Bio


I came to Stanford in 2010, after completing my PhD at the University of Wisconsin-Madison, focusing on hamstring muscle injury and recovery in athletes. After completing my postdoc in 2013, I began working in the Human Performance Lab where we work help understand and improve the health of the Stanford and surrounding communities. My research interests include better understanding human locomotion patterns and efficiency, as well as understanding the mechanisms behind acute athletic injuries in order to optimize recovery.

Current Role at Stanford


Associate Director
Human Performance Lab

Honors & Awards


  • Postdoctoral Deans Fellowship, Stanford University (2010-2012)
  • Young Investigator Award, Multi-Scale Muscle Mechanics Workshop (2009)
  • Young Scientist Pre-Doctoral Award, American Society of Biomechanics (2009)
  • National Collegiate Athletic Association Best Poster on Student-Athlete Wellbeing Award Winner, North American Congress on Biomechanics (2008)
  • New Investigator Award for Excellence in Clinical/Applied Aging Research, University of Wisconsin - Madison Institute on Aging (2007)
  • National Science Foundation Predoctoral Fellowship, National Science Foundation (2005-2008)
  • John P. Holton Wisconsin Distinguished Graduate Fellowship, University of Wisconsin - Madison (2004)

Education & Certifications


  • PhD, University of Wisconsin - Madison, Biomedical Engineering (2009)
  • MS, University of Wisconsin - Madison, Biomedical Engineering (2006)
  • BSE, Michigan State University, Biosystems Engineering (2003)

Personal Interests


running, cycling, fishing/camping, and watching TV

All Publications


  • 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

  • Differences in ACL biomechanical risk factors between field hockey and lacrosse female athletes KNEE SURGERY SPORTS TRAUMATOLOGY ARTHROSCOPY Braun, H. J., Shultz, R., Malone, M., Leatherwood, W. E., Silder, A., Dragoo, J. L. 2015; 23 (4): 1065-1070

    Abstract

    Previous investigations have revealed a greater incidence of anterior cruciate ligament (ACL) injuries in female lacrosse versus field hockey players. Lacrosse is played in an upright posture with overhead throwing and catching, while field hockey is almost exclusively played in a crouched, forward-flexed position. Biomechanical factors, including decreased knee, hip, and trunk flexion angles, have been identified as risk factors for ACL injury. The purpose of this study was to assess ACL biomechanical risk factors in female field hockey and lacrosse players to determine whether sport-specific posture might contribute to the increased incidence of ACL injury observed in lacrosse athletes.Thirty-one Division I NCAA females from field hockey and lacrosse completed four tasks, three times per leg: bilateral drop jump, single-leg drop jump (SDJ), single-leg jump onto a Bosu ball (SDB), and a 45° anticipated cut. Kinematic and force plate data were used to evaluate knee flexion angle, knee adduction moment, hip flexion angle, and trunk flexion and sway angles. Muscle activity of the lateral hamstrings and vastus lateralis was used to estimate peak hamstring activity and the quadriceps/hamstring ratio at the time of peak quadriceps activity (co-contraction ratio).During the SDJ and SDB, peak knee flexion angles were greater in field hockey compared with lacrosse. During cutting, field hockey players were more flexed at the trunk and had greater trunk sway, compared with the lacrosse players. No significant difference was observed for the co-contraction ratio for any of the tasks.Decreased knee flexion angle during landing, consistent with sport-specific playing postures, may contribute to the higher incidence of ACL injury in lacrosse players relative to field hockey. Sport-specific training injury prevention programmes may benefit from considering these differences between specialized athletes.II.

    View details for DOI 10.1007/s00167-014-2873-0

    View details for Web of Science ID 000351611700021

    View details for PubMedID 24493257

  • Unstable Surface Improves Quadriceps:Hamstring Co-contraction for Anterior Cruciate Ligament Injury Prevention Strategies. Sports health Shultz, R., Silder, A., Malone, M., Braun, H. J., Dragoo, J. L. 2015; 7 (2): 166-171

    Abstract

    Increasing quadriceps:hamstring muscular co-contraction at the knee may reduce the risk of anterior cruciate ligament (ACL) injury. The purpose of this investigation was to examine muscle activation in the quadriceps and hamstrings and peak kinematics of the knee, hip, and trunk when performing a single-leg drop (SLD) on to a Bosu ball (unstable surface) compared with on to the floor (stable surface).(1) The SLD on an unstable surface would lower the quadriceps to hamstrings electromyographic (EMG) activation ratio (Q:H EMG activation ratio) compared with being performed on the floor. (2) Lower Q:H EMG activation ratio would be caused by a relative increase in hamstring activation, with no significant change in quadriceps activation.Controlled laboratory study.Thirty-nine Division I National Collegiate Athletic Association (NCAA) female athletes performed 3 SLDs per leg onto a Bosu ball and onto the floor. Muscle activity of the vastus lateralis and lateral hamstrings were used to estimate peak quadriceps and hamstring activation, along with the Q:H EMG activation ratio. Kinematic measures at the knee, hip, and trunk were also estimated. Differences between landings were assessed using a 2-level analysis of variance (limb and surface).The maximum Q:H EMG activation ratio was significantly reduced when athletes performed an SLD onto the Bosu ball (20%, P < 0.001) compared with the floor. Peak hamstring activity was higher when athletes landed on a Bosu ball (18% higher, P = 0.029) compared with when they landed on the floor.Compared with landing on the floor (a stable surface), landing on a Bosu ball (unstable surface) changed the athlete's co-contraction at the knee and increased hamstring activity. However, landing on a Bosu ball also decreased the athlete's knee flexion, which was an undesired effect.These findings highlight the potential utility of unstable surfaces as a training tool to reduce the risk of ACL injury in female athletes.

    View details for DOI 10.1177/1941738114565088

    View details for PubMedID 25984263

  • 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

  • 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 Web of Science ID 000325591800024

  • 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 Web of Science ID 000319350500004

    View details for PubMedID 23494804

  • Clinical and Morphological Changes Following 2 Rehabilitation Programs for Acute Hamstring Strain Injuries: A Randomized Clinical Trial JOURNAL OF ORTHOPAEDIC & SPORTS PHYSICAL THERAPY Silder, A., Sherry, M. A., Sanfilippo, J., Tuite, M. J., Hetzel, S. J., Heiderscheit, B. C. 2013; 43 (5): 284-299

    Abstract

    Randomized, double-blind, parallel-group clinical trial.To assess differences between a progressive agility and trunk stabilization rehabilitation program and a progressive running and eccentric strengthening rehabilitation program in recovery characteristics following an acute hamstring injury, as measured via physical examination and magnetic resonance imaging (MRI).Determining the type of rehabilitation program that most effectively promotes muscle and functional recovery is essential to minimize reinjury risk and to optimize athlete performance.Individuals who sustained a recent hamstring strain injury were randomly assigned to 1 of 2 rehabilitation programs: (1) progressive agility and trunk stabilization or (2) progressive running and eccentric strengthening. MRI and physical examinations were conducted before and after completion of rehabilitation.Thirty-one subjects were enrolled, 29 began rehabilitation, and 25 completed rehabilitation. There were few differences in clinical or morphological outcome measures between rehabilitation groups across time, and reinjury rates were low for both rehabilitation groups after return to sport (4 of 29 subjects had reinjuries). Greater craniocaudal length of injury, as measured on MRI before the start of rehabilitation, was positively correlated with longer return-to-sport time. At the time of return to sport, although all subjects showed a near-complete resolution of pain and return of muscle strength, no subject showed complete resolution of injury as assessed on MRI.The 2 rehabilitation programs employed in this study yielded similar results with respect to hamstring muscle recovery and function at the time of return to sport. Evidence of continuing muscular healing is present after completion of rehabilitation, despite the appearance of normal physical strength and function on clinical examination.Therapy, level 1b-.

    View details for DOI 10.2519/jospt.2013.4452

    View details for Web of Science ID 000325781800002

    View details for PubMedID 23485730

  • Hamstring Strength and Morphology Progression after Return to Sport from Injury MEDICINE AND SCIENCE IN SPORTS AND EXERCISE Sanfilippo, J. L., Silder, A., Sherry, M. A., Tuite, M. J., Heiderscheit, B. C. 2013; 45 (3): 448-454

    Abstract

    Hamstring strain reinjury rates can reach 30% within the initial 2 wk after return to sport (RTS). Incomplete recovery of strength may be a contributing factor. However, relative strength of the injured and unaffected limbs at RTS is currently unknown.The purpose was to characterize hamstring strength and morphology at the time of RTS and 6 months later.Twenty-five athletes who experienced an acute hamstring strain injury participated after completion of a controlled rehabilitation program. Bilateral isokinetic strength testing and magnetic resonance imaging (MRI) were performed at RTS and 6 months later. Strength (knee flexion peak torque, work, and angle of peak torque) and MRI (muscle and tendon volumes) measures were compared between limbs and over time using repeated-measures ANOVA.The injured limb showed a peak torque deficit of 9.6% compared to the uninjured limb at RTS (60°·s, P < 0.001) but not 6 months after. The knee flexion angle of peak torque decreased over time for both limbs (60°·s, P < 0.001). MRI revealed that 20.4% of the muscle cross-sectional area showed signs of edema at RTS with full resolution by the 6-month follow-up. Tendon volume of the injured limb tended to increase over time (P = 0.108), whereas muscle volume decreased between 4% and 5% in both limbs (P < 0.001).Residual edema and deficits in isokinetic knee flexion strength were present at RTS but resolved during the subsequent 6 months. This occurred despite MRI evidence of scar tissue formation (increased tendon volume) and muscle atrophy, suggesting that neuromuscular factors may contribute to the return of strength.

    View details for DOI 10.1249/MSS.0b013e3182776eff

    View details for Web of Science ID 000315268700007

    View details for PubMedID 23059864

  • 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

  • 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

  • Reduced Knee Adduction Moment and Improved Symptoms Following a Six-Week Gait Retraining Program for Medial Compartment Knee Osteoarthritis Journal of Orthopaedic Research Shull, P. B., Silder, A., Shultz, R., Besier, T. F., Delp, S. L., Cutkosky, M. R. 2013; 46: 122-128
  • 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

  • Influence of Bicycle Seat Tube Angle and Hand Position on Lower Extremity Kinematics and Neuromuscular Control: Implications for Triathlon Running Performance JOURNAL OF APPLIED BIOMECHANICS Slider, A., Gleason, K., Thelen, D. G. 2011; 27 (4): 297-305
  • Hamstring Strains: Basic Science and Clinical Research Applications for Preventing the Recurrent Injury STRENGTH AND CONDITIONING JOURNAL Sherry, M. A., Best, T. M., Silder, A., Thelen, D. G., Heiderscheit, B. C. 2011; 33 (3): 56-71
  • The influence of prior hamstring injury on lengthening muscle tissue mechanics JOURNAL OF BIOMECHANICS Silder, A., Reeder, S. B., Thelen, D. G. 2010; 43 (12): 2254-2260

    Abstract

    Hamstring strain injuries often occur near the proximal musculotendon junction (MTJ) of the biceps femoris. Post-injury remodeling can involve scar tissue formation, which may alter contraction mechanics and influence re-injury risk. The purpose of this study was to assess the affect of prior hamstring strain injury on muscle tissue displacements and strains during active lengthening contractions. Eleven healthy and eight subjects with prior biceps femoris injuries were tested. All previously injured subjects had since returned to sport and exhibited evidence of residual scarring along the proximal aponeurosis. Subjects performed cyclic knee flexion-extension on an MRI-compatible device using elastic and inertial loads, which induced active shortening and lengthening contractions, respectively. CINE phase-contrast imaging was used to measure tissue velocities within the biceps femoris during these tasks. Numerical integration of the velocity information was used to estimate two-dimensional tissue displacement and strain fields during muscle lengthening. The largest tissue motion was observed along the distal MTJ, with the active lengthening muscle exhibiting significantly greater and more homogeneous tissue displacements. First principal strain magnitudes were largest along the proximal MTJ for both loading conditions. The previously injured subjects exhibited less tissue motion and significantly greater strains near the proximal MTJ. We conclude that localized regions of high tissue strains during active lengthening contractions may predispose the proximal biceps femoris to injury. Furthermore, post-injury remodeling may alter the in-series stiffness seen by muscle tissue and contribute to the relatively larger localized tissue strains near the proximal MTJ, as was observed in this study.

    View details for DOI 10.1016/j.jbiomech.2010.02.038

    View details for Web of Science ID 000282112300002

    View details for PubMedID 20472238

  • Influence of a Prior Hamstring Strain Injury on Strength, Flexibility, and Biomechanical Function Journal of Clinical Biomechanics Silder, A., Thelen, D. G., Heiderscheit, B. C. 2010; 25: 681-686
  • Acute Hamstring Injuries: Recommendations for Diagnosis, Rehabilitation and Injury Reduction Journal of Orthopedic and Sports and Physical Therapy Heiderscheit, B. C., Sherry, M. A., Silder, A., Chumanov, E. S., Thelen, D. G. 2010; 40: 67-81
  • Differences in lower-extremity muscular activation during walking between healthy older and young adults JOURNAL OF ELECTROMYOGRAPHY AND KINESIOLOGY Schmitz, A., Silder, A., Heiderscheit, B., Mahoney, J., Thelen, D. G. 2009; 19 (6): 1085-1091

    Abstract

    Previous studies have identified differences in gait kinetics between healthy older and young adults. However, the underlying factors that cause these changes are not well understood. The objective of this study was to assess the effects of age and speed on the activation of lower-extremity muscles during human walking. We recorded electromyography (EMG) signals of the soleus, gastrocnemius, biceps femoris, medial hamstrings, tibialis anterior, vastus lateralis, and rectus femoris as healthy young and older adults walked over ground at slow, preferred and fast walking speeds. Nineteen healthy older adults (age, 73+/-5 years) and 18 healthy young adults (age, 26+/-3 years) participated. Rectified EMG signals were normalized to mean activities over a gait cycle at the preferred speed, allowing for an assessment of how the activity was distributed over the gait cycle and modulated with speed. Compared to the young adults, the older adults exhibited greater activation of the tibialis anterior and soleus during mid-stance at all walking speeds and greater activation of the vastus lateralis and medial hamstrings during loading and mid-stance at the fast walking speed, suggesting increased coactivation across the ankle and knee. In addition, older adults depend less on soleus muscle activation to push off at faster walking speeds. We conclude that age-related changes in neuromuscular activity reflect a strategy of stiffening the limb during single support and likely contribute to reduced push off power at fast walking speeds.

    View details for DOI 10.1016/j.jelekin.2008.10.008

    View details for Web of Science ID 000272367200009

    View details for PubMedID 19081734

  • Effect of age on center of mass motion during human walking GAIT & POSTURE Hernandez, A., Silder, A., Heiderscheit, B. C., Thelen, D. G. 2009; 30 (2): 217-222

    Abstract

    The objective of this study was to investigate the effects of age and speed on body center of mass (COM) motion over a gait cycle. Whole body kinematics and ground reactions were recorded for 21 healthy young (21-32 y) and 20 healthy older adults (66-81 y) walking at 80%, 100% and 120% of preferred speed. The limb-induced COM accelerations and the work done on the COM by the limbs were computed. Despite walking with similar gait speeds, older adults did significantly (p<0.05) less positive work on the COM during push-off but then performed more positive work on the COM during midstance. As a result, older adults induced lower tri-axial COM accelerations via the trailing limb and higher vertical COM acceleration via the leading limb during double support. Older adults also reduced the mediolateral COM acceleration induced by the leading limb during the last third of double support. The forward and vertical components of the limb-induced COM accelerations were highly correlated (p<0.005) but were not correlated to the mediolateral component during double support, at any speed. Together, these results suggest that older adults use the leading limb to compensate for reduced vertical support and work done by the trailing limb. Further, older adults seem to adapt their gait patterns to reduce mediolateral COM accelerations. These findings are relevant for understanding the factors that underlie walking performance and lateral balance in old age.

    View details for DOI 10.1016/j.gaitpost.2009.05.006

    View details for Web of Science ID 000268559300017

    View details for PubMedID 19502061

  • A MR-Compatible Loading Device for Dynamically Imaging Shortening and Lengthening Muscle Contraction Mechanics Journal of Medical Devices Silder, A., Westphal, C. J., Thelen, D. G. 2009: 034504-1
  • MR observations of long-term musculotendon remodeling following a hamstring strain injury SKELETAL RADIOLOGY Silder, A., Heiderscheit, B. C., Thelen, D. G., Enright, T., Tuite, M. J. 2008; 37 (12): 1101-1109

    Abstract

    The objective of this study was to use magnetic resonance (MR) imaging to investigate long-term changes in muscle and tendon morphology following a hamstring strain injury.MR images were obtained from 14 athletes who sustained a clinically diagnosed grade I-II hamstring strain injury between 5 and 23 months prior as well as five healthy controls. Qualitative bilateral comparisons were used to assess the presence of fatty infiltration and changes in morphology that may have arisen as a result of the previous injury. Hamstring muscle and tendon-scar volumes were quantified in both limbs for the biceps femoris long head (BFLH), biceps femoris short head (BFSH), the proximal semimembranosus tendon, and the proximal conjoint biceps femoris and semitendinosus tendon. Differences in muscle and tendon volume between limbs were statistically compared between the previously injured and healthy control subjects.Increased low-intensity signal was present along the musculotendon junction adjacent to the site of presumed prior injury for 11 of the 14 subjects, suggestive of persistent scar tissue. The 13 subjects with biceps femoris injuries displayed a significant decrease in BFLH volume (p < 0.01), often accompanied by an increase in BFSH volume. Two of these subjects also presented with fatty infiltration within the previously injured BFLH.The results of this study provide evidence of long-term musculotendon remodeling following a hamstring strain injury. Additionally, many athletes are likely returning to sport with residual atrophy of the BFLH and/or hypertrophy of the BFSH. It is possible that long-term changes in musculotendon structure following injury alters contraction mechanics during functional movement, such as running and may contribute to reinjury risk.

    View details for DOI 10.1007/s00256-008-0546-0

    View details for Web of Science ID 000260379900006

    View details for PubMedID 18649077

  • The contribution of passive-elastic mechanisms to lower extremity joint kinetics during human walking GAIT & POSTURE Whittington, B., Silder, A., Heiderscheit, B., Thelen, D. G. 2008; 27 (4): 628-634

    Abstract

    The purpose of this study was to investigate the contribution of passive mechanisms to lower extremity joint kinetics in normal walking at slow, comfortable, and fast speeds. Twenty healthy young adults participated in a passive testing protocol in which the relaxed lower limb was manipulated through full sagittal hip, knee, and ankle ranges of motion while kinematics and applied forces were simultaneously measured. The relationship between passive joint moments and angles was modeled by a set of exponential functions that accounted for the stretch of uniarticular structures and biarticular muscles. Subject specific walking kinematics (80%, 100%, and 120% of preferred speed) were input into the passive models to estimate joint moments, power, and work attributable to passive mechanisms. Passive hip flexion moments were substantial from late stance through early swing, absorbing approximately 40% of the net negative work done during hip extension and producing over half of the net positive work done during the hip flexor power burst (H3). Passive ankle plantarflexor moments were also produced during pre-swing, but generated a smaller percentage ( approximately 10%) of the net ankle plantarflexor power burst (A2). The joint work attributed to passive structures increased significantly (p<0.05) with walking speed. The biarticular rectus femoris and gastrocnemius allowed for net passive energy absorption at the knee and subsequent return at the hip and ankle (p<0.05). Together, these results suggest that passive-elastic mechanisms can contribute substantially to normal human walking and that biarticular muscles play a role in passively transferring energy between joints.

    View details for DOI 10.1016/j.gaitpost.2007.08.005

    View details for Web of Science ID 000255732700014

    View details for PubMedID 17928228

  • Active and passive contributions to joint kinetics during walking in older adults JOURNAL OF BIOMECHANICS Silder, A., Heiderscheit, B., Thelen, D. G. 2008; 41 (7): 1520-1527

    Abstract

    The objectives of this study were to characterize the active and passive contributions to joint kinetics during walking in healthy young and older adults, and assess whether isokinetic ankle strength is associated with ankle power output during walking. Twenty healthy young (18-35 years) and 20 healthy older (65-85 years) adults participated in this study. We measured subject-specific passive-elastic joint moment-angle relationships in the lower extremity and tested maximum isokinetic ankle strength at 30 deg/s. Passive moment-angle relationships were used to estimate active and passive joint moment, power, and work quantities during walking at 80%, 100% and 120% of preferred walking speed. There were no significant differences in walking speed, step length, or cadence between the older and young adults. However, the older adults produced significantly more net positive work at the hip but less net positive work at the ankle at all walking speeds. Passive contributions to hip and ankle work did not significantly differ between groups, inferring that the older adults generated the additional hip work actively. Maximum isokinetic ankle strength was significantly less in the older adults, and correlated with peak positive plantar-flexor power at both the preferred and fast walking speeds. The results of this study suggest that age-related shifts in joint kinetics do not arise as a result of increased passive hip joint stiffness, but seem to be reflected in plantar-flexor weakness.

    View details for DOI 10.1016/j.jbiomech.2008.02.016

    View details for Web of Science ID 000256769200016

    View details for PubMedID 18420214

  • Identification of passive elastic joint moment-angle relationships in the lower extremity JOURNAL OF BIOMECHANICS Silder, A., Whittington, B., Heiderscheit, B., Thelen, D. G. 2007; 40 (12): 2628-2635

    Abstract

    The purpose of this study was to develop a method for identifying subject-specific passive elastic joint moment-angle relationships in the lower extremity, which could subsequently be used to estimate passive contributions to joint kinetics during gait. Twenty healthy young adults participated in the study. Subjects were positioned side-lying with their dominant limb supported on a table via low-friction carts. A physical therapist slowly manipulated the limb through full sagittal hip, knee, and ankle ranges of motion using two hand-held 3D load cells. Lower extremity kinematics, measured with a passive marker motion capture system, and load cell readings were used to compute joint angles and associated passive joint moments. We formulated a passive joint moment-angle model that included eight exponential functions to account for forces generated via the passive stretch of uni-articular structures and bi-articular muscles. Model parameters were estimated for individual subjects by minimizing the sum of squared errors between model predicted and experimentally measured moments. The model predictions closely replicated measured joint moments with average root-mean-squared errors of 2.5, 1.4, and 0.7 Nm about the hip, knee, and ankle respectively. We show that the models can be coupled with gait kinematics to estimate passive joint moments during walking. Passive hip moments were substantial from terminal stance through initial swing, with energy being stored as the hip extended and subsequently returned during pre- and initial swing. We conclude that the proposed methodology could provide quantitative insights into the potentially important role that passive mechanisms play in both normal and abnormal gait.

    View details for DOI 10.1016/j.jbiomech.2006.12.017

    View details for Web of Science ID 000249508900006

    View details for PubMedID 17359981