Steve Collins is an Associate Professor of Mechanical Engineering at Stanford University, where he teaches courses on design and robotics and directs the Stanford Biomechatronics Laboratory. His primary focus is to speed and systematize the design and prescription of prostheses and exoskeletons using versatile device emulator hardware and human-in-the-loop optimization algorithms (Zhang et al. 2017, Science). Another interest is efficient autonomous devices, such as highly energy-efficient walking robots (Collins et al. 2005, Science) and exoskeletons that use no energy yet reduce the metabolic energy cost of human walking (Collins et al. 2015, Nature).

Prof. Collins received his B.S. in Mechanical Engineering in 2002 from Cornell University, where he performed undergraduate research on passive dynamic walking robots. He received his Ph.D. in Mechanical Engineering in 2008 from the University of Michigan, where he performed research on the dynamics and control of human walking. He performed postdoctoral research on humanoid robots at T. U. Delft in the Netherlands. He was a professor of Mechanical Engineering and Robotics at Carnegie Mellon University for seven years. In 2017, he joined the faculty of Mechanical Engineering at Stanford University.

Prof. Collins is a member of the Scientific Board of Dynamic Walking and the Editorial Board of Science Robotics. He has received the Young Scientist Award from the American Society of Biomechanics, the Best Medical Devices Paper from the International Conference on Robotics and Automation, and the student-voted Professor of the Year in his department.

Academic Appointments

Honors & Awards

  • Chambers Faculty Scholar, Stanford School of Engineering (2020-2024)
  • Teaching Honor Roll, Tau Beta Pi of Stanford University (2020)
  • Best Medical Robotics Paper Award, International Conference on Robotics and Automation (ICRA) (2015)
  • Professor of the Year (student-voted), Department of Mechanical Engineering, Carnegie Mellon University (2014)
  • Young Scientist Award, Post-Doctoral, American Society for Biomechanics (2013)
  • Struminger Faculty Teaching Fellow, Department of Mechanical Engineering, Carnegie Mellon University (2012)
  • McManus Design Award, Department of Mechanical Engineering, Cornell University (2002)

Boards, Advisory Committees, Professional Organizations

  • Editorial Board, Science Robotics (2019 - Present)
  • Associate Editor, International Journal of Robotics Research (2017 - 2019)
  • Scientific Board, Dynamic Walking (2008 - Present)

Professional Education

  • Ph.D., University of Michigan, Mechanical Engineering (2008)
  • B.S., Cornell University, Mechanical Engineering (2002)

2023-24 Courses

Stanford Advisees

All Publications

  • Elastic energy-recycling actuators for efficient robots. Science robotics Krimsky, E., Collins, S. H. 2024; 9 (88): eadj7246


    Electric motors are widely used in robots but waste energy in many applications. We introduce an elastic energy-recycling actuator that maintains the versatility of motors while improving energy efficiency in cyclic tasks. The actuator comprises a motor in parallel with an array of springs that can be individually engaged and disengaged, while retaining stored energy, by pairs of low-power electroadhesive clutches. We developed a prototype actuator and tested it in five repetitive tasks with features common in robotic applications but difficult to perform efficiently. The actuator reduced power consumption by at least 50% in all cases and by 97% in the best case. Elastic energy recovery, controlled by low-power clutches, can improve the efficiency of mobile robots, assistive devices, and other engineered systems.

    View details for DOI 10.1126/scirobotics.adj7246

    View details for PubMedID 38507474

  • 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


    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

  • Human-in-the-loop optimization of exoskeleton assistance during walking SCIENCE Zhang, J., Fiers, P., Witte, K. A., Jackson, R. W., Poggensee, K. L., Atkeson, C. G., Collins, S. H. 2017; 356: 1280-1284


    Exoskeletons and active prostheses promise to enhance human mobility, but few have succeeded. Optimizing device characteristics on the basis of measured human performance could lead to improved designs. We have developed a method for identifying the exoskeleton assistance that minimizes human energy cost during walking. Optimized torque patterns from an exoskeleton worn on one ankle reduced metabolic energy consumption by 24.2 ± 7.4% compared to no torque. The approach was effective with exoskeletons worn on one or both ankles, during a variety of walking conditions, during running, and when optimizing muscle activity. Finding a good generic assistance pattern, customizing it to individual needs, and helping users learn to take advantage of the device all contributed to improved economy. Optimization methods with these features can substantially improve performance.

    View details for DOI 10.1126/science.aal5054

  • Reducing the energy cost of human walking using an unpowered exoskeleton NATURE Collins, S. H., Wiggin, M. B., Sawicki, G. S. 2015; 522 (7555): 212-?


    With efficiencies derived from evolution, growth and learning, humans are very well-tuned for locomotion. Metabolic energy used during walking can be partly replaced by power input from an exoskeleton, but is it possible to reduce metabolic rate without providing an additional energy source? This would require an improvement in the efficiency of the human-machine system as a whole, and would be remarkable given the apparent optimality of human gait. Here we show that the metabolic rate of human walking can be reduced by an unpowered ankle exoskeleton. We built a lightweight elastic device that acts in parallel with the user's calf muscles, off-loading muscle force and thereby reducing the metabolic energy consumed in contractions. The device uses a mechanical clutch to hold a spring as it is stretched and relaxed by ankle movements when the foot is on the ground, helping to fulfil one function of the calf muscles and Achilles tendon. Unlike muscles, however, the clutch sustains force passively. The exoskeleton consumes no chemical or electrical energy and delivers no net positive mechanical work, yet reduces the metabolic cost of walking by 7.2 ± 2.6% for healthy human users under natural conditions, comparable to savings with powered devices. Improving upon walking economy in this way is analogous to altering the structure of the body such that it is more energy-effective at walking. While strong natural pressures have already shaped human locomotion, improvements in efficiency are still possible. Much remains to be learned about this seemingly simple behaviour.

    View details for DOI 10.1038/nature14288

    View details for Web of Science ID 000356016700038

    View details for PubMedID 25830889

    View details for PubMedCentralID PMC4481882

  • Efficient bipedal robots based on passive-dynamic walkers SCIENCE Collins, S., Ruina, A., Tedrake, R., Wisse, M. 2005; 307 (5712): 1082-1085


    Passive-dynamic walkers are simple mechanical devices, composed of solid parts connected by joints, that walk stably down a slope. They have no motors or controllers, yet can have remarkably humanlike motions. This suggests that these machines are useful models of human locomotion; however, they cannot walk on level ground. Here we present three robots based on passive-dynamics, with small active power sources substituted for gravity, which can walk on level ground. These robots use less control and less energy than other powered robots, yet walk more naturally, further suggesting the importance of passive-dynamics in human locomotion.

    View details for DOI 10.1126/science.1107799

    View details for Web of Science ID 000227197300040

    View details for PubMedID 15718465

  • Lower limb biomechanics of fully trained exoskeleton users reveal complex mechanisms behind the reductions in energy cost with human-in-the-loop optimization. Frontiers in robotics and AI Poggensee, K. L., Collins, S. H. 2024; 11: 1283080


    Exoskeletons that assist in ankle plantarflexion can improve energy economy in locomotion. Characterizing the joint-level mechanisms behind these reductions in energy cost can lead to a better understanding of how people interact with these devices, as well as to improved device design and training protocols. We examined the biomechanical responses to exoskeleton assistance in exoskeleton users trained with a lengthened protocol. Kinematics at unassisted joints were generally unchanged by assistance, which has been observed in other ankle exoskeleton studies. Peak plantarflexion angle increased with plantarflexion assistance, which led to increased total and biological mechanical power despite decreases in biological joint torque and whole-body net metabolic energy cost. Ankle plantarflexor activity also decreased with assistance. Muscles that act about unassisted joints also increased activity for large levels of assistance, and this response should be investigated over long-term use to prevent overuse injuries.

    View details for DOI 10.3389/frobt.2024.1283080

    View details for PubMedID 38357293

    View details for PubMedCentralID PMC10864513

  • Optimizing exoskeleton assistance to improve walking speed and energy economy for older adults. Journal of neuroengineering and rehabilitation Lakmazaheri, A., Song, S., Vuong, B. B., Biskner, B., Kado, D. M., Collins, S. H. 2024; 21 (1): 1


    Walking speed and energy economy tend to decline with age. Lower-limb exoskeletons have demonstrated potential to improve either measure, but primarily in studies conducted on healthy younger adults. Promising techniques like optimization of exoskeleton assistance have yet to be tested with older populations, while speed and energy consumption have yet to be simultaneously optimized for any population.We investigated the effectiveness of human-in-the-loop optimization of ankle exoskeletons with older adults. Ten healthy adults > 65 years of age (5 females; mean age: 72 ± 3 yrs) participated in approximately 240 min of training and optimization with tethered ankle exoskeletons on a self-paced treadmill. Multi-objective human-in-the-loop optimization was used to identify assistive ankle plantarflexion torque patterns that simultaneously improved self-selected walking speed and metabolic rate. The effects of optimized exoskeleton assistance were evaluated in separate trials.Optimized exoskeleton assistance improved walking performance for older adults. Both objectives were simultaneously improved; self-selected walking speed increased by 8% (0.10 m/s; p = 0.001) and metabolic rate decreased by 19% (p = 0.007), resulting in a 25% decrease in energetic cost of transport (p = 8e-4) compared to walking with exoskeletons applying zero torque. Compared to younger participants in studies optimizing a single objective, our participants required lower exoskeleton torques, experienced smaller improvements in energy use, and required more time for motor adaptation.Our results confirm that exoskeleton assistance can improve walking performance for older adults and show that multiple objectives can be simultaneously addressed through human-in-the-loop optimization.

