity cadaveric specimens which ranged in size from a 5'0" female to a 6'4" male. D(s) accurately reflected the differences in peak moment arms across different muscles, explaining 93-99% of the variation in peaks between muscles in the same specimen. D(s) also explained between 55% and 88% of the interspecimen variation in peak moment arms for brachioradialis, biceps, and ECRL. Triceps peak moment arm was significantly correlated to the anterior-posterior dimension of the ulna measured at the olecranon (r(2)=0.61, p=0.008). Radius length provides a good measure of the interspecimen variation in peaks for brachioradialis, biceps, and ECRL. However, bone lengths were not significantly correlated to triceps moment arm or anterior-posterior bone dimensions. This work advances our understanding of the variability and scaling dimensions for elbow muscle moment arms across subjects of different sizes.

  View details for Web of Science ID 000173539000002

  View details for PubMedID 11747879

• Three-dimensional dynamic simulation of total knee replacement motion during a step-up task JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME Piazza, S. J., Delp, S. L. 2001; 123 (6): 599-606

  Abstract

  A three-dimensional dynamic model of the tibiofemoral and patellofemoral articulations was developed to predict the motions of knee implants during a step-up activity. Patterns of muscle activity, initial joint angles and velocities, and kinematics of the hip and tinkle were measured experimentally and used as inputs to the simulation. Prosthetic knee kinematics were determined by integration of dynamic equations of motion subject to forces generated by muscles, ligaments, and contact at both the tibiofemoral and patellofemoral articulations. The modeling of contacts between implants did not rely upon explicit constraint equations; thus, changes in the number of contact points were allowed without modification to the model formulation. The simulation reproduced experimentally measured flexion-extension angle of the knee (within one standard deviation), but translations at the tibiofemoral articulations were larger during the simulated step-up task than those reported for patients with total knee replacements.

  View details for Web of Science ID 000172872100011

  View details for PubMedID 11783731

• Three-dimensional isometric strength of neck muscles in humans SPINE Vasavada, A. N., Li, S. P., Delp, S. L. 2001; 26 (17): 1904-1909

  Abstract

  Three-dimensional moments were measured experimentally during maximum voluntary contractions of neck muscles in humans.To characterize the maximum moments with attention paid to subject size and gender, to calculate moments at different locations in the neck, and to quantify the relative magnitudes of extension, flexion, lateral bending, and axial rotation moments.Few studies of neck strength have measured moments in directions other than extension, and it is difficult to compare results among studies because moments often are resolved at different locations in the cervical spine. Further, it is not clear how subject size, gender, and neck geometry relate to variations in the moment-generating capacity of neck muscles.Maximum moments were measured in 11 men and 5 women with an average age of 31 years (range, 20-42 years). Anatomic landmarks were digitized to resolve moments at different locations in the cervical spine.When moments were resolved about axes through the midpoint of the line between the C7 spinous process and the sternal notch, the maximum moments were as follows: extension (men, 52 +/- 11 Nm; women, 21 +/- 12 Nm), flexion (men, 30 +/- 5 Nm; women, 15 +/- 4 Nm), lateral bending (men, 36 +/- 8 Nm; women, 16 +/- 8 Nm), and axial rotation (men 15 +/- 4; women, 6 +/- 3) Nm). The magnitudes of extension, flexion, and lateral bending moments decreased linearly with vertical distance from the lower cervical spine to the mastoid process.Moments in three dimensions were quantified with regard to subject size and location along the cervical spine. These data are needed to characterize neck strength for biomechanical analysis of normal and pathologic conditions.

  View details for Web of Science ID 000170914000015

  View details for PubMedID 11568704

• Rotational moment arms of the medial hamstrings and adductors vary with femoral geometry and limb position: implications for the treatment of internally rotated gait JOURNAL OF BIOMECHANICS Arnold, A. S., Delp, S. L. 2001; 34 (4): 437-447

  Abstract

  Persons with cerebral palsy frequently walk with a crouched, internally rotated gait. Spastic medial hamstrings or adductors are presumed to contribute to excessive hip internal rotation in some patients; however, the capacity of these muscles to produce internal rotation has not been adequately investigated. The purpose of this study was to determine the hip rotation moment arms of the medial hamstrings and adductors in persons with femoral anteversion deformities who walk with a crouched, internally rotated gait. A musculoskeletal model with a "deformable" femur was developed. This model was used, in conjunction with kinematic data obtained from gait analysis, to calculate the muscle moment arms for combinations of joint angles and anteversion deformities exhibited by 21 subjects with cerebral palsy and excessive hip internal rotation. We found that the semimembranosus, semitendinosus, and gracilis muscles in our model had negligible or external rotation moment arms when the hip was internally rotated or the knee was flexed -- the body positions assumed by the subjects during walking. When the femur was excessively anteverted, the rotational moment arms of the adductor brevis, adductor longus, pectineus, and proximal compartments of the adductor magnus in our model shifted toward external rotation. These results suggest that neither the medial hamstrings nor the adductors are likely to contribute substantially to excessive internal rotation of the hip and that other causes of internal rotation should be considered when planning treatments for these patients.

  View details for Web of Science ID 000168088700004

  View details for PubMedID 11266666

• Evaluation of a deformable musculoskeletal model for estimating muscle-tendon lengths during crouch gait ANNALS OF BIOMEDICAL ENGINEERING Arnold, A. S., Blemker, S. S., Delp, S. L. 2001; 29 (3): 263-274

  Abstract

  The hamstrings and psoas muscles are often lengthened surgically in an attempt to correct crouch gait in persons with cerebral palsy. The purpose of this study was to determine if, and under what conditions, medial hamstrings and psoas lengths estimated with a "deformable" musculoskeletal model accurately characterize the lengths of the muscles during walking in individuals with crouch gait. Computer models of four subjects with crouch gait were developed from magnetic resonance (MR) images. These models were used in conjunction with the subjects' measured gait kinematics to calculate the muscle-tendon lengths at the body positions corresponding to walking. The lengths calculated with the MR-based models were normalized and were compared to the lengths estimated using a deformable generic model. The deformable model was either left undeformed and unscaled, or was deformed or scaled to more closely approximate the femoral geometry or bone dimensions of each subject. In most cases, differences between the normalized lengths of the medial hamstrings computed with the deformable and MR-based models were less than 5 mm. Differences in the psoas lengths computed with the deformable and MR-based models were also small (<3 mm) when the deformable model was adjusted to represent the femoral geometry of each subject. This work demonstrates that a deformable musculoskeletal model, in combination with a few subject-specific parameters and simple normalization techniques, can provide rapid and accurate estimates of medial hamstrings and psoas lengths in persons with neuromuscular disorders.

  View details for Web of Science ID 000168012800009

  View details for PubMedID 11310788

• Architecture of the rectus abdominis, quadratus lumborum, and erector spinae JOURNAL OF BIOMECHANICS Delp, S. L., Suryanarayanan, S., Murray, W. M., Uhlir, J., Triolo, R. J. 2001; 34 (3): 371-375

  Abstract

  Quantitative descriptions of muscle architecture are needed to characterize the force-generating capabilities of muscles. This study reports the architecture of three major trunk muscles: the rectus abdominis, quadratus lumborum, and three columns of the erector spinae (spinalis thoracis, longissimus thoracis and iliocostalis lumborum). Musculotendon lengths, muscle lengths, fascicle lengths, sarcomere lengths, pennation angles, and muscle masses were measured in five cadavers. Optimal fascicle lengths (the fascicle length at which the muscle generates maximum force) and physiologic cross-sectional areas (the ratio of muscle volume to optimal fascicle length) were computed from these measurements. The rectus abdominis had the longest fascicles of the muscles studied, with a mean (S.D.) optimal fascicle length of 28.3 (4.2)cm. The three columns of the erector spinae had mean optimal fascicle lengths that ranged from 6.4 (0.6)cm in the spinalis thoracis to 14.2 (2.1)cm in the iliocostalis lumborum. The proximal portion of the quadratus lumborum had a mean optimal fascicle length of 8.5 (1.5)cm and the distal segment of this muscle had a mean optimal fascicle length of 5.6 (0.9)cm. The physiologic cross-sectional area of the rectus abdominis was 2.6 (0.9)cm(2), the combined physiologic cross-sectional area of the erector spinae was 11.6 (1.8)cm(2), and the physiologic cross-sectional area of the quadratus lumborum was 2.8 (0.5)cm(2). These data provide the basis for estimation of the force-generating potential of these muscles.

  View details for Web of Science ID 000167482500011

  View details for PubMedID 11182129

• A computational framework for simulating and analyzing human and animal movement COMPUTING IN SCIENCE & ENGINEERING Delp, S. L., Loan, J. P. 2000; 2 (5): 46-55

  View details for Web of Science ID 000089283900008

• The isometric functional capacity of muscles that cross the elbow JOURNAL OF BIOMECHANICS Murray, W. M., Buchanan, T. S., Delp, S. L. 2000; 33 (8): 943-952

  Abstract

  We hypothesized that muscles crossing the elbow have fundamental differences in their capacity for excursion, force generation, and moment generation due to differences in their architecture, moment arm, and the combination of their architecture and moment arm. Muscle fascicle length, sarcomere length, pennation angle, mass, and tendon displacement with elbow flexion were measured for the major elbow muscles in 10 upper extremity specimens. Optimal fascicle length, physiological cross-sectional area (PCSA), moment arm, operating range on the force-length curve, and moment-generating capacity were estimated from these data. Brachioradialis and pronator teres had the longest (17.7cm) and shortest (5.5cm) fascicles, respectively. Triceps brachii (combined heads) and brachioradialis had the greatest (14.9cm(2)) and smallest (1.2cm(2)) PCSAs, respectively. Despite a comparable fascicle length, long head of biceps brachii operates over a broader range of the force-length curve (length change=56% of optimal length, 12.8cm) than the long head of triceps brachii (length change=28% of optimal length, 12. 7cm) because of its larger moment arm (4.7cm vs. 2.3cm). Although brachioradialis has a small PCSA, it has a relatively large moment-generating capacity (6.8cm(3)) due to its large moment arm (average peak=7.7cm). These results emphasize the need to consider the interplay of architecture and moment arm when evaluating the functional capabilities of a muscle.

  View details for Web of Science ID 000087474600005

  View details for PubMedID 10828324

• The use of basis functions in modelling joint articular surfaces: application to the knee joint JOURNAL OF BIOMECHANICS Dhaher, Y. Y., Delp, S. L., Rymer, W. Z. 2000; 33 (7): 901-907

  Abstract

  This article introduces a new method to represent bone surface geometry for simulations of joint contact. The method uses the inner product of two basis functions to provide a mathematical representation of the joint surfaces. This method guarantees a continuous transition in the direction of the surface normals, an important property for computation of joint contact. Our formulation handles experimental data that are not evenly distributed, a common characteristic of digitized data of musculoskeletal morphologies. The method makes it possible to represent highly curved surfaces, which are encountered in many anatomical structures. The accuracy of this method is demonstrated by modeling the human knee joint. The mean relative percentage error in the representation of the patellar track surface was 0.25% (range 0-1.56%) which corresponded to an absolute error of 0.17mm (range 0-0.16mm).

  View details for Web of Science ID 000087392900014

  View details for PubMedID 10831766

• Do the hamstrings and adductors contribute to excessive internal rotation of the hip in persons with cerebral palsy? GAIT & POSTURE Arnold, A. S., Asakawa, D. J., Delp, S. L. 2000; 11 (3): 181-190

  Abstract

  Children with cerebral palsy frequently walk with excessive internal rotation of the hip. Spastic medial hamstrings or adductors are presumed to contribute to the excessive internal rotation in some patients; however, the capacity of these muscles to produce internal rotation during walking in individuals with cerebral palsy has not been adequately investigated. The purpose of this study was to determine the hip rotation moment arms of the medial hamstrings and adductors in persons who walk with a crouched, internally-rotated gait. Highly accurate computer models of three subjects with cerebral palsy were created from magnetic resonance images. These subject-specific models were used in conjunction with joint kinematics obtained from gait analysis to calculate the rotational moment arms of the muscles at body positions corresponding to each subject's internally-rotated gait. Analysis of the models revealed that the medial hamstrings, adductor brevis, and gracilis had negligible or external rotation moment arms throughout the gait cycle in all three subjects. The adductor longus had an internal rotation moment arm in two of the subjects, but the moment arm was small (<4 mm) in each case. These findings indicate that neither the medial hamstrings nor the adductor brevis, adductor longus, or gracilis are likely to be important contributors to excessive internal rotation of the hip. This suggests that these muscles should not be lengthened to treat excessive internal rotation of the hip and that other factors are more likely to cause internally-rotated gait in these patients.

  View details for Web of Science ID 000087261900002

  View details for PubMedID 10802430

• Accuracy of muscle moment arms estimated from MRI-based musculoskeletal models of the lower extremity. Computer aided surgery Arnold, A. S., SALINAS, S., Asakawa, D. J., Delp, S. L. 2000; 5 (2): 108-119

  Abstract

  Biomechanical models that compute the lengths and moment arms of soft tissues are broadly applicable to the treatment of movement abnormalities and the planning of orthopaedic surgical procedures. The goals of this study were to: (i) develop methods to construct subject-specific biomechanical models from magnetic resonance (MR) images, (ii) create models of three lower-extremity cadaveric specimens, and (iii) quantify the accuracy of muscle-tendon lengths and moment arms estimated using these models.Models describing the paths of the medial hamstrings and psoas muscles for a wide range of body positions were developed from MR images in one joint configuration by defining kinematic models of the hip and knee, and by specifying "wrapping surfaces" that simulate interactions between the muscles and underlying structures. Our methods for constructing these models were evaluated by comparing hip and knee flexion moment arms estimated from models of three specimens to the moment arms determined experimentally on the same specimens. Because a muscle's moment arm determines its change in length with joint rotation, these comparisons also tested the accuracy with which the models could estimate muscle-tendon lengths over a range of hip and knee motions.Errors in the moment arms calculated with the models, averaged over functional ranges of hip and knee flexion, were less than 4 mm (within 10% of experimental values).The combination of MR imaging and graphics-based musculoskeletal modeling provides an accurate and efficient means of estimating muscle-tendon lengths and moment arms in vivo.

  View details for PubMedID 10862133

• Moment arm and force-generating capacity of the extensor carpi ulnaris after transfer to the extensor carpi radialis brevis JOURNAL OF HAND SURGERY-AMERICAN VOLUME Herrmann, A. M., Delp, S. L. 1999; 24A (5): 1083-1090

  Abstract

  Tendon transfers to the extensor carpi radialis brevis (ECRB) are often performed to augment wrist extension. This study was conducted to analyze how transfer of the extensor carpi ulnaris (ECU) to the ECRB affects the moment arms, force-generating capacity, and moment-generating capacity of the ECU over a range of wrist flexion-extension. A graphics-based computer model was developed from anatomic measurements of the muscle-tendon paths before and after transfer. This model calculates the lengths and moment arms of the muscles over a range of wrist flexion-extension and represents the muscles' force-generating characteristics from previous measurements of their physiologic cross-sectional areas, fiber lengths, and pennation angles. Analysis of the computer model revealed that the maximum isometric extension moment of the ECU at the neutral wrist position increased from 0.50 N-m to 1.72 N-m after transfer to the ECRB. The deviation moment shifted from 2.72 N-m ulnar deviation to 1.42 N-m radial deviation. The extension moment generated by the ECU varied more with wrist flexion angle after transfer due to its broadened operating range on the muscle force-length relationship. The simulations highlight the need for proper intraoperative tensioning of the ECU to maximize the force-generating potential of the transferred muscle over the functional range of motion.

  View details for Web of Science ID 000082698400026

• Moment arm and force-generating capacity of the extensor carpi ulnaris after transfer to the extensor carpi radialis brevis. The Journal of hand surgery Herrmann, A. M., Delp, S. L. 1999; 24 (5): 1083-90

  Abstract

  Tendon transfers to the extensor carpi radialis brevis (ECRB) are often performed to augment wrist extension. This study was conducted to analyze how transfer of the extensor carpi ulnaris (ECU) to the ECRB affects the moment arms, force-generating capacity, and moment-generating capacity of the ECU over a range of wrist flexion-extension. A graphics-based computer model was developed from anatomic measurements of the muscle-tendon paths before and after transfer. This model calculates the lengths and moment arms of the muscles over a range of wrist flexion-extension and represents the muscles' force-generating characteristics from previous measurements of their physiologic cross-sectional areas, fiber lengths, and pennation angles. Analysis of the computer model revealed that the maximum isometric extension moment of the ECU at the neutral wrist position increased from 0.50 N-m to 1.72 N-m after transfer to the ECRB. The deviation moment shifted from 2.72 N-m ulnar deviation to 1.42 N-m radial deviation. The extension moment generated by the ECU varied more with wrist flexion angle after transfer due to its broadened operating range on the muscle force-length relationship. The simulations highlight the need for proper intraoperative tensioning of the ECU to maximize the force-generating potential of the transferred muscle over the functional range of motion.

  View details for DOI 10.1053/jhsu.1999.1083

  View details for PubMedID 10509289

• Variation of rotation moment arms with hip flexion JOURNAL OF BIOMECHANICS Delp, S. L., Hess, W. E., Hungerford, D. S., Jones, L. C. 1999; 32 (5): 493-501

  Abstract

  Excessive flexion and internal rotation of the hip is a common gait abnormality among individuals with cerebral palsy. The purpose of this study was to examine the influence of hip flexion on the rotational moment arms of the hip muscles. We hypothesized that flexion of the hip would increase internal rotation moment arms and decrease external rotation moment arms of the primary hip rotators. To test this hypothesis we measured rotational moment arms of the gluteus maximus (six compartments), gluteus medius (four compartments), gluteus minimus (three compartments) iliopsoas, piriformis, quadratus femoris, obturator internus, and obturator externus. Moment arms were measured at hip flexion angles of 0, 20, 45, 60, and 90 degrees in four cadavers. A three-dimensional computer model of the hip muscles was developed and compared to the experimental measurements. The experimental results and the computer model showed that the internal rotation moment arms of some muscles increase with flexion; the external rotation moment arms of other muscles decrease, and some muscles switch from external rotation to internal rotation as the hip is flexed. This trend toward internal rotation with hip flexion was apparent in 15 of the 18 muscle compartments we examined, suggesting that excessive hip flexion may exacerbate internal rotation of the hip. The gluteus maximus was found to have a large capacity for external rotation. Enhancing the activation of the gluteus maximus, a muscle that is frequently underactive in persons with cerebral palsy, may help correct excessive flexion and internal rotation of the hip.

  View details for Web of Science ID 000079982200006

  View details for PubMedID 10327003

• Length changes of the hamstrings and adductors resulting from derotational osteotomies of the femur JOURNAL OF ORTHOPAEDIC RESEARCH Schmidt, D. J., Arnold, A. S., Carroll, N. C., Delp, S. L. 1999; 17 (2): 279-285

  Abstract

  Derotational osteotomies of the femur are frequently performed to treat persons with cerebral palsy who walk with excessive internal rotation of the hip. However, whether these procedures stretch or slacken the surrounding muscles appreciably is unknown. Determination of how muscle lengths are altered by derotational osteotomies is difficult because the length changes depend not only on the osteotomy site and the degree of derotation, but also on the anteversion angle of the femur and the rotational position of the hip. We have developed a three-dimensional computer simulation of derotational osteotomies, tested by anatomical experiments, to examine how femoral anteversion, hip internal rotation, and derotation affect the lengths of the semitendinosus, semimembranosus, biceps femoris long head, adductor longus, adductor brevis, and gracilis muscles. Simulation of derotational osteotomies at the intertrochanteric, subtrochanteric, or supracondylar levels decreased the origin-to-insertion lengths of the hamstrings and gracilis in our model by less than 8 mm (1.8%). Hence, the lengths of the hamstrings and gracilis are not likely to be altered substantially by these procedures. The origin-to-insertion lengths of the adductor longus and adductor brevis decreased less than 4 mm (1.9%) with subtrochanteric correction in our model, but the length of adductor brevis increased 8 mm (6.3%) with 60 degrees of intertrochanteric derotation. These muscles are also unlikely to be affected by derotational osteotomies, unless a large degree of intertrochanteric derotation is performed.

  View details for Web of Science ID 000079838500017

  View details for PubMedID 10221846

• Muscular resistance to varus and valgus loads at the elbow JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME Buchanan, T. S., Delp, S. L., Solbeck, J. A. 1998; 120 (5): 634-639

  Abstract

  Although the contributions of passive structures to stability of the elbow have been well documented, the role of active muscular resistance of varus and valgus loads at the elbow remains unclear. We hypothesized that muscles: (1) can produce substantial varus and valgus moments about the elbow, and (2) are activated in response to sustained varus and valgus loading of the elbow. To test the first hypothesis, we developed a detailed musculoskeletal model to estimate the varus and valgus moment-generating capacity of the muscles about the elbow. To test the second hypothesis, we measured EMGs from 11 muscles in four subjects during a series of isometric tasks that included flexion, extension, varus, and valgus moments about the elbow. The EMG recordings were used as inputs to the elbow model to estimate the contributions of individual muscles to flexion-extension and varus-valgus moments. Analysis of the model revealed that nearly all of the muscles that cross the elbow are capable of producing varus or valgus moments; the capacity of the muscles to produce varus moment (34 Nm) and valgus moment (35 Nm) is roughly half of the maximum flexion moment (70 Nm). Analysis of the measured EMGs showed that the anconeus was the most significant contributor to valgus moments and the pronator teres was the largest contributor to varus moments. Although our results show that muscles were activated in response to static varus and valgus loads, their activations were modest and were not sufficient to balance the applied load.

  View details for Web of Science ID 000076377600012

  View details for PubMedID 10412442

• Computer assisted knee replacement CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Delp, S. L., Stulberg, S. D., Davies, B., Picard, F., Leitner, F. 1998: 49-56

  Abstract

  Accurate alignment of knee implants is essential for the success of total knee replacement. Although mechanical alignment guides have been designed to improve alignment accuracy, there are several fundamental limitations of this technology that will inhibit additional improvements. Various computer assisted techniques have been developed to examine the potential to install knee implants more accurately and consistently than can be done with mechanical guides. For example, computer integrated instrumentation incorporates highly accurate measurement devices to locate joint centers, track surgical tools, and align prosthetic components. Image guided knee replacement provides a three-dimensional preoperative plan that guides the placement of the cutting blocks and prosthetic components. Robot assisted knee replacement allows one to machine bones accurately without the use of standard cutting blocks. The rationale for the development of computer assisted knee replacement systems is presented, the operation of several different systems is described, the advantages and disadvantages of different approaches are discussed, and areas for future research are suggested.

  View details for Web of Science ID 000075898000007

  View details for PubMedID 9755763

• Posterior tilting of the tibial component decreases femoral rollback in posterior-substituting knee replacement: A computer simulation study JOURNAL OF ORTHOPAEDIC RESEARCH Piazza, S. J., Delp, S. L., Stulberg, S. D., Stern, S. H. 1998; 16 (2): 264-270

  Abstract

  Posterior tilting of the tibial component is thought to increase the range of motion in posterior cruciate-retaining total knee replacement, but its effect on implant motion in posterior cruciate-substituting total knee replacement is unknown. This issue has become of interest recently because manufacturers have introduced instrumentation that produces a posteriorly tilted tibial cut for both implant types. The purpose of this study was to investigate how motion of posterior cruciate-substituting total knee replacement is affected when the tibial component is installed with posterior tilt. Sagittal plane implant motions were predicted from prosthesis geometry with use of a computer simulation in which the femoral condyles were assumed to sit in the bottoms of the tibial condylar wells when the knee was in extension. Rollback of the femoral component was produced by a cam-spine mechanism at higher angles of flexion. The simulations revealed that even small degrees of posterior tilt reduced rollback by limiting the interaction between the cam and spine. Tilting the component posteriorly by 5 degrees caused the cam to contact the spine at a knee flexion angle that was 18 degrees higher than with the untilted component. The results suggest that posterior tilting of the tibial component in posterior cruciate-substituting knee replacement may not produce the same beneficial effects that have been reported for the tilting of tibial components in posterior cruciate-retaining knee replacement.

  View details for Web of Science ID 000073905500013

  View details for PubMedID 9621901

• Influence of muscle morphometry and moment arms on the moment-generating capacity of human neck muscles SPINE Vasavada, A. N., Li, S. P., Delp, S. L. 1998; 23 (4): 412-422

  Abstract

  The function of neck muscles was quantified by incorporating experimentally measured morphometric parameters into a three-dimensional biomechanical model.To analyze how muscle morphometry and moment arms influence moment-generating capacity of human neck muscles in physiologic ranges of motion.Previous biomechanical analyses of the head-neck system have used simplified representations of the musculoskeletal anatomy. The force- and moment-generating properties of individual neck muscles have not been reported.A computer graphics model was developed that incorporates detailed neck muscle morphometric data into a model of cervical musculoskeletal anatomy and intervertebral kinematics. Moment arms and force-generating capacity of neck muscles were calculated for a range of head positions.With the head in the upright neutral position, the muscles with the largest moment arms and moment-generating capacities are sternocleidomastoid in flexion and lateral bending, semispinalis capitis and splenius capitis in extension, and trapezius in axial rotation. The moment arms of certain neck muscles (e.g., rectus capitis posterior major in axial rotation) change considerably in the physiologic range of motion. Most neck muscles maintain at least 80% of their peak force-generating capacity throughout the range of motion; however, the force-generating capacities of muscles with large moment arms and/or short fascicles (e.g., splenius capitis) vary substantially with head posture.These results quantify the contributions of individual neck muscles to moment-generating capacity and demonstrate that variations in force-generating capacity and moment arm throughout the range of motion can alter muscle moment-generating capacities.

  View details for Web of Science ID 000072276600002

  View details for PubMedID 9516695

• Building biomechanical models based on medical image data: An assessment of model accuracy 1st International Conference on Medical Image Computing and Computer-Assisted Intervention (MICCAI 98) Murray, W. M., Arnold, A. S., Salinas, S., Durbhakula, M. M., Buchanan, T. S., Delp, S. L. SPRINGER-VERLAG BERLIN. 1998: 539–549

  View details for Web of Science ID 000082115900058

• Graphics-based modeling and analysis of gait abnormalities BIO-MEDICAL MATERIALS AND ENGINEERING Delp, S. L., Arnold, A. S., Piazza, S. J. 1998; 8 (3-4): 227-240

  View details for Web of Science ID 000078402500013

  View details for PubMedID 10065889

• A bipedal, closed-chain dynamic model of the human lower extremities and pelvis for simulation-based development of standing and mobility neuroprostheses 10th Annual International Conference of the IEEE-Engineering-in-Medicine-and-Biology-Society Zhao, W. F., Kirsch, R. F., Triolo, R. J., Delp, S. IEEE. 1998: 2605–2608

  View details for Web of Science ID 000079210400711

• How muscle architecture and moment arms affect wrist flexion-extension moments JOURNAL OF BIOMECHANICS Gonzalez, R. V., Buchanan, T. S., Delp, S. L. 1997; 30 (7): 705-712

  Abstract

  The purpose of this investigation was to determine how the moment arms and architecture of the wrist muscles influence their isometric moment-generating characteristics. A three-dimensional computer graphic model was developed that estimates the moment arms, maximum isometric forces, and maximum isometric flexion-extension moments generated by 15 muscles about the wrist over a range of wrist flexion angles. In combination with measurements of muscle strength, we used this model to answer three questions: (1) why is peak wrist flexion moment greater than peak extension moment, (2) why does flexion moment vary more with wrist flexion angle than does extension moment, and (3) why does flexion moment peak with the wrist in a flexed position? Analysis of the model revealed that the peak flexion moment is greater than the peak extension moment primarily because of the larger (110%) summed physiologic cross-sectional area of the flexors. The larger variation of flexion moment with flexion angle is caused mainly by greater variation of the moment arms of the major wrist flexors with flexion angle. The location of the peak flexion moment is determined by the wrist flexion moment arms (which tend to increase with wrist flexion) in combination with the force-length characteristics of these muscles.

  View details for Web of Science ID A1997XK22300006

  View details for PubMedID 9239550

• Kinematics of the freely moving head and neck in the alert cat EXPERIMENTAL BRAIN RESEARCH Keshner, E. A., Statler, K. D., Delp, S. L. 1997; 115 (2): 257-266

  Abstract

  In this study we examined connections between the moment-generating capacity of the neck muscles and their patterns of activation during voluntary head-tracking movements. Three cats lying prone were trained to produce sinusoidal (0.25 Hz) tracking movements of the head in the sagittal plane, and 22.5 degrees and 45 degrees away from the sagittal plane. Radio-opaque markers were placed in the cervical vertebrae, and intramuscular patch electrodes were implanted in five neck muscles, including biventer cervicis, complexus, splenius capitis, occipitoscapularis, and rectus capitis posterior major. Videofluoroscopic images of cervical vertebral motion and muscle electromyographic responses were simultaneously recorded. A three-dimensional biomechanical model was developed to estimate how muscle moment arms and force-generating capacities change during the head-tracking movement. Experimental results demonstrated that the head and vertebrae moved synchronously, but neither the muscle activation patterns nor vertebral movements were constant across trials. Analysis of the biomechanical model revealed that, in some cases, modification of muscle activation patterns was consistent with changes in muscle moment arms or force-generating potential. In other cases, however, changes in muscle activation patterns were observed without changes in muscle moment arms or force-generating potential. This suggests that the moment-generating potential of muscles is just one of the variables that influences which muscles the central nervous system will select to participate in a movement.

  View details for Web of Science ID A1997XG49000006

  View details for PubMedID 9224854

• Surgical simulation: An emerging technology for training in emergency medicine PRESENCE-TELEOPERATORS AND VIRTUAL ENVIRONMENTS Delp, S. L., Loan, P., Basdogan, C., Rosen, J. M. 1997; 6 (2): 147-159

  View details for Web of Science ID A1997WW65500003

• The action of the rectus femoris muscle following distal tendon transfer: Does it generate knee flexion moment? DEVELOPMENTAL MEDICINE AND CHILD NEUROLOGY Riewald, S. A., Delp, S. L. 1997; 39 (2): 99-105

  Abstract

  Rectus femoris transfer surgery involves detaching the rectus femoris from the patella and reattaching it posterior to the knee. While this procedure is thought to convert the rectus femoris from a knee extensor to a knee flexor, the moments generated by this muscle after transfer have never been measured. We used intramuscular electrodes to stimulate the rectus femoris in four subjects, two after transfer to the semitendinosus and two after transfer to the iliotibial band, while measuring the resultant knee moment. Electromyographic activity was monitored in the quadriceps, hamstrings, and gastrocnemius muscles to verify that the rectus femoris was the only muscle activated by the stimulus. We found that the rectus femoris generated a knee extension moment in all of the subjects tested. This finding suggests that transfer surgery does not convert the rectus femoris to a knee flexor, and that a mechanism exists which may transmit the force generated by the rectus femoris anterior to the knee joint center after distal tendon transfer.

  View details for Web of Science ID A1997WM28200006

  View details for PubMedID 9062424

• Internal rotation gait: A compensatory mechanism to restore abduction capacity decreased by bone deformity? DEVELOPMENTAL MEDICINE AND CHILD NEUROLOGY Arnold, A. S., KOMATTU, A. V., Delp, S. L. 1997; 39 (1): 40-44

  Abstract

  Children with excessive femoral anteversion frequently walk with abnormal internal rotation of the hip. The authors hypothesized that excessive anteversion decreases the abduction moment arm of the gluteus medius and that this moment arm is restored with internal rotation; hence internal rotation may be a compensatory mechanism to preserve abduction capacity. To test this hypothesis a three-dimensional computer model of an adult lower limb was developed to determine how changes in femoral anteversion angle, neck-shaft angle, and hip internal rotation angle affect the abduction moment arm of the gluteus medius. Analysis of the model revealed that anteversion and valgus deformities of the femur can decrease the abduction moment arm of the gluteus medius substantially. In particular, increasing the anteversion angle of the model by 30 to 40 degrees caused a 40 to 50% decrease in the abduction moment arm of the gluteus medius - enough to impair walking. Internal rotation of the hip by 30 degrees restored the abduction moment arm of the gluteus medius to within 5% of the moment arm of the model in its normal, undeformed state. These results support the authors' hypothesis and are consistent with the theory that internal rotation may be a compensatory mechanism adopted by children with femoral deformities to achieve the abduction moment arm needed for walking.

  View details for Web of Science ID A1997WA30100008

  View details for PubMedID 9003728

• Maximum isometric moments generated by the wrist muscles in flexion-extension and radial-ulnar deviation JOURNAL OF BIOMECHANICS Delp, S. L., Grierson, A. E., Buchanan, T. S. 1996; 29 (10): 1371-1375

  Abstract

  Maximum isometric and passive moments about the wrist were measured for a range of flexion-extension and radial-ulnar deviation angles in 10 healthy adult males. Each subject was seated in a test apparatus with his shoulder abducted 90 degrees, elbow flexed 90 degrees, and body and forearm constrained. Peak flexion moments ranged from 5.2 to 18.7 N m (mean = 12.2, SD = 3.7), while peak extension moments ranged from 3.4 to 9.4 N m (mean = 7.1, SD = 2.1). The average flexion moment peaked at 40 degrees of flexion, whereas the average extension moment was relatively constant from 30 degrees flexion to 70 degrees extension. Peak moments generated by the radial and ulnar deviators ranged from 7.9 to 15.3 N m (mean = 11.0, SD = 2.0) and 5.9 to 11.9 N m (mean = 9.5, SD = 2.2), respectively. Passive moments in flexion-extension were near zero in the central 150 degrees of motion, but increased at the end of the range of motion. The average passive moment was 0.5 N m in 90 degrees flexion and 1.2 N m in 90 degrees extension. Average passive moments about the radial-ulnar deviation axis were near zero with the wrist radially deviated and at neutral, but increased to 0.9 N m in full ulnar deviation.

  View details for Web of Science ID A1996VH06300015

  View details for PubMedID 8884484

• How superior placement of the joint center in hip arthroplasty affects the abductor muscles CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Delp, S. L., Wixson, R. L., KOMATTU, A. V., KOCMOND, J. H. 1996: 137-146

  Abstract

  This study examines the effects of a superiorly placed hip center on the strength of the abductor muscles. A 3-dimensional computer model of the hip and the surrounding musculature was used to calculate the moment arms, forces, and moments generated when the hip abductor muscles are maximally activated. A representation of a hip prosthesis was implanted into the computer model with altered hip center positions and a range of prosthetic neck lengths. Analysis of these simulated hip replacements demonstrated that superolateral placement of the hip center (2 cm superior and 2 cm lateral) decreases the moment arms of the hip abductor muscles by an average of 28%. This decrease in moment arm cannot be restored by increasing prosthetic neck length, resulting in an unrecoverable loss of abduction strength with superolateral displacement. By contrast, a 2-cm superior displacement of the hip center changes the moment arms and force generating capacities of the abductors by less than 10% if prosthetic neck length is increased to compensate for decreased muscle length. The results of this study suggest that superior positioning of the hip center, without lateral placement, does not have major, adverse effects on abduction moment arms or force generating capacities when the neck length is appropriately increased.

  View details for Web of Science ID A1996UV55600022

  View details for PubMedID 8653946

• The influence of muscles on knee flexion during the swing phase of gait JOURNAL OF BIOMECHANICS Piazza, S. J., Delp, S. L. 1996; 29 (6): 723-733

  Abstract

  Although the movement of the leg during swing phase is often compared to the unforced motion of a compound pendulum, the muscles of the leg are active during swing and presumably influence its motion. To examine the roles of muscles in determining swing phase knee flexion, we developed a muscle-actuated forward dynamic simulation of the swing phase of normal gait. Joint angles and angular velocities at toe-off were derived from experimental measurements, as were pelvis motions and muscle excitations. Joint angles and joint moments resulting from the simulation corresponded to experimental measurements made during normal gait. Muscular joint moments and initial joint angular velocities were altered to determine the effects of each upon peak knee flexion in swing phase. As expected, the simulation demonstrated that either increasing knee extension moment or decreasing toe-off knee flexion velocity decreased peak knee flexion. Decreasing hip flexion moment or increasing toe-off hip flexion velocity also caused substantial decreases in peak knee flexion. The rectus femoris muscle played an important role in regulating knee flexion; removal of the rectus femoris actuator from the model resulted in hyperflexion of the knee, whereas an increase in the excitation input to the rectus femoris actuator reduced knee flexion. These findings confirm that reduced knee flexion during the swing phase (stiff-knee gait) may be caused by overactivity of the rectus femoris. The simulations also suggest that weakened hip flexors and stance phase factors that determine the angular velocities of the knee and hip at toe-off may be responsible for decreased knee flexion during swing phase.

  View details for Web of Science ID A1996UK44200003

  View details for PubMedID 9147969

• Trochanteric transfer in total hip replacement: Effects on the moment arms and force-generating capacities of the hip abductors JOURNAL OF ORTHOPAEDIC RESEARCH Free, S. A., Delp, S. L. 1996; 14 (2): 245–50

  Abstract

  A three-dimensional computer model of the pelvis, femur, gluteus medius, and gluteus minimus was used to evaluate the changes in muscle moment arms and force-generating capacities caused by alterations in the location of the greater trochanter. In the first part of this study, the hip center and all other aspects of joint geometry remained unaltered, while we examined changes in abduction moment arms that resulted from transfer of the trochanteric fragment to a wide variety of positions on the femur. The largest increase in average abduction moment arm was 11% (0.5 cm), which occurred with an anterolateral transfer. Most transfers resulted in moment arm changes of less than 5%. In the second part of this study, the hip center was displaced 2 cm superiorly, and the effects of a distal trochanteric transfer on the moment arms and force-generating capacities of the abductors were analyzed. The superior displacement caused a 13% decrease in the moment arm of the abductors and a 43% decrease in their force-generating capacity. The moment arm was not restored by distal transfer of the greater trochanter; however, distal transfer had the major advantage of restoring muscle lengths and force-generating capacities. These results suggest that trochanteric transfer should be considered primarily as a means to restore muscle length because it has limited potential to increase the moment arms of the two primary hip abductors.

  View details for DOI 10.1002/jor.1100140212

  View details for Web of Science ID A1996UK21800011

  View details for PubMedID 8648502

• Hamstrings and psoas lengths during normal and crouch gait: Implications for muscle-tendon surgery JOURNAL OF ORTHOPAEDIC RESEARCH Delp, S. L., Arnold, A. S., Speers, R. A., Moore, C. A. 1996; 14 (1): 144-151

  Abstract

  Crouch gait, one of the most common movement abnormalities among children with cerebral palsy, is characterized by persistent flexion of the knee during the stance phase. Short hamstrings are thought to be the cause of crouch gait; thus, crouch gait is often treated by surgical lengthening of the hamstrings. In this study, a graphics-based model of the lower extremity was used in conjunction with three-dimensional kinematic data obtained from gait analysis to estimate the lengths of the hamstrings and psoas muscles during normal and crouch gaits. Only three of 14 subjects with crouch gait (four of 20 limbs with knee flexion of 20 degrees or more throughout stance) had hamstrings that were shorter than normal by more than 1 SD during walking. Most (80%) of the subjects with crouch gait had hamstrings of normal length or longer, despite persistent knee flexion during stance. This occurred because the excessive knee flexion was typically accompanied by excessive hip flexion throughout the gait cycle. All of the subjects with crouch gait had a psoas that was shorter than normal by more than 1 SD during walking. These results emphasize the need to consider the geometry and kinematics of multiple joints before performing surgical procedures aimed at correcting crouch gait.

  View details for Web of Science ID A1996UA58800022

  View details for PubMedID 8618157

• Virtual reality and medicine: From training systems to performance machines IEEE 1996 Virtual Reality Annual International Symposium Rosen, J. M., Laub, D. R., Pieper, S. D., Mecinski, A. M., Soltanian, H., McKenna, M. A., Chen, D., Delp, S. L., Loan, J. P., Basdogan, C. I E E E, COMPUTER SOC PRESS. 1996: 5–13

  View details for Web of Science ID A1996BF53U00002

• TRADEOFFS BETWEEN MOTION AND STABILITY IN POSTERIOR SUBSTITUTING KNEE ARTHROPLASTY DESIGN JOURNAL OF BIOMECHANICS Delp, S. L., KOCMOND, J. H., Stern, S. H. 1995; 28 (10): 1155-?

  Abstract

  The purpose of this study was to examine how changes in component geometry of posterior substituting knees affect tibiofemoral kinematics and prosthesis stability. Most posterior cruciate ligament substituting prostheses rely on an articulation between a femoral cam and tibial spine to provide anterior-posterior stability of the knee. Failure of this ligament substitution mechanism has resulted in knee dislocations with several different posterior substituting designs. A computer model of a generic posterior substituting prosthesis was altered to analyze the effects of five design parameters (tibial spine height, spine anterior-posterior position, femoral component posterior radius, and femoral cam anterior-posterior and distal-proximal position) on prosthesis stability, tibiofemoral kinematics, and maximum obtainable knee flexion. Prosthesis stability was characterized by a 'dislocation safety factor', defined as the vertical distance from the bottom of the femoral cam to the top of the tibial spine. Computer simulations revealed that posterior substituting knees are most likely to dislocate at maximum knee flexion. Prosthesis stability can be improved by increasing the tibial spine height and moving the femoral cam posteriorly. Our results suggest there is a tradeoff between maximum knee flexion and prosthesis stability. We found that relatively small gains in maximum knee flexion, made through design changes, may cause substantial decreases in prosthesis stability.

  View details for Web of Science ID A1995RT92200002

  View details for PubMedID 8550634

• STABILITY AND RANGE OF MOTION OF INSALL-BURSTEIN CONDYLAR PROSTHESES - A COMPUTER-SIMULATION STUDY JOURNAL OF ARTHROPLASTY KOCMOND, J. H., Delp, S. L., Stern, S. H. 1995; 10 (3): 383-388

  Abstract

  The Insall-Burstein Posterior Stabilized Prosthesis (Zimmer, Warsaw, IN) uses an articulation between a femoral cam and tibial spine to provide anteroposterior stability to the knee. Dislocation can occur if the femoral cam translocates anteriorly and over the tibial spine. A computer model was used to examine the effects of design changes made between the Insall-Burstein I (IB I), Insall-Burstein II (IB II), and revised Insall-Burstein II (IB IIR) knees. The effects of these design changes were determined from their influence on knee stability and maximum obtainable knee flexion. Knee stability was characterized by a dislocation safety factor, defined as the vertical distance from the top of the tibial spine to the bottom of the femoral cam. Our analysis showed that the dislocation safety factor is greatest at approximately 70 degrees of knee flexion for all IB knees. As knee flexion is increased from this angle, the dislocation safety factor decreases, reducing knee stability. The simulations highlighted a trade-off between improving knee flexion and improving knee stability. The geometry of the IB II knee allowed greater knee flexion. The maximum flexion achieved with the IB II knee was 125 degrees compared with 115 degrees and 117 degrees for the IB I and IB IIR knees, respectively. However, the simulations indicate that the IB I and IB IIR knees are less likely to dislocate because they have greater dislocation safety factors than the IB II knees.

  View details for Web of Science ID A1995RE00700019

  View details for PubMedID 7673919

• VARIATION OF MUSCLE MOMENT ARMS WITH ELBOW AND FOREARM POSITION JOURNAL OF BIOMECHANICS Murray, W. M., Delp, S. L., Buchanan, T. S. 1995; 28 (5): 513-525

  Abstract

  We hypothesized that the moment arms of muscles crossing the elbow vary substantially with forearm and elbow position and that these variations could be represented using a three-dimensional computer model. Flexion/extension and pronation/supination moment arms of the brachioradialis, biceps, brachialis, pronator teres, and triceps were calculated from measurements of tendon displacement and joint angle in two anatomic specimens and were estimated using a computer model of the elbow joint. The anatomical measurements revealed that the flexion/extension moment arms varied by at least 30% over a 95 degrees range of motion. The changes in flexion/extension moment arm magnitudes with elbow flexion angle were represented well by the computer model. The anatomical studies and the computer model demonstrate that the biceps flexion moment arm peaks in a more extended elbow position and has a larger peak when the forearm is supinated. Also, the peak biceps supination moment arm decreases as the elbow is extended. These results emphasize the need to account for the variation of muscle moment arms with elbow flexion and forearm position.

  View details for Web of Science ID A1995QM87800003

  View details for PubMedID 7775488

• A GRAPHICS-BASED SOFTWARE SYSTEM TO DEVELOP AND ANALYZE MODELS OF MUSCULOSKELETAL STRUCTURES COMPUTERS IN BIOLOGY AND MEDICINE Delp, S. L., Loan, J. P. 1995; 25 (1): 21-34

  Abstract

  We have created a graphics-based software system that enables users to develop and analyze musculoskeletal models without programming. To define a model using this system one specifies the surfaces of the bones, the kinematics of the joints and the lines of action and force-generating parameters of the muscles. Once a model is defined, the function of each muscle can be analyzed by computing its length, moment arms, force and joint moments. The software has been implemented on a computer graphics workstation so that users can view the model from any perspective and graphically manipulate the joint kinematics and musculoskeletal geometry. Models can also be animated to visualize the results of motion analysis experiments. Since the software can be used to study models of many different musculoskeletal structures, it can enhance the productivity of investigators working on diverse problems in biomechanics.

  View details for Web of Science ID A1995QV12500002

  View details for PubMedID 7600758

• PRESERVING PLANTAR FLEXION STRENGTH AFTER SURGICAL-TREATMENT FOR CONTRACTURE OF THE TRICEPS SURAE - A COMPUTER-SIMULATION STUDY JOURNAL OF ORTHOPAEDIC RESEARCH Delp, S. L., STATLER, K., Carroll, N. C. 1995; 13 (1): 96-104

  Abstract

  Contractures of the triceps surae commonly are treated by surgical lengthening of the gastrocnemius aponeurosis or the Achilles tendon. Although these procedures generally relieve contractures, patients sometimes are left with dramatically decreased plantar flexion strength (i.e., decreased capacity to generate plantar flexion moment). The purpose of this study was to examine the trade-off between restoring range of motion and maintaining plantar flexion strength after surgical treatment for contracture of the triceps surae. A computer model representing the normal moment-generating characteristics of the triceps surae was altered to represent two conditions: isolated contracture of the gastrocnemius and contracture of both the gastrocnemius and the soleus. The effects of lengthening the gastrocnemius aponeurosis and the Achilles tendon were simulated for each condition. The simulations showed that nearly normal moment-generating characteristics could be restored when isolated gastrocnemius contracture was treated with lengthening of the gastrocnemius aponeurosis. However, when isolated gastrocnemius contracture was treated with lengthening of the Achilles tendon, the moment-generating capacity of the plantar flexors decreased greatly. This suggests that lengthening of the Achilles tendon should be avoided in persons with isolated gastrocnemius contracture. Our simulations also suggest that neither lengthening of the gastrocnemius aponeurosis nor lengthening of the Achilles tendon by itself is an effective treatment for combined contracture of the gastrocnemius and soleus. Lengthening the gastrocnemius aponeurosis did not decrease the excessive passive moment developed by the contracted soleus. Lengthening the Achilles tendon restored the normal passive range of motion but substantially decreased the active force-generating capacity of the muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

  View details for Web of Science ID A1995QG26900014

  View details for PubMedID 7853110

• SUPERIOR DISPLACEMENT OF THE HIP IN TOTAL JOINT REPLACEMENT - EFFECTS OF PROSTHETIC NECK LENGTH, NECK-STEM ANGLE, AND ANTEVERSION ANGLE ON THE MOMENT-GENERATING CAPACITY OF THE MUSCLES JOURNAL OF ORTHOPAEDIC RESEARCH Delp, S. L., KOMATTU, A. V., Wixson, R. L. 1994; 12 (6): 860-870

  Abstract

  The purpose of this study was to determine the effects of superior displacement of the hip center and changes in three prosthetic parameters (neck length, neck-stem angle, and anteversion angle) on the capacity of muscles to generate force and moment about the hip. A three-dimensional model that calculates the maximum isometric forces and moments generated by 25 muscles crossing the hip over a wide range of body positions was used to evaluate the effects of a 2 cm elevation of the hip center and changes in the prosthetic parameters. After superior displacement of the hip center, the neck length was increased from 0 to 3 cm, the neck-stem angle was varied between 110 and 150 degrees, and the anteversion angle was varied between 0 and 40 degrees. Our analysis showed that a 2 cm superior displacement of the hip center would decrease the moment-generating capacity of the four muscle groups studied (abductors, adductors, flexors, and extensors) if neck length were not increased to compensate for decreased muscle length. In the computer model of an adult man that we used, a 2 cm increase in neck length restored the moment-generating capacity of the muscles by increasing muscle length and force-generating capacity. However, a 3 cm increase in neck length increased passive muscle forces substantially, which potentially could limit joint motion. An increased neck-stem angle (i.e. a valgus neck) decreased the abduction moment arm but increased the moment-generating capacity of the other muscle groups. A change in the anteversion angle from 0 to 40 degrees had a relatively small effect on the isometric moment-generating capacity of the muscles studied.

  View details for Web of Science ID A1994PW53900013

  View details for PubMedID 7983561

• TRANSFER OF THE RECTUS FEMORIS - EFFECTS OF TRANSFER SITE ON MOMENT ARMS ABOUT THE KNEE AND HIP JOURNAL OF BIOMECHANICS Delp, S. L., RINGWELSKI, D. A., Carroll, N. C. 1994; 27 (10): 1201-?

  Abstract

  Decreased range of knee motion during gait is often treated by surgically releasing the rectus femoris from the patella and transferring it to one of four sites: semitendinosus, gracilis, sartorius, or the iliotibial tract. This study was conducted to determine if there are differences between these four tendon transfer sites in terms of post-surgical moment arms about the knee and hip. A graphics-based model of the lower extremity was used to simulate the origin-to-insertion path of the rectus femoris after transfer. Anatomical studies were conducted to evaluate the accuracy of the simulated tendon transfers by comparing knee flexion moment arms calculated with the computer model to moment arms measured in two anatomical specimens. The computer simulations and anatomical studies revealed substantial differences in the knee moment arms between the four sites. We found that the rectus femoris has the largest peak knee flexion moment arm (4-5 cm) after transfer to the semitendinosus. In contrast, after transfer to the iliotibial tract the rectus femoris has a slight (0-5 mm) knee extension moment arm. None of the transfers to muscle-tendon complexes on the medial side of the knee (semitendinosus, gracilis, sartorius) substantially affect the hip rotation moment arm of the rectus femoris. Transferring to the iliotibial tract increases hip internal rotation moment arm of the rectus femoris, but only when the hip is externally rotated.

  View details for Web of Science ID A1994PD27800001

  View details for PubMedID 7962008

• COMPENSATING FOR CHANGES IN MUSCLE LENGTH IN TOTAL HIP-ARTHROPLASTY - EFFECTS ON THE MOMENT GENERATING CAPACITY OF THE MUSCLES CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Vasavada, A. N., Delp, S. L., Maloney, W. J., Schurman, D. J., Zajac, F. E. 1994: 121-133

  Abstract

  Alterations in the location of the hip center may change the lengths and moment arms of the muscles, and thereby affect their capacity to generate force and moment about the hip. This study demonstrates some of the differences between compensating and not compensating for changes in muscle length that arise from displacement of the hip center. A computer model was developed to estimate the maximum isometric moment generating capacity of the hip muscles under two conditions. In the compensated condition, the hip center was displaced, but the muscles were restored to their original lengths and orientations by altering proximal femoral geometry. In the uncompensated condition, femoral geometry remained constant; thus, muscle lengths and orientations changed with displacement of the hip center. The computer simulations showed large differences between the two conditions. For example, a 2-cm superior displacement of the hip center decreased the moment generating capacity of the hip abductors 18% with compensation and 49% without compensation. Similarly, a 1-cm medial displacement of the hip center increased the moment generating capacity of the abductors 17% with compensation, but decreased it 4% without compensation. In contrast, a 1-cm inferior displacement decreased the moment generating capacity of flexors 6% with compensation, but increased it 12% without compensation. The results presented here demonstrate that compensating for changes in muscle length can be important in terms of preserving the moment generating capacity of the muscles when the hip center is displaced superiorly and medially, but not when the hip center is displaced in the inferior direction.

  View details for Web of Science ID A1994NL07400020

  View details for PubMedID 8168289

• EFFECTS OF HIP CENTER LOCATION ON THE MOMENT-GENERATING CAPACITY OF THE MUSCLES JOURNAL OF BIOMECHANICS Delp, S. L., Maloney, W. 1993; 26 (4-5): 485-499

  Abstract

  We have developed a three-dimensional biomechanical model of the human lower extremity to study how the location of the hip center affects the moment-generating capacity of four muscle groups: the hip abductors, adductors, flexors, and extensors. The model computes the maximum isometric force and the resulting joint moments that each of 25 muscle-tendon complexes develops at any body position. Abduction, adduction, flexion, and extension moments calculated with the model correspond closely with isometric joint moments measured during maximum voluntary contractions. We used the model to determine (1) the hip center locations that maximize and minimize the moment-generating capacity of each muscle group and (2) the effects of superior-inferior, anterior-posterior, and medial-lateral displacement of the hip center on the moment arms, maximum isometric muscle forces, and maximum isometric moments generated by each muscle group. We found that superior-inferior displacement of the hip center has the greatest effect on the force- and moment-generating capacity of the muscles. A 2 cm superior displacement decreases abduction force (44%), moment arm (12%), and moment (49%), while a 2 cm inferior displacement increases abduction force (20%), moment arm (7%) and moment (26%). Similarly, a 2 cm superior displacement decreases flexion force (27%), moment arm (6%), and moment (22%), while inferior displacement increases all three variables. Anterior-posterior displacement alters the moment-generating capacity of the flexors and extensors considerably, primarily due to moment arm changes. Medial-lateral displacement has a large effect on the moment-generating capacity of the adductors only. A 2 cm medial displacement decreases adduction moment arm (20%), force (26%) and moment (40%). These results demonstrate that the force- and moment-generating capacities of the muscles are sensitive to the location of the hip center.

  View details for Web of Science ID A1993KX57600011

  View details for PubMedID 8478351

• FORCE-GENERATING AND MOMENT-GENERATING CAPACITY OF LOWER-EXTREMITY MUSCLES BEFORE AND AFTER TENDON LENGTHENING CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Delp, S. L., Zajac, F. E. 1992: 247-259

  View details for Web of Science ID A1992JX20400035

• AN INTERACTIVE GRAPHICS-BASED MODEL OF THE LOWER-EXTREMITY TO STUDY ORTHOPEDIC SURGICAL-PROCEDURES IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Delp, S. L., Loan, J. P., HOY, M. G., Zajac, F. E., Topp, E. L., Rosen, J. M. 1990; 37 (8): 757-767

  Abstract

  We have developed a model of the human lower extremity to study how surgical changes in musculoskeletal geometry and musculotendon parameters affect muscle force and its moment about the joints. The lines of action of 43 musculotendon actuators were defined based on their anatomical relationships to three-dimensional bone surface representations. A model for each actuator was formulated to compute its isometric force-length relation. The kinematics of the lower extremity were defined by modeling the hip, knee, ankle, subtalar, and metatarsophalangeal joints. Thus, the force and joint moment that each musculotendon actuator develops can be computed for any body position. The joint moments calculated with the model compare well with experimentally measured isometric joint moments. We developed a graphical interface to the model that allows the user to visualize the musculoskeletal geometry and to manipulate the model parameters to study the biomechanical consequences of orthopaedic surgical procedures. For example, tendon transfer and lengthening procedures can be simulated by adjusting the model parameters according to various surgical techniques. Results of the simulated surgeries can be analyzed quickly in terms of postsurgery muscle forces and other biomechanical variables. Just as interactive graphics have enhanced engineering design and analysis, we have found that graphics-based musculoskeletal models are effective tools for designing and analyzing surgical procedures.

  View details for Web of Science ID A1990DV77000003

  View details for PubMedID 2210784

• BIOMECHANICAL ANALYSIS OF THE CHIARI PELVIC OSTEOTOMY - PRESERVING HIP ABDUCTOR STRENGTH CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Delp, S. L., BLECK, E. E., Zajac, F. E., Bollini, G. 1990: 189-198

  Abstract

  Although the Chiari osteotomy is usually effective in reducing pain, many patients are left with a long-term limp. The postoperative limp can at times be caused by hip abductors that have strength insufficient to counteract the torque from body weight during single-leg stance. To study how the surgical technique affects the hip abductor muscles, a biomechanical model was developed that computes the postsurgery pelvic geometry and the resulting hip abductor torque given three surgical parameters: angulation of the osteotomy, distance of medical displacement, and angle of internal rotation. The computer simulations of the Chiari osteotomy showed that some sets of surgical parameters conserve abductor torque while others greatly reduce it. Simulated surgeries with high angulation and large medial displacement reduce gluteus medius abductor torque by up to 65%. Therefore, this combination of surgical parameters may account for some instances of the postoperative limp. In the model, high angulation reduces the length of the gluteus medius and is the primary cause of reduced abductor strength. Simulated horizontal osteotomies (0 degrees to 10 degrees) were found to best conserve both muscle length and abductor torque.

  View details for Web of Science ID A1990DB35400027

  View details for PubMedID 2323130