    View details for DOI 10.1186/s12984-023-01287-5

    View details for PubMedID 38167151

    View details for PubMedCentralID 6620279

  • 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


    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

  • 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


    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, 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

  • 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


    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, 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


    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

  • Trajectory and Sway Prediction Towards Fall Prevention. IEEE International Conference on Robotics and Automation : ICRA : [proceedings]. IEEE International Conference on Robotics and Automation Wang, W., Raitor, M., Collins, S., Liu, C. K., Kennedy, M. 2023; 2023: 10483-10489


    Falls are the leading cause of fatal and non-fatal injuries, particularly for older persons. Imbalance can result from the body's internal causes (illness), or external causes (active or passive perturbation). Active perturbation results from applying an external force to a person, while passive perturbation results from human motion interacting with a static obstacle. This work proposes a metric that allows for the monitoring of the persons torso and its correlation to active and passive perturbations. We show that large changes in the torso sway can be strongly correlated to active perturbations. We also show that we can reasonably predict the future path and expected change in torso sway by conditioning the expected path and torso sway on the past trajectory, torso motion, and the surrounding scene. This could have direct future applications to fall prevention. Results demonstrate that the torso sway is strongly correlated with perturbations. And our model is able to make use of the visual cues presented in the panorama and condition the prediction accordingly.

    View details for DOI 10.1109/icra48891.2023.10161361

    View details for PubMedID 38009123

    View details for PubMedCentralID PMC10671274

  • Trajectory and Sway Prediction Towards Fall Prevention Wang, W., Raitor, M., Collins, S., Liu, C., Kennedy, M. 2023: 10483-10489
  • Human Perception of Wrist Flexion and Extension Torque During Upper and Lower Extremity Movement. IEEE transactions on haptics Welker, C. G., Collins, S. H., Okamura, A. M. 2022; PP


    Real-world application of haptic feedback from kinesthetic devices is implemented while the user is in motion, but human wrist torque magnitude discrimination has previously only been characterized while users are stationary. In this study, we measured wrist torque discrimination in conditions relevant to activities of daily living, using a previously developed backdrivable wrist exoskeleton capable of applying wrist flexion and extension torque. We implemented a torque comparison test using a two-alternative forced-choice paradigm while participants were both seated and walking on a treadmill, with both a stationary and a moving wrist. Like most kinesthetic haptic devices, the wrist exoskeleton output torque is commanded in an open-loop manner. Thus, the study design was informed by Monte Carlo simulations to verify that the errors in the wrist exoskeleton output torque would not significantly affect the results. Results from ten participants show that although both walking and moving wrist conditions result in higher Weber Fractions (worse perception), participants were able to detect relatively small changes in torque of 12-19% on average in all grouped conditions. The results provide insight regarding the torque magnitudes necessary to make wrist-worn kinesthetic haptic devices noticeable and meaningful to the user in various conditions relevant to activities of daily living.

    View details for DOI 10.1109/TOH.2022.3219031

    View details for PubMedID 36343009

  • Robotic Emulation of Candidate Prosthetic Foot Designs May Enable Efficient, Evidence-Based, and Individualized Prescriptions. Journal of prosthetics and orthotics : JPO Caputo, J. M., Dvorak, E., Shipley, K., Miknevich, M. A., Adamczyk, P. G., Collins, S. H. 2022; 34 (4): 202-212


    Introduction: The design and selection of lower-limb prosthetic devices is currently hampered by a shortage of evidence to drive the choice of prosthetic foot parameters. We propose a new approach wherein prostheses could be designed, specified, and provided based on individualized measurements of the benefits provided by candidate feet. In this manuscript, we present a pilot test of this evidence-based and personalized process.Methods: We previously developed a "prosthetic foot emulator," a wearable robotic system that provides users with the physical sensation of trying on different prosthetic feet before definitive fitting. Here we detail preliminary demonstrations of two possible approaches to personalizing foot design: 1) an emulation and test-drive strategy of representative commercial foot models, and 2) a prosthetist-driven tuning procedure to optimize foot parameters.Results: The first experiment demonstrated large and sometimes surprising differences in optimal prosthetic foot parameters across a variety of subjects, walking conditions, and outcome measures. The second experiment demonstrated a quick and effective simple manual tuning procedure for identifying preferred prosthetic foot parameters.Conclusions: Emulator-based approaches could improve individualization of prosthetic foot prescription. The present results motivate future clinical studies of the validity, efficacy, and economics of the approach across larger and more diverse subject populations.Clinical Relevance: Today, emulator technology is being used to accelerate research and development of novel prosthetic and orthotic devices. In the future, after further refinement and validation, this technology could benefit clinical practice by providing a means for rapid test-driving and optimal selection of clinically available prosthetic feet.

    View details for DOI 10.1097/JPO.0000000000000409

    View details for PubMedID 36157327

  • The Effects of Incline Level on Optimized Lower-Limb Exoskeleton Assistance: a Case Series. IEEE transactions on neural systems and rehabilitation engineering : a publication of the IEEE Engineering in Medicine and Biology Society Franks, P. W., Bryan, G. M., Reyes, R., O'Donovan, M. P., Gregorczyk, K. N., Collins, S. H. 2022; PP


    For exoskeletons to be successful in real-world settings, they will need to be effective across a variety of terrains, including on inclines. While some single-joint exoskeletons have assisted incline walking, recent successes in level-ground assistance suggest that greater improvements may be possible by optimizing assistance of the whole leg. To understand how exoskeleton assistance should change with incline, we used human-in-the-loop optimization to find whole-leg exoskeleton assistance torques that minimized metabolic cost on a range of grades. We optimized assistance for three able-bodied, expert participants on 5 degree, 10 degree, and 15 degree inclines using a hip-knee-ankle exoskeleton emulator. For all assisted conditions, the cost of transport was reduced by at least 50% relative to walking in the device with no assistance, which is a large improvement to walking comparable to the benefits of whole-leg assistance on level-ground (N = 3). Optimized extension torque magnitudes and exoskeleton power increased with incline. Hip extension, knee extension and ankle plantarflexion often grew as large as allowed by comfort-based limits. Applied powers on steep inclines were double the powers applied during level-ground walking, indicating that greater exoskeleton power may be optimal in scenarios where biological powers and costs are higher. Future exoskeleton devices could deliver large improvements in walking performance across a range of inclines if they have sufficient torque and power capabilities.

    View details for DOI 10.1109/TNSRE.2022.3196665

    View details for PubMedID 35930513

  • The split-belt rimless wheel INTERNATIONAL JOURNAL OF ROBOTICS RESEARCH Butterfield, J. K., Simha, S. N., Donelan, J., Collins, S. H. 2022
  • Characterizing the relationship between peak assistance torque and metabolic cost reduction during running with ankle exoskeletons. Journal of neuroengineering and rehabilitation Miller, D. E., Tan, G. R., Farina, E. M., Sheets-Singer, A. L., Collins, S. H. 2022; 19 (1): 46


    BACKGROUND: Reducing the energy cost of running with exoskeletons could improve enjoyment, reduce fatigue, and encourage participation among novice and ageing runners. Previously, tethered ankle exoskeleton emulators with offboard motors were used to greatly reduce the energy cost of running with powered ankle plantarflexion assistance. Through a process known as "human-in-the-loop optimization", the timing and magnitude of assistance torque was optimized to maximally reduce metabolic cost. However, to achieve the maximum net benefit in energy cost outside of the laboratory environment, it is also necessary to consider the tradeoff between the magnitude of device assistance and the metabolic penalty of carrying a heavier, more powerful exoskeleton.METHODS: In this study, tethered ankle exoskeleton emulators were used to characterize the effect of peak assistance torque on metabolic cost during running. Three recreational runners participated in human-in-the-loop optimization at four fixed peak assistance torque levels to obtain their energetically optimal assistance timing parameters at each level.RESULTS: We found that the relationship between metabolic rate and peak assistance torque was nearly linear but with diminishing returns at higher torque magnitudes, which is well-approximated by an asymptotic exponential function. At the highest assistance torque magnitude of 0.8 Nm/kg, participants' net metabolic rate was 24.8 ± 2.3% (p = 4e-6) lower than running in the unpowered devices. Optimized timing of peak assistance torque was as late as allowed during stance (80% of stance) and optimized timing of torque removal was at toe-off (100% of stance); similar assistance timing was preferred across participants and torque magnitudes.CONCLUSIONS: These results allow exoskeleton designers to predict the energy cost savings for candidate devices with different assistance torque capabilities, thus informing the design of portable ankle exoskeletons that maximize net metabolic benefit.

    View details for DOI 10.1186/s12984-022-01023-5

    View details for PubMedID 35549977

  • General variability leads to specific adaptation toward optimal movement policies. Current biology : CB Abram, S. J., Poggensee, K. L., Sanchez, N., Simha, S. N., Finley, J. M., Collins, S. H., Donelan, J. M. 2022


    Our nervous systems can learn optimal control policies in response to changes to our bodies, tasks, and movement contexts. For example, humans can learn to adapt their control policy in walking contexts where the energy-optimal policy is shifted along variables such as step frequency or step width. However, it is unclear how the nervous system determines which ways to adapt its control policy. Here, we asked how human participants explore through variations in their control policy to identify more optimal policies in new contexts. We created new contexts using exoskeletons that apply assistive torques to each ankle at each walking step. We analyzed four variables that spanned the levels of the whole movement, the joint, and the muscle: step frequency, ankle angle range, total soleus activity, and total medial gastrocnemius activity. We found that, across all of these analyzed variables, variability increased upon initial exposure to new contexts and then decreased with experience. This led to adaptive changes in the magnitude of specific variables, and these changes were correlated with reduced energetic cost. The timescales by which adaptive changes progressed and variability decreased were faster for some variables than others, suggesting a reduced search space within which the nervous system continues to optimize its policy. These collective findings support the principle that exploration through general variability leads to specific adaptation toward optimal movement policies.

    View details for DOI 10.1016/j.cub.2022.04.015

    View details for PubMedID 35537453

  • The energy cost of split-belt walking for a variety of belt speed combinations. Journal of biomechanics Butterfield, J. K., Collins, S. H. 2022; 132: 110905


    Walking on a split-belt treadmill is often compared to walking on tied belts at the average speed, but the relationship between the metabolic energy costs of split- and tied-belt walking remains largely unexplored. Recent simulation work has suggested that people could take advantage of a belt speed difference and lower their energy costs, but this effect has not yet been observed experimentally. To relate metabolic energy costs across a range of belt speed combinations, we had 10 participants each complete 14 tied-belt and 39 split-belt walking trials, with early split-belt trials incorporating additional time for training. The average speeds ranged from 0.6 to 1.8 m/s with belt speed differences up to 1.4 m/s. We used ANOVA to determine differences in energy cost due to average speed and belt speed difference. We fit a linear model to estimate the tied-belt speed with the same energy cost as a given pair of split belt speeds. The cost of split-belt walking was on average just 0.13 ± 0.32 W/kg more expensive than the cost of tied-belt walking at the average speed. Contrary to predictions from simple dynamical models, increased belt speed difference resulted in increased energy cost, and the energetically equivalent tied-belt speed could be estimated as veq=vavg+0.065⋅Δv. Clinicians designing rehabilitation protocols can balance the therapeutic benefits of higher belt speed difference with increased energetic demands. Open questions remain about why people cannot fully take advantage of mechanical work performed by a split-belt treadmill.

    View details for DOI 10.1016/j.jbiomech.2021.110905

    View details for PubMedID 34998181

  • Five years of Science Robotics. Science robotics Yang, G., Collins, S. H., Dario, P., Fischer, P., Goldberg, K., Laschi, C., McNutt, M. K. 1800; 6 (61): eabn2720


    [Figure: see text].

    View details for DOI 10.1126/scirobotics.abn2720

    View details for PubMedID 34910531

  • Comparing optimized exoskeleton assistance of the hip, knee, and ankle in single and multi-joint configurations. Wearable technologies Franks, P. W., Bryan, G. M., Martin, R. M., Reyes, R., Lakmazaheri, A. C., Collins, S. H. 2021; 2: e16


    Exoskeletons that assist the hip, knee, and ankle joints have begun to improve human mobility, particularly by reducing the metabolic cost of walking. However, direct comparisons of optimal assistance of these joints, or their combinations, have not yet been possible. Assisting multiple joints may be more beneficial than the sum of individual effects, because muscles often span multiple joints, or less effective, because single-joint assistance can indirectly aid other joints. In this study, we used a hip-knee-ankle exoskeleton emulator paired with human-in-the-loop optimization to find single-joint, two-joint, and whole-leg assistance that maximally reduced the metabolic cost of walking. Hip-only and ankle-only assistance reduced the metabolic cost of walking by 26 and 30% relative to walking in the device unassisted, confirming that both joints are good targets for assistance (N = 3). Knee-only assistance reduced the metabolic cost of walking by 13%, demonstrating that effective knee assistance is possible (N = 3). Two-joint assistance reduced the metabolic cost of walking by between 33 and 42%, with the largest improvements coming from hip-ankle assistance (N = 3). Assisting all three joints reduced the metabolic cost of walking by 50%, showing that at least half of the metabolic energy expended during walking can be saved through exoskeleton assistance (N = 4). Changes in kinematics and muscle activity indicate that single-joint assistance indirectly assisted muscles at other joints, such that the improvement from whole-leg assistance was smaller than the sum of its single-joint parts. Exoskeletons can assist the entire limb for maximum effect, but a single well-chosen joint can be more efficient when considering additional factors such as weight and cost.

    View details for DOI 10.1017/wtc.2021.14

    View details for PubMedID 38486633

    View details for PubMedCentralID PMC10936256

  • Optimized hip-knee-ankle exoskeleton assistance reduces the metabolic cost of walking with worn loads. Journal of neuroengineering and rehabilitation Bryan, G. M., Franks, P. W., Song, S., Reyes, R., O'Donovan, M. P., Gregorczyk, K. N., Collins, S. H. 2021; 18 (1): 161


    BACKGROUND: Load carriage is common in a wide range of professions, but prolonged load carriage is associated with increased fatigue and overuse injuries. Exoskeletons could improve the quality of life of these professionals by reducing metabolic cost to combat fatigue and reducing muscle activity to prevent injuries. Current exoskeletons have reduced the metabolic cost of loaded walking by up to 22% relative to walking in the device with no assistance when assisting one or two joints. Greater metabolic reductions may be possible with optimized assistance of the entire leg.METHODS: We used human-in the-loop optimization to optimize hip-knee-ankle exoskeleton assistance with no additional load, a light load (15% of body weight), and a heavy load (30% of body weight) for three participants. All loads were applied through a weight vest with an attached waist belt. We measured metabolic cost, exoskeleton assistance, kinematics, and muscle activity. We performed Friedman's tests to analyze trends across worn loads and paired t-tests to determine whether changes from the unassisted conditions to the assisted conditions were significant.RESULTS: Exoskeleton assistance reduced the metabolic cost of walking relative to walking in the device without assistance for all tested conditions. Exoskeleton assistance reduced the metabolic cost of walking by 48% with no load (p = 0.05), 41% with the light load (p = 0.01), and 43% with the heavy load (p = 0.04). The smaller metabolic reduction with the light load may be due to insufficient participant training or lack of optimizer convergence. The total applied positive power was similar for all tested conditions, and the positive knee power decreased slightly as load increased. Optimized torque timing parameters were consistent across participants and load conditions while optimized magnitude parameters varied.CONCLUSIONS: Whole-leg exoskeleton assistance can reduce the metabolic cost of walking while carrying a range of loads. The consistent optimized timing parameters across participants and conditions suggest that metabolic cost reductions are sensitive to torque timing. The variable torque magnitude parameters could imply that torque magnitude should be customized to the individual, or that there is a range of useful torque magnitudes. Future work should test whether applying the load to the exoskeleton rather than the person's torso results in larger benefits.

    View details for DOI 10.1186/s12984-021-00955-8

    View details for PubMedID 34743714

  • Optimized hip-knee-ankle exoskeleton assistance at a range of walking speeds. Journal of neuroengineering and rehabilitation Bryan, G. M., Franks, P. W., Song, S., Voloshina, A. S., Reyes, R., O'Donovan, M. P., Gregorczyk, K. N., Collins, S. H. 2021; 18 (1): 152


    BACKGROUND: Autonomous exoskeletons will need to be useful at a variety of walking speeds, but it is unclear how optimal hip-knee-ankle exoskeleton assistance should change with speed. Biological joint moments tend to increase with speed, and in some cases, optimized ankle exoskeleton torques follow a similar trend. Ideal hip-knee-ankle exoskeleton torque may also increase with speed. The purpose of this study was to characterize the relationship between walking speed, optimal hip-knee-ankle exoskeleton assistance, and the benefits to metabolic energy cost.METHODS: We optimized hip-knee-ankle exoskeleton assistance to reduce metabolic cost for three able-bodied participants walking at 1.0 m/s, 1.25 m/s and 1.5 m/s. We measured metabolic cost, muscle activity, exoskeleton assistance and kinematics. We performed Friedman's tests to analyze trends across walking speeds and paired t-tests to determine if changes from the unassisted conditions to the assisted conditions were significant.RESULTS: Exoskeleton assistance reduced the metabolic cost of walking compared to wearing the exoskeleton with no torque applied by 26%, 47% and 50% at 1.0, 1.25 and 1.5 m/s, respectively. For all three participants, optimized exoskeleton ankle torque was the smallest for slow walking, while hip and knee torque changed slightly with speed in ways that varied across participants. Total applied positive power increased with speed for all three participants, largely due to increased joint velocities, which consistently increased with speed.CONCLUSIONS: Exoskeleton assistance is effective at a range of speeds and is most effective at medium and fast walking speeds. Exoskeleton assistance was less effective for slow walking, which may explain the limited success in reducing metabolic cost for patient populations through exoskeleton assistance. Exoskeleton designers may have more success when targeting activities and groups with faster walking speeds. Speed-related changes in optimized exoskeleton assistance varied by participant, indicating either the benefit of participant-specific tuning or that a wide variety of torque profiles are similarly effective.

    View details for DOI 10.1186/s12984-021-00943-y

    View details for PubMedID 34663372

  • How adaptation, training, and customization contribute to benefits from exoskeleton assistance. Science robotics Poggensee, K. L., Collins, S. H. 2021; 6 (58): eabf1078


    [Figure: see text].

    View details for DOI 10.1126/scirobotics.abf1078

    View details for PubMedID 34586837

  • Shortcomings of human-in-the-loop optimization of an ankle-foot prosthesis emulator: a case series. Royal Society open science Welker, C. G., Voloshina, A. S., Chiu, V. L., Collins, S. H. 2021; 8 (5): 202020


    Human-in-the-loop optimization allows for individualized device control based on measured human performance. This technique has been used to produce large reductions in energy expenditure during walking with exoskeletons but has not yet been applied to prosthetic devices. In this series of case studies, we applied human-in-the-loop optimization to the control of an active ankle-foot prosthesis used by participants with unilateral transtibial amputation. We optimized the parameters of five control architectures that captured aspects of successful exoskeletons and commercial prostheses, but none resulted in significantly lower metabolic rate than generic control. In one control architecture, we increased the exposure time per condition by a factor of five, but the optimized controller still resulted in higher metabolic rate. Finally, we optimized for self-reported comfort instead of metabolic rate, but the resulting controller was not preferred. There are several reasons why human-in-the-loop optimization may have failed for people with amputation. Control architecture is an unlikely cause given the variety of controllers tested. The lack of effect likely relates to changes in motor adaptation, learning, or objectives in people with amputation. Future work should investigate these potential causes to determine whether human-in-the-loop optimization for prostheses could be successful.

    View details for DOI 10.1098/rsos.202020

    View details for PubMedID 34035945

    View details for PubMedCentralID PMC8097204

  • The effects of ground-irregularity-cancelling prosthesis control on balance over uneven surfaces. Royal Society open science Chiu, V. L., Voloshina, A. S., Collins, S. H. 2021; 8 (1): 201235


    Over half of individuals with a lower-limb amputation are unable to walk on uneven terrain. Using a prosthesis emulator system, we developed an irregularity-cancelling controller intended to reduce the effect of disturbances resulting from uneven surfaces. This controller functions by changing the neutral angles of two forefoot digits in response to local terrain heights. To isolate the effects of the controller, we also programmed a spring-like controller that maintained fixed neutral angles. Five participants with transtibial amputation walked on a treadmill with an uneven walking surface. Compared with the spring-like controller, the irregularity-cancelling controller reduced ankle torque variability by 41% in the sagittal plane and 64% in the frontal plane. However, user outcomes associated with balance were mostly unaffected; only trunk movement variability was reduced, whereas metabolic rate, mediolateral centre of mass motion, and variabilities in step width, step length and step time were unchanged. We conclude that reducing ankle torque variability of the affected limb is not sufficient for reducing the overall effect of disturbances due to uneven terrain. It is possible that other factors, such as changes in step height or disturbances to the intact limb, play a larger role in difficulty balancing while walking over uneven surfaces.

    View details for DOI 10.1098/rsos.201235

    View details for PubMedID 33614071

    View details for PubMedCentralID PMC7890502

  • Optimizing Exoskeleton Assistance for Faster Self-Selected Walking IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING Song, S., Collins, S. H. 2021; 29: 786-795


    Self-selected walking speed is an important aspect of mobility. Exoskeletons can increase walking speed, but the mechanisms behind these changes and the upper limits on performance are unknown. Human-in-the-loop optimization is a technique for identifying exoskeleton characteristics that maximize the benefits of assistance, which has been critical to achieving large improvements in energy economy. In this study, we used human-in-the-loop optimization to test whether large improvements in self-selected walking speed are possible through ankle exoskeleton assistance. Healthy participants (N =10) were instructed to walk at a comfortable speed on a self-paced treadmill while wearing tethered ankle exoskeletons. An algorithm sequentially applied different patterns of exoskeleton torque and estimated the speed-optimal pattern, which was then evaluated in separate trials. With torque optimized for speed, participants walked 42% faster than in normal shoes (1.83 ms-1 vs. 1.31 ms-1; Tukey HSD, p = 4 ×10-8 ), with speed increases ranging from 6% to 91%. Participants walked faster with speed-optimized torque than with torque optimized for energy consumption (1.55 ms-1) or torque chosen to induce slow walking (1.18 ms-1). Gait characteristics with speed-optimized torque were highly variable across participants, and changes in metabolic cost of transport ranged from a 31% decrease to a 78% increase, with a decrease of 2% on average. These results demonstrate that ankle exoskeletons can facilitate large increases in self-selected walking speed, which could benefit older adults and others with reduced walking speed.

    View details for DOI 10.1109/TNSRE.2021.3074154

    View details for Web of Science ID 000647325200001

    View details for PubMedID 33877982

  • The Iterative Learning Gain That Optimizes Real-Time Torque Tracking for Ankle Exoskeletons in Human Walking Under Gait Variations. Frontiers in neurorobotics Zhang, J., Collins, S. H. 2021; 15: 653409


    Lower-limb exoskeletons often use torque control to manipulate energy flow and ensure human safety. The accuracy of the applied torque greatly affects how well the motion is assisted and therefore improving it is always of interest. Feed-forward iterative learning, which is similar to predictive stride-wise integral control, has proven an effective compensation to feedback control for torque tracking in exoskeletons with complicated dynamics during human walking. Although the intention of iterative learning was initially to benefit average tracking performance over multiple strides, we found that, after proper gain tuning, it can also help improve real-time torque tracking. We used theoretical analysis to predict an optimal iterative-learning gain as the inverse of the passive actuator stiffness. Walking experiments resulted in an optimum gain equal to 0.99 ± 0.38 times the predicted value, confirming our hypothesis. The results of this study provide guidance for the design of torque controllers in robotic legged locomotion systems and will help improve the performance of robots that assist gait.

    View details for DOI 10.3389/fnbot.2021.653409

    View details for PubMedID 34122032

  • Human Perception of Wrist Torque Magnitude During Upper and Lower Extremity Movement Welker, C., Collins, S. H., Okamura, A. M., IEEE IEEE. 2021: 870
  • Comparing optimized exoskeleton assistance of the hip, knee, and ankle in single and multi-joint configurations Wearable Technologies Franks, P. W., Bryan, G. M., Martin, R. M., Reyes, R., Lakmazaheri, A. C., Collins, S. H. 2021; 2

    View details for DOI 10.1017/wtc.2021.14

  • Weighted Shoes in the Wild: Initial Insights into the Relationship Between the Effort of Walking and the Amount of Walking Performed Wu, M. R., Adamczyk, P. G., Collins, S. H. bioRxiv. 2021
  • Design of a hip exoskeleton with actuation in frontal and sagittal planes Transactions on Medical Robotics and Bionics Chiu, V., Raitor, M., Collins, S. H. 2021; 3: 773-782
  • Robotic emulation of candidate prosthetic foot designs may enable efficient, evidence-based, and individualized prescriptions Journal of Prosthetics and Orthotics Caputo, J. M., Dvorak, E., Shipley, K., Miknevich, M., Adamczyk, P. G., Collins, S. H. 2021: 11 pages
  • 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


    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 hip–knee–ankle exoskeleton emulator for studying gait assistance The International Journal of Robotics Research Bryan, G. M., Franks, P. W., Klein, S. C., Peuchen, R. J., Collins, S. H. 2021

    View details for DOI 10.1177/0278364920961452

  • Self-selected step length asymmetry is not explained by energy cost minimization in individuals with chronic stroke. Journal of neuroengineering and rehabilitation Nguyen, T. M., Jackson, R. W., Aucie, Y., de Kam, D., Collins, S. H., Torres-Oviedo, G. 2020; 17 (1): 119


    BACKGROUND: Asymmetric gait post-stroke is associated with decreased mobility, yet individuals with chronic stroke often self-select an asymmetric gait despite being capable of walking more symmetrically. The purpose of this study was to test whether self-selected asymmetry could be explained by energy cost minimization. We hypothesized that short-term deviations from self-selected asymmetry would result in increased metabolic energy consumption, despite being associated with long-term rehabilitation benefits. Other studies have found no difference in metabolic rate across different levels of enforced asymmetry among individuals with chronic stroke, but used methods that left some uncertainty to be resolved.METHODS: In this study, ten individuals with chronic stroke walked on a treadmill at participant-specific speeds while voluntarily altering step length asymmetry. We included only participants with clinically relevant self-selected asymmetry who were able to significantly alter asymmetry using visual biofeedback. Conditions included targeting zero asymmetry, self-selected asymmetry, and double the self-selected asymmetry. Participants were trained with the biofeedback system in one session, and data were collected in three subsequent sessions with repeated measures. Self-selected asymmetry was consistent across sessions. A similar protocol was conducted among unimpaired participants.RESULTS: Participants with chronic stroke substantially altered step length asymmetry using biofeedback, but this did not affect metabolic rate (ANOVA, p=0.68). In unimpaired participants, self-selected step length asymmetry was close to zero and corresponded to the lowest metabolic energy cost (ANOVA, p=6e-4). While the symmetry of unimpaired gait may be the result of energy cost minimization, self-selected step length asymmetry in individuals with chronic stroke cannot be explained by a similar least-effort drive.CONCLUSIONS: Interventions that encourage changes in step length asymmetry by manipulating metabolic energy consumption may be effective because these therapies would not have to overcome a metabolic penalty for altering asymmetry.

    View details for DOI 10.1186/s12984-020-00733-y

    View details for PubMedID 32847596

  • Prosthesis Inversion/Eversion Stiffness Reduces Balance-Related Variability During Level Walking. Journal of biomechanical engineering Kim, M., Lyness, H., Chen, T., Collins, S. 2020


    Prosthesis features that enhance balance are desirable to people with below-knee amputation. Ankle inversion/eversion compliance is intended to improve balance on uneven ground, but its effects remain unclear on level ground. We posited that increasing ankle inversion/eversion stiffness during level-ground walking would reduce balance-related effort by assisting in recovery from small disturbances in frontal-plane motions. We performed tests with an ankle-foot prosthesis emulator programmed to apply inversion/eversion torques in proportion to the deviation from a nominal inversion/eversion position trajectory. We applied a range of stiffnesses, hypothesizing that positive stiffness would reduce effort while negative stiffness would increase effort. Nominal joint angle trajectories were calculated online as a moving average over several steps. In experiments with K3 ambulators with unilateral transtibial amputation (N = 5), stiffness affected step width variability, average step width, margin of stability, intact-foot center of pressure variability, and user satisfaction (p < 0.05, Friedman's test), but not intact-limb evertor average, intact-limb evertor variability, and metabolic rate (p > 0.38, Friedman's test). Compared to zero stiffness, high positive stiffness reduced step width variability by 13%, step width by 3%, margin of stability by 3%, and intact-foot center of pressure variability by 14%, whereas high negative stiffness had opposite effects and decreased satisfaction by 63%. Positive ankle inversion stiffness seems to reduce active control requirements during level walking.

    View details for DOI 10.1115/1.4046881

    View details for PubMedID 32280955

  • Combating COVID-19-The role of robotics in managing public health and infectious diseases SCIENCE ROBOTICS Yang, G., Nelson, B. J., Murphy, R. R., Choset, H., Christensen, H., Collins, S. H., Dario, P., Goldberg, K., Ikuta, K., Jacobstein, N., Kragic, D., Taylor, R. H., McNutt, M. 2020; 5 (40)
  • Improving the energy economy of human running with powered and unpowered ankle exoskeleton assistance. Science robotics Witte, K. A., Fiers, P., Sheets-Singer, A. L., Collins, S. H. 2020; 5 (40)


    Exoskeletons that reduce energetic cost could make recreational running more enjoyable and improve running performance. Although there are many ways to assist runners, the best approaches remain unclear. In our study, we used a tethered ankle exoskeleton emulator to optimize both powered and spring-like exoskeleton characteristics while participants ran on a treadmill. We expected powered conditions to provide large improvements in energy economy and for spring-like patterns to provide smaller benefits achievable with simpler devices. We used human-in-the-loop optimization to attempt to identify the best exoskeleton characteristics for each device type and individual user, allowing for a well-controlled comparison. We found that optimized powered assistance improved energy economy by 24.7 ± 6.9% compared with zero torque and 14.6 ± 7.7% compared with running in normal shoes. Optimized powered torque patterns for individuals varied substantially, but all resulted in relatively high mechanical work input (0.36 ± 0.09 joule kilogram-1 per step) and late timing of peak torque (75.7 ± 5.0% stance). Unexpectedly, spring-like assistance was ineffective, improving energy economy by only 2.1 ± 2.4% compared with zero torque and increasing metabolic rate by 11.1 ± 2.8% compared with control shoes. The energy savings we observed imply that running velocity could be increased by as much as 10% with no added effort for the user and could influence the design of future products.

    View details for DOI 10.1126/scirobotics.aay9108

    View details for PubMedID 33022600

  • Bump'em: an Open-Source, Bump-Emulation System for Studying Human Balance and Gait Tan, G., Raitor, M., Collins, S. H., IEEE IEEE. 2020: 9093-9099
  • Testing Simulated Assistance Strategies on a Hip-Knee-Ankle Exoskeleton: a Case Study International Conference on Biomedical Robotics and Biomechatronics Franks, P. W., Bianco, N. A., Bryan, G. M., Hicks, J. L., Delp, S. L., Collins, S. H. 2020: 700–707
  • Optimal Control of an Energy-Recycling Actuator for Mobile Robotics Applications International Conference on Robotics and Automation Krimsky, E., Collins, S. H. 2020: 3559–3565
  • Teleoperation of an ankle-foot prosthesis with a wrist exoskeleton. IEEE transactions on bio-medical engineering Welker, C. G., Chiu, V. L., Voloshina, A. n., Collins, S. n., Okamura, A. M. 2020; PP


    We aimed to develop a system for people with amputation that non-invasively restores missing control and sensory information for an ankle-foot prosthesis.In our approach, a wrist exoskeleton allows people with amputation to control and receive feedback from their prosthetic ankle via teleoperation. We implemented two control schemes: position control with haptic feedback of ankle torque at the wrist; and torque control that allows the user to modify a baseline torque profile by moving their wrist against a virtual spring. We measured tracking error and frequency response for the ankle-foot prosthesis and the wrist exoskeleton. To demonstrate feasibility and evaluate system performance, we conducted an experiment in which one participant with a transtibial amputation tracked desired wrist trajectories during walking, while we measured wrist and ankle response.Benchtop testing demonstrated that for relevant walking frequencies, system error was below human perceptual error. During the walking experiment, the participant was able to voluntarily follow different wrist trajectories with an average RMS error of 1.55° after training. The ankle was also able to track desired trajectories below human perceptual error for both position control (RMSE = 0.8°) and torque control (RMSE = 8.4%).We present a system that allows a user with amputation to control an ankle-foot prosthesis and receive feedback about its state using a wrist exoskeleton, with accuracy comparable to biological neuromotor control.This bilateral teleoperation system enables novel prosthesis control and feedback strategies that could improve prosthesis control and aid motor learning.

    View details for DOI 10.1109/TBME.2020.3046357

    View details for PubMedID 33347402

  • Bump’em: an Open-Source, Bump-Emulation System for Studying Human Balance and Gait International Conference on Robotics and Automation Tan, G. R., Raitor, M., Collins, S. H. 2020: 9093–9099
  • An Ankle-Foot Prosthesis Emulator Capable of Modulating Center of Pressure IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Chiu, V. L., Voloshina, A. S., Collins, S. H. 2020; 67 (1): 166–76


    Several powered ankle-foot prostheses have demonstrated moderate reductions in energy expenditure by restoring pushoff work in late stance or by assisting with balance. However, it is possible that center of pressure trajectory modulation could provide even further improvements in user performance. Here, we describe the design of a prosthesis emulator with two torque-controlled forefoot digits and a torque-controlled heel digit. Independent actuation of these three digits can modulate the origin and magnitude of the total ground reaction force vector.The emulator was designed to be compact and lightweight while exceeding the range of motion and torque requirements of the biological ankle during walking. We ran a series of tests to determine torque-measurement accuracy, closed-loop torque control bandwidth, torque-tracking error, and center of pressure control accuracy.Each of the three digits demonstrated less than 2 Nm of RMS torque measurement error, a 90% rise time of 19 ms, and a bandwidth of 33 Hz. The untethered end-effector has a mass of 1.2 kg. During walking trials, the emulator demonstrated less than 2 Nm of RMS torque-tracking error and was able to maintain full digit ground contact for 56% of stance. In fixed, standing, and walking conditions, the emulator was able to control center of pressure along a prescribed pattern with RMS errors of about 10% the length of the pattern.The proposed emulator system meets all design criteria and can effectively modulate center of pressure and ground reaction force magnitude.This emulator system will enable rapid development of controllers designed to enhance user balance and reduce user energy expenditure. Experiments conducted using this emulator could identify beneficial control behaviors that can be implemented on autonomous devices, thus improving mobility and quality of life of individuals with amputation.

    View details for DOI 10.1109/TBME.2019.2910071

    View details for Web of Science ID 000505526300016

    View details for PubMedID 30969914

  • Using force data to self-pace an instrumented treadmill and measure self-selected walking speed. Journal of neuroengineering and rehabilitation Song, S. n., Choi, H. n., Collins, S. H. 2020; 17 (1): 68


    Self-selected speed is an important functional index of walking. A self-pacing controller that reliably matches walking speed without additional hardware can be useful for measuring self-selected speed in a treadmill-based laboratory.We adapted a previously proposed self-pacing controller for force-instrumented treadmills and validated its use for measuring self-selected speeds. We first evaluated the controller's estimation of subject speed and position from the force-plates by comparing it to those from motion capture data. We then compared five tests of self-selected speed. Ten healthy adults completed a standard 10-meter walk test, a 150-meter walk test, a commonly used manual treadmill speed selection test, a two-minute self-paced treadmill test, and a 150-meter self-paced treadmill test. In each case, subjects were instructed to walk at or select their comfortable speed. We also assessed the time taken for a trial and a survey on comfort and ease of choosing a speed in all the tests.The self-pacing algorithm estimated subject speed and position accurately, with root mean square differences compared to motion capture of 0.023 m s -1 and 0.014 m, respectively. Self-selected speeds from both self-paced treadmill tests correlated well with those from the 10-meter walk test (R>0.93,p<1×10-13). Subjects walked slower on average in the self-paced treadmill tests (1.23±0.27 ms-1) than in the 10-meter walk test (1.32±0.18 ms-1) but the speed differences within subjects were consistent. These correlations and walking speeds are comparable to those from the manual treadmill speed selection test (R=0.89,p=3×10-11;1.18±0.24 ms-1). Comfort and ease of speed selection were similar in the self-paced tests and the manual speed selection test, but the self-paced tests required only about a third of the time to complete. Our results demonstrate that these self-paced treadmill tests can be a strong alternative to the commonly used manual treadmill speed selection test.The self-paced force-instrumented treadmill well adapts to subject walking speed and reliably measures self-selected walking speeds. We provide the self-pacing software to facilitate use by gait researchers and clinicians.

    View details for DOI 10.1186/s12984-020-00683-5

    View details for PubMedID 32493426

  • Heuristic-Based Ankle Exoskeleton Control for Co-Adaptive Assistance of Human Locomotion IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING Jackson, R. W., Collins, S. H. 2019; 27 (10): 2059–69


    Assisting human locomotion with exoskeletons is challenging, largely due to the complexity of the neuromusculoskeletal system, the time-varying dynamics that accompany motor learning, and the uniqueness of every individual's response to device assistance. Assistance strategies designed to keep the human "in-the-loop" can help overcome many of these challenges. The purpose of this study was to develop a human-in-the-loop assistance strategy that uses co-adaptive control to slowly and continuously respond to biomechanical changes thought to encode the user's needs. Online measurements of muscle activity and joint kinematics were used to guide the evolution of an exoskeleton torque pattern based on the following heuristics: 1) muscle activity that acts cooperatively with the exoskeleton indicates the user wants more torque; 2) muscle activity that acts antagonistically to the exoskeleton indicates the user wants less torque; and 3) torque should stop increasing if the user is not adapting. We applied our controller to tethered, bilateral ankle exoskeletons worn by naïve participants as they walked on a treadmill at 1.25 m · s-1 for 30 minutes. The evolved torque profiles reduced the root-mean-square of soleus muscle activity by 35±12% and metabolic rate by 22±8% compared to walking with the exoskeletons while they provided no torque. This was equivalent to a 9±12% reduction in metabolic rate when compared to normal walking. Furthermore, the algorithm was responsive to changes in each user's coordination patterns. These results confirm the effectiveness of the controller and suggest a new approach to exoskeleton assistance aimed at fostering co-adaptation with the user. This technique might particularly benefit individuals with age-related muscle weakness.

    View details for DOI 10.1109/TNSRE.2019.2936383

    View details for Web of Science ID 000497685900013

    View details for PubMedID 31425120

  • 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


    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

  • Design of lower-limb exoskeletons and emulator systems Wearable Robotics: Systems and Applications Witte, K. A., Collins, S. H. 2019: 251–274
  • 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


    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

  • A review of design and control approaches in lower-limb prosthetic devices Wearable Robotics: Systems and Applications Voloshina, A. S., Collins, S. H. 2019: 469–486
  • The effects of electroadhesive clutch design parameters on performance characteristics JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES Diller, S. B., Collins, S. H., Majidi, C. 2018; 29 (19): 3804–28
  • An Ankle-Foot Prosthesis Emulator With Control of Plantarflexion and Inversion-Eversion Torque IEEE TRANSACTIONS ON ROBOTICS Kim, M., Chen, T., Chen, T., Collins, S. H. 2018; 34 (5): 1183–94
  • The Passive Series Stiffness That Optimizes Torque Tracking for a Lower-Limb Exoskeleton in Human Walking FRONTIERS IN NEUROROBOTICS Zhang, J., Collins, S. H. 2017; 11: 68


    This study uses theory and experiments to investigate the relationship between the passive stiffness of series elastic actuators and torque tracking performance in lower-limb exoskeletons during human walking. Through theoretical analysis with our simplified system model, we found that the optimal passive stiffness matches the slope of the desired torque-angle relationship. We also conjectured that a bandwidth limit resulted in a maximum rate of change in torque error that can be commanded through control input, which is fixed across desired and passive stiffness conditions. This led to hypotheses about the interactions among optimal control gains, passive stiffness and desired quasi-stiffness. Walking experiments were conducted with multiple angle-based desired torque curves. The observed lowest torque tracking errors identified for each combination of desired and passive stiffnesses were shown to be linearly proportional to the magnitude of the difference between the two stiffnesses. The proportional gains corresponding to the lowest observed errors were seen inversely proportional to passive stiffness values and to desired stiffness. These findings supported our hypotheses, and provide guidance to application-specific hardware customization as well as controller design for torque-controlled robotic legged locomotion.

    View details for PubMedID 29326580

  • Step-to-Step Ankle Inversion/Eversion Torque Modulation Can Reduce Effort Associated with Balance. Frontiers in neurorobotics Kim, M., Collins, S. H. 2017; 11: 62


    Below-knee amputation is associated with higher energy expenditure during walking, partially due to difficulty maintaining balance. We previously found that once-per-step push-off work control can reduce balance-related effort, both in simulation and in experiments with human participants. Simulations also suggested that changing ankle inversion/eversion torque on each step, in response to changes in body state, could assist with balance. In this study, we investigated the effects of ankle inversion/eversion torque modulation on balance-related effort among amputees (N = 5) using a multi-actuated ankle-foot prosthesis emulator. In stabilizing conditions, changes in ankle inversion/eversion torque were applied so as to counteract deviations in side-to-side center-of-mass acceleration at the moment of intact-limb toe off; higher acceleration toward the prosthetic limb resulted in a corrective ankle inversion torque during the ensuing stance phase. Destabilizing controllers had the opposite effect, and a zero gain controller made no changes to the nominal inversion/eversion torque. To separate the balance-related effects of step-to-step control from the potential effects of changes in average mechanics, average ankle inversion/eversion torque and prosthesis work were held constant across conditions. High-gain stabilizing control lowered metabolic cost by 13% compared to the zero gain controller (p = 0.05). We then investigated individual responses to subject-specific stabilizing controllers following an enforced exploration period. Four of five participants experienced reduced metabolic rate compared to the zero gain controller (-15, -14, -11, -6, and +4%) an average reduction of 9% (p = 0.05). Average prosthesis mechanics were unchanged across all conditions, suggesting that improvements in energy economy might have come from changes in step-to-step corrections related to balance. Step-to-step modulation of inversion/eversion torque could be used in new, active ankle-foot prostheses to reduce walking effort associated with maintaining balance.

    View details for DOI 10.3389/fnbot.2017.00062

    View details for PubMedID 29184493

    View details for PubMedCentralID PMC5694462

  • Muscle recruitment and coordination with an ankle exoskeleton JOURNAL OF BIOMECHANICS Steele, K. M., Jackson, R. W., Shuman, B. R., Collins, S. H. 2017; 59: 50–58


    Exoskeletons have the potential to assist and augment human performance. Understanding how users adapt their movement and neuromuscular control in response to external assistance is important to inform the design of these devices. The aim of this research was to evaluate changes in muscle recruitment and coordination for ten unimpaired individuals walking with an ankle exoskeleton. We evaluated changes in the activity of individual muscles, cocontraction levels, and synergistic patterns of muscle coordination with increasing exoskeleton work and torque. Participants were able to selectively reduce activity of the ankle plantarflexors with increasing exoskeleton assistance. Increasing exoskeleton net work resulted in greater reductions in muscle activity than increasing exoskeleton torque. Patterns of muscle coordination were not restricted or constrained to synergistic patterns observed during unassisted walking. While three synergies could describe nearly 95% of the variance in electromyography data during unassisted walking, these same synergies could describe only 85-90% of the variance in muscle activity while walking with the exoskeleton. Synergies calculated with the exoskeleton demonstrated greater changes in synergy weights with increasing exoskeleton work versus greater changes in synergy activations with increasing exoskeleton torque. These results support the theory that unimpaired individuals do not exclusively use central pattern generators or other low-level building blocks to coordinate muscle activity, especially when learning a new task or adapting to external assistance, and demonstrate the potential for using exoskeletons to modulate muscle recruitment and coordination patterns for rehabilitation or performance.

    View details for PubMedID 28623037

    View details for PubMedCentralID PMC5644499

  • 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


    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

  • Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power. Journal of neuroengineering and rehabilitation Galle, S., Malcolm, P., Collins, S. H., De Clercq, D. 2017; 14 (1): 35-?


    Powered ankle-foot exoskeletons can reduce the metabolic cost of human walking to below normal levels, but optimal assistance properties remain unclear. The purpose of this study was to test the effects of different assistance timing and power characteristics in an experiment with a tethered ankle-foot exoskeleton.Ten healthy female subjects walked on a treadmill with bilateral ankle-foot exoskeletons in 10 different assistance conditions. Artificial pneumatic muscles assisted plantarflexion during ankle push-off using one of four actuation onset timings (36, 42, 48 and 54% of the stride) and three power levels (average positive exoskeleton power over a stride, summed for both legs, of 0.2, 0.4 and 0.5 W∙kg(-1)). We compared metabolic rate, kinematics and electromyography (EMG) between conditions.Optimal assistance was achieved with an onset of 42% stride and average power of 0.4 W∙kg(-1), leading to 21% reduction in metabolic cost compared to walking with the exoskeleton deactivated and 12% reduction compared to normal walking without the exoskeleton. With suboptimal timing or power, the exoskeleton still reduced metabolic cost, but substantially less so. The relationship between timing, power and metabolic rate was well-characterized by a two-dimensional quadratic function. The assistive mechanisms leading to these improvements included reducing muscular activity in the ankle plantarflexors and assisting leg swing initiation.These results emphasize the importance of optimizing exoskeleton actuation properties when assisting or augmenting human locomotion. Our optimal assistance onset timing and average power levels could be used for other exoskeletons to improve assistance and resulting benefits.

    View details for DOI 10.1186/s12984-017-0235-0

    View details for PubMedID 28449684

  • Once-Per-Step Control of Ankle Push-Off Work Improves Balance in a Three-Dimensional Simulation of Bipedal Walking IEEE TRANSACTIONS ON ROBOTICS Kim, M., Collins, S. H. 2017; 33 (2): 406-418
  • Design of a lightweight, tethered, torque-controlled knee exoskeleton International Conference on Rehabilitation Robotics (ICORR) Witte, K. A., Fatschel, A. M., Collins, S. H. 2017: 1646–53
  • Torque control in legged locomotion Bio-Inspired Legged Locomotion: Models, Concepts, Control and Applications Zhang, J., Collins, S. H. 2017: 347–395
  • Increasing ankle push-off work with a powered prosthesis does not necessarily reduce metabolic rate for transtibial amputees JOURNAL OF BIOMECHANICS Quesada, R. E., Caputo, J. M., Collins, S. H. 2016; 49 (14): 3452-3459


    Amputees using passive ankle-foot prostheses tend to expend more metabolic energy during walking than non-amputees, and reducing this cost has been a central motivation for the development of active ankle-foot prostheses. Increased push-off work at the end of stance has been proposed as a way to reduce metabolic energy use, but the effects of push-off work have not been tested in isolation. In this experiment, participants with unilateral transtibial amputation (N=6) walked on a treadmill at a constant speed while wearing a powered prosthesis emulator. The prosthesis delivered different levels of ankle push-off work across conditions, ranging from the value for passive prostheses to double the value for non-amputee walking, while all other prosthesis mechanics were held constant. Participants completed six acclimation sessions prior to a data collection in which metabolic rate, kinematics, kinetics, muscle activity and user satisfaction were recorded. Metabolic rate was not affected by net prosthesis work rate (p=0.5; R(2)=0.007). Metabolic rate, gait mechanics and muscle activity varied widely across participants, but no participant had lower metabolic rate with higher levels of push-off work. User satisfaction was affected by push-off work (p=0.002), with participants preferring values of ankle push-off slightly higher than in non-amputee walking, possibly indicating other benefits. Restoring or augmenting ankle push-off work is not sufficient to improve energy economy for lower-limb amputees. Additional necessary conditions might include alternate timing or control, individualized tuning, or particular subject characteristics.

    View details for DOI 10.1016/j.jbiomech.2016.09.015

    View details for Web of Science ID 000386984400043

    View details for PubMedID 27702444

  • A lightweight, low-power electroadhesive clutch and spring for exoskeleton actuation International Conference on Robotics and Automation Diller, S., Majidi, C., Collins, S. H. 2016: 682–689
  • An experimental comparison of the relative benefits of work and torque assistance in ankle exoskeletons JOURNAL OF APPLIED PHYSIOLOGY Jackson, R. W., Collins, S. H. 2015; 119 (5): 541-557


    Techniques proposed for assisting locomotion with exoskeletons have often included a combination of active work input and passive torque support, but the physiological effects of different assistance techniques remain unclear. We performed an experiment to study the independent effects of net exoskeleton work and average exoskeleton torque on human locomotion. Subjects wore a unilateral ankle exoskeleton and walked on a treadmill at 1.25 m·s(-1) while net exoskeleton work rate was systematically varied from -0.054 to 0.25 J·kg(-1)·s(-1), with constant (0.12 N·m·kg(-1)) average exoskeleton torque, and while average exoskeleton torque was systematically varied from approximately zero to 0.18 N·m·kg(-1), with approximately zero net exoskeleton work. We measured metabolic rate, center-of-mass mechanics, joint mechanics, and muscle activity. Both techniques reduced effort-related measures at the assisted ankle, but this form of work input reduced metabolic cost (-17% with maximum net work input) while this form of torque support increased metabolic cost (+13% with maximum average torque). Disparate effects on metabolic rate seem to be due to cascading effects on whole body coordination, particularly related to assisted ankle muscle dynamics and the effects of trailing ankle behavior on leading leg mechanics during double support. It would be difficult to predict these results using simple walking models without muscles or musculoskeletal models that assume fixed kinematics or kinetics. Data from this experiment can be used to improve predictive models of human neuromuscular adaptation and guide the design of assistive devices.

    View details for DOI 10.1152/japplphysiol.01133.2014

    View details for Web of Science ID 000360694300014

    View details for PubMedID 26159764

  • Once-per-step control of ankle-foot prosthesis push-off work reduces effort associated with balance during walking JOURNAL OF NEUROENGINEERING AND REHABILITATION Kim, M., Collins, S. H. 2015; 12


    Individuals with below-knee amputation have more difficulty balancing during walking, yet few studies have explored balance enhancement through active prosthesis control. We previously used a dynamical model to show that prosthetic ankle push-off work affects both sagittal and frontal plane dynamics, and that appropriate step-by-step control of push-off work can improve stability. We hypothesized that this approach could be applied to a robotic prosthesis to partially fulfill the active balance requirements of human walking, thereby reducing balance-related activity and associated effort for the person using the device.We conducted experiments on human participants (N = 10) with simulated amputation. Prosthetic ankle push-off work was varied on each step in ways expected to either stabilize, destabilize or have no effect on balance. Average ankle push-off work, known to affect effort, was kept constant across conditions. Stabilizing controllers commanded more push-off work on steps when the mediolateral velocity of the center of mass was lower than usual at the moment of contralateral heel strike. Destabilizing controllers enforced the opposite relationship, while a neutral controller maintained constant push-off work regardless of body state. A random disturbance to landing foot angle and a cognitive distraction task were applied, further challenging participants' balance. We measured metabolic rate, foot placement kinematics, center of pressure kinematics, distraction task performance, and user preference in each condition. We expected the stabilizing controller to reduce active control of balance and balance-related effort for the user, improving user preference.The best stabilizing controller lowered metabolic rate by 5.5% (p = 0.003) and 8.5% (p = 0.02), and step width variability by 10.0% (p = 0.009) and 10.7% (p = 0.03) compared to conditions with no control and destabilizing control, respectively. Participants tended to prefer stabilizing controllers. These effects were not due to differences in average push-off work, which was unchanged across conditions, or to average gait mechanics, which were also unchanged. Instead, benefits were derived from step-by-step adjustments to prosthesis behavior in response to variations in mediolateral velocity at heel strike.Once-per-step control of prosthetic ankle push-off work can reduce both active control of foot placement and balance-related metabolic energy use during walking.

    View details for DOI 10.1186/s12984-015-0027-3

    View details for Web of Science ID 000354253500001

    View details for PubMedID 25928176

  • The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking JOURNAL OF NEUROENGINEERING AND REHABILITATION Malcolm, P., Quesada, R. E., Caputo, J. M., Collins, S. H. 2015; 12


    Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity.We tested 10 able-bodied participants walking on a treadmill at 1.25 m · s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46% and 56% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work.When push-off began at or after leading leg contact, metabolic rate was about 10% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work.Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing.

    View details for DOI 10.1186/s12984-015-0014-8

    View details for Web of Science ID 000353186900001

    View details for PubMedID 25889201

  • Experimental comparison of torque control methods on an ankle exoskeleton during human walking Zhang, J., Cheah, C., Collins, S. H., IEEE IEEE COMPUTER SOC. 2015: 5584–89
  • Informing Ankle-Foot Prosthesis Prescription through Haptic Emulation of Candidate Devices Caputo, J. M., Adamczyk, P. G., Collins, S. H., IEEE IEEE COMPUTER SOC. 2015: 6445–50


    Robotic prostheses can improve walking performance for amputees, but prescription of these devices has been hindered by their high cost and uncertainty about the degree to which individuals will benefit. The typical prescription process cannot well predict how an individual will respond to a device they have never used because it bases decisions on subjective assessment of an individual's current activity level. We propose a new approach in which individuals 'test drive' candidate devices using a prosthesis emulator while their walking performance is quantitatively assessed and results are distilled to inform prescription. In this system, prosthesis behavior is controlled by software rather than mechanical implementation, so users can quickly experience a broad range of devices. To test the viability of the approach, we developed a prototype emulator and assessment protocol, leveraging hardware and methods we previously developed for basic science experiments. We demonstrated emulations across the spectrum of commercially available prostheses, including traditional (e.g. SACH), dynamic-elastic (e.g. FlexFoot), and powered robotic (e.g. BiOM® T2) prostheses. Emulations exhibited low error with respect to reference data and provided subjectively convincing representations of each device. We demonstrated an assessment protocol that differentiated device classes for each individual based on quantitative performance metrics, providing feedback that could be used to make objective, personalized device prescriptions.

    View details for PubMedID 27570639

    View details for PubMedCentralID PMC4996637

  • An Ankle-Foot Prosthesis Emulator with Control of Plantarflexion and Inversion-Eversion Torque Collins, S. H., Kim, M., Chen, T., Chen, T., IEEE IEEE COMPUTER SOC. 2015: 1210–16
  • Design of Two Lightweight, High-Bandwidth Torque-Controlled Ankle Exoskeletons Witte, K., Zhang, J., Jackson, R. W., Collins, S. H., IEEE IEEE COMPUTER SOC. 2015: 1223–28
  • Prosthetic ankle push-off work reduces metabolic rate but not collision work in non-amputee walking SCIENTIFIC REPORTS Caputo, J. M., Collins, S. H. 2014; 4


    Individuals with unilateral below-knee amputation expend more energy than non-amputees during walking and exhibit reduced push-off work and increased hip work in the affected limb. Simple dynamic models of walking suggest a possible solution, predicting that increasing prosthetic ankle push-off should decrease leading limb collision, thereby reducing overall energy requirements. We conducted a rigorous experimental test of this idea wherein ankle-foot prosthesis push-off work was incrementally varied in isolation from one-half to two-times normal levels while subjects with simulated amputation walked on a treadmill at 1.25 m · s(-1). Increased prosthesis push-off significantly reduced metabolic energy expenditure, with a 14% reduction at maximum prosthesis work. In contrast to model predictions, however, collision losses were unchanged, while hip work during swing initiation was decreased. This suggests that powered ankle push-off reduces walking effort primarily through other mechanisms, such as assisting leg swing, which would be better understood using more complete neuromuscular models.

    View details for DOI 10.1038/srep07213

    View details for Web of Science ID 000346260100001

    View details for PubMedID 25467389

  • A Universal Ankle-Foot Prosthesis Emulator for Human Locomotion Experiments JOURNAL OF BIOMECHANICAL ENGINEERING-TRANSACTIONS OF THE ASME Caputo, J. M., Collins, S. H. 2014; 136 (3)


    Robotic prostheses have the potential to significantly improve mobility for people with lower-limb amputation. Humans exhibit complex responses to mechanical interactions with these devices, however, and computational models are not yet able to predict such responses meaningfully. Experiments therefore play a critical role in development, but have been limited by the use of product-like prototypes, each requiring years of development and specialized for a narrow range of functions. Here we describe a robotic ankle-foot prosthesis system that enables rapid exploration of a wide range of dynamical behaviors in experiments with human subjects. This emulator comprises powerful off-board motor and control hardware, a flexible Bowden cable tether, and a lightweight instrumented prosthesis, resulting in a combination of low mass worn by the human (0.96 kg) and high mechatronic performance compared to prior platforms. Benchtop tests demonstrated closed-loop torque bandwidth of 17 Hz, peak torque of 175 Nm, and peak power of 1.0 kW. Tests with an anthropomorphic pendulum "leg" demonstrated low interference from the tether, less than 1 Nm about the hip. This combination of low worn mass, high bandwidth, high torque, and unrestricted movement makes the platform exceptionally versatile. To demonstrate suitability for human experiments, we performed preliminary tests in which a subject with unilateral transtibial amputation walked on a treadmill at 1.25 ms-1 while the prosthesis behaved in various ways. These tests revealed low torque tracking error (RMS error of 2.8 Nm) and the capacity to systematically vary work production or absorption across a broad range (from -5 to 21 J per step). These results support the use of robotic emulators during early stage assessment of proposed device functionalities and for scientific study of fundamental aspects of human-robot interaction. The design of simple, alternate end-effectors would enable studies at other joints or with additional degrees of freedom.

    View details for DOI 10.1115/1.4026225

    View details for Web of Science ID 000333099700010

    View details for PubMedID 24337103

  • Emulating prosthetic feet during the prescription process to improve outcomes and justifications Caputo, J. M., Collins, S. H., Adamczyk, P. G., IEEE IEEE. 2014: 127–28
  • Two Independent Contributions to Step Variability during Over-Ground Human Walking PLOS ONE Collins, S. H., Kuo, A. D. 2013; 8 (8)


    Human walking exhibits small variations in both step length and step width, some of which may be related to active balance control. Lateral balance is thought to require integrative sensorimotor control through adjustment of step width rather than length, contributing to greater variability in step width. Here we propose that step length variations are largely explained by the typical human preference for step length to increase with walking speed, which itself normally exhibits some slow and spontaneous fluctuation. In contrast, step width variations should have little relation to speed if they are produced more for lateral balance. As a test, we examined hundreds of overground walking steps by healthy young adults (N = 14, age < 40 yrs.). We found that slow fluctuations in self-selected walking speed (2.3% coefficient of variation) could explain most of the variance in step length (59%, P < 0.01). The residual variability not explained by speed was small (1.5% coefficient of variation), suggesting that step length is actually quite precise if not for the slow speed fluctuations. Step width varied over faster time scales and was independent of speed fluctuations, with variance 4.3 times greater than that for step length (P < 0.01) after accounting for the speed effect. That difference was further magnified by walking with eyes closed, which appears detrimental to control of lateral balance. Humans appear to modulate fore-aft foot placement in precise accordance with slow fluctuations in walking speed, whereas the variability of lateral foot placement appears more closely related to balance. Step variability is separable in both direction and time scale into balance- and speed-related components. The separation of factors not related to balance may reveal which aspects of walking are most critical for the nervous system to control.

    View details for DOI 10.1371/journal.pone.0073597

    View details for Web of Science ID 000323733800103

    View details for PubMedID 24015308

  • Inducing Self-Selected Human Engagement in Robotic Locomotion Training Collins, S. H., Jackson, R. W., IEEE IEEE. 2013
  • What Do Walking Humans Want From Mechatronics? Collins, S. H., IEEE IEEE. 2013
  • An Experimental Robotic Testbed for Accelerated Development of Ankle Prostheses Caputo, J. M., Collins, S. H., IEEE IEEE. 2013: 2645–50
  • Stable Human-Robot Interaction Control for Upper-limb Rehabilitation Robotics Zhang, J., Cheah, C., Collins, S. H., IEEE IEEE. 2013: 2201–6
  • The Effect of Foot Compliance Encoded in the Windlass Mechanism on the Energetics of Human Walking Song, S., LaMontagna, C., Collins, S. H., Geyer, H., IEEE IEEE. 2013: 3179–82


    The human foot, which is the part of the body that interacts with the environment during locomotion, consists of rich biomechanical design. One of the unique designs of human feet is the windlass mechanism. In a previous simulation study, we found that the windlass mechanism seems to improve the energy efficiency of walking. To better understand the origin of this efficiency, we here conduct both simulation and experimental studies exploring the influence of foot compliance, which is one of the functionalities that the windlass mechanism embeds, on the energetics of walking. The studies show that walking with compliant feet incurs more energetic costs than walking with stiff feet. The preliminary results suggest that the energy saved by introducing the windlass mechanism does not originate from the compliance it embeds. We speculate that the energy savings of the windlass mechanism are related more to its contribution to reducing the effective foot length in swing than to providing compliance in stance.

    View details for Web of Science ID 000341702103155

    View details for PubMedID 24110403

  • Stabilization of a Three-Dimensional Limit Cycle Walking Model through Step-to-Step Ankle Control Kim, M., Collins, S. H., IEEE IEEE. 2013
  • The effects of a controlled energy storage and return prototype prosthetic foot on transtibial amputee ambulation HUMAN MOVEMENT SCIENCE Segal, A. D., Zelik, K. E., Klute, G. K., Morgenroth, D. C., Hahn, M. E., Orendurff, M. S., Adamczyk, P. G., Collins, S. H., Kuo, A. D., Czerniecki, J. M. 2012; 31 (4): 918–31


    The lack of functional ankle musculature in lower limb amputees contributes to the reduced prosthetic ankle push-off, compensations at other joints and more energetically costly gait commonly observed in comparison to non-amputees. A variety of energy storing and return prosthetic feet have been developed to address these issues but have not been shown to sufficiently improve amputee biomechanics and energetic cost, perhaps because the timing and magnitude of energy return is not controlled. The goal of this study was to examine how a prototype microprocessor-controlled prosthetic foot designed to store some of the energy during loading and return it during push-off affects amputee gait. Unilateral transtibial amputees wore the Controlled Energy Storage and Return prosthetic foot (CESR), a conventional foot (CONV), and their previously prescribed foot (PRES) in random order. Three-dimensional gait analysis and net oxygen consumption were collected as participants walked at constant speed. The CESR foot demonstrated increased energy storage during early stance, increased prosthetic foot peak push-off power and work, increased prosthetic limb center of mass (COM) push-off work and decreased intact limb COM collision work compared to CONV and PRES. The biological contribution of the positive COM work for CESR was reduced compared to CONV and PRES. However, the net metabolic cost for CESR did not change compared to CONV and increased compared to PRES, which may partially reflect the greater weight, lack of individualized size and stiffness and relatively less familiarity for CESR and CONV. Controlled energy storage and return enhanced prosthetic push-off, but requires further design modifications to improve amputee walking economy.

    View details for DOI 10.1016/j.humov.2011.08.005

    View details for Web of Science ID 000312971800014

    View details for PubMedID 22100728

    View details for PubMedCentralID PMC4302415

  • The effect of ankle foot orthosis stiffness on the energy cost of walking: A simulation study CLINICAL BIOMECHANICS Bregman, D. J., van der Krogt, M. M., de Groot, V., Harlaar, J., Wisse, M., Collins, S. H. 2011; 26 (9): 955–61


    In stroke and multiple sclerosis patients, gait is frequently hampered by a reduced ability to push-off with the ankle caused by weakness of the plantar-flexor muscles. To enhance ankle push-off and to decrease the high energy cost of walking, spring-like carbon-composite Ankle Foot Orthoses are frequently prescribed. However, it is unknown what Ankle Foot Orthoses stiffness should be used to obtain the most efficient gait. The aim of this simulation study was to gain insights into the effect of variation in Ankle Foot Orthosis stiffness on the amount of energy stored in the Ankle Foot Orthosis and the energy cost of walking.We developed a two-dimensional forward-dynamic walking model with a passive spring at the ankle representing the Ankle Foot Orthosis and two constant torques at the hip for propulsion. We varied Ankle Foot Orthosis stiffness while keeping speed and step length constant.We found an optimal stiffness, at which the energy delivered at the hip joint was minimal. Energy cost decreased with increasing energy storage in the ankle foot orthosis, but the most efficient gait did not occur with maximal energy storage. With maximum storage, push-off occurred too late to reduce the impact of the contralateral leg with the floor. Maximum return prior to foot strike was also suboptimal, as push-off occurred too early and its effects were subsequently counteracted by gravity. The optimal Ankle Foot Orthosis stiffness resulted in significant push-off timed just prior to foot strike and led to greater ankle plantar-flexion velocity just before contralateral foot strike.Our results suggest that patient energy cost might be reduced by the proper choice of Ankle Foot Orthosis stiffness.

    View details for DOI 10.1016/j.clinbiomech.2011.05.007

    View details for Web of Science ID 000296410400011

    View details for PubMedID 21723012

  • The effect of prosthetic foot push-off on mechanical loading associated with knee osteoarthritis in lower extremity amputees GAIT & POSTURE Morgenroth, D. C., Segal, A. D., Zelik, K. E., Czerniecki, J. M., Klute, G. K., Adamczyk, P. G., Orendurff, M. S., Hahn, M. E., Collins, S. H., Kuo, A. D. 2011; 34 (4): 502-507


    Lower extremity amputation not only limits mobility, but also increases the risk of knee osteoarthritis of the intact limb. Dynamic walking models of non-amputees suggest that pushing-off from the trailing limb can reduce collision forces on the leading limb. These collision forces may determine the peak knee external adduction moment (EAM), which has been linked to the development of knee OA in the general population. We therefore hypothesized that greater prosthetic push-off would lead to reduced loading and knee EAM of the intact limb in unilateral transtibial amputees. Seven unilateral transtibial amputees were studied during gait under three prosthetic foot conditions that were intended to vary push-off. Prosthetic foot-ankle push-off work, intact limb knee EAM and ground reaction impulses for both limbs during step-to-step transition were measured. Overall, trailing limb prosthetic push-off work was negatively correlated with leading intact limb 1st peak knee EAM (slope=-.72±.22; p=.011). Prosthetic push-off work and 1st peak intact knee EAM varied significantly with foot type. The prosthetic foot condition with the least push-off demonstrated the largest knee EAM, which was reduced by 26% with the prosthetic foot producing the most push-off. Trailing prosthetic limb push-off impulse was negatively correlated with leading intact limb loading impulse (slope=-.34±.14; p=.001), which may help explain how prosthetic limb push-off can affect intact limb loading. Prosthetic feet that perform more prosthetic push-off appear to be associated with a reduction in 1st peak intact knee EAM, and their use could potentially reduce the risk and burden of knee osteoarthritis in this population.

    View details for DOI 10.1016/j.gaitpost.2011.07.001

    View details for Web of Science ID 000296484800011

    View details for PubMedID 21803584

  • Systematic Variation of Prosthetic Foot Spring Affects Center-of-Mass Mechanics and Metabolic Cost During Walking IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING Zelik, K. E., Collins, S. H., Adamczyk, P. G., Segal, A. D., Klute, G. K., Morgenroth, D. C., Hahn, M. E., Orendurff, M. S., Czerniecki, J. M., Kuo, A. D. 2011; 19 (4): 411-419


    Lower-limb amputees expend more energy to walk than non-amputees and have an elevated risk of secondary disabilities. Insufficient push-off by the prosthetic foot may be a contributing factor. We aimed to systematically study the effect of prosthetic foot mechanics on gait, to gain insight into fundamental prosthetic design principles. We varied a single parameter in isolation, the energy-storing spring in a prototype prosthetic foot, the controlled energy storage and return (CESR) foot, and observed the effect on gait. Subjects walked on the CESR foot with three different springs. We performed parallel studies on amputees and on non-amputees wearing prosthetic simulators. In both groups, spring characteristics similarly affected ankle and body center-of-mass (COM) mechanics and metabolic cost. Softer springs led to greater energy storage, energy return, and prosthetic limb COM push-off work. But metabolic energy expenditure was lowest with a spring of intermediate stiffness, suggesting biomechanical disadvantages to the softest spring despite its greater push-off. Disadvantages of the softest spring may include excessive heel displacements and COM collision losses. We also observed some differences in joint kinetics between amputees and non-amputees walking on the prototype foot. During prosthetic push-off, amputees exhibited reduced energy transfer from the prosthesis to the COM along with increased hip work, perhaps due to greater energy dissipation at the knee. Nevertheless, the results indicate that spring compliance can contribute to push-off, but with biomechanical trade-offs that limit the degree to which greater push-off might improve walking economy.

    View details for DOI 10.1109/TNSRE.2011.2159018

    View details for Web of Science ID 000293754800009

    View details for PubMedID 21708509

  • An Exoskeleton Using Controlled Energy Storage and Release to Aid Ankle Propulsion Wiggin, M., Sawicki, G. S., Collins, S. H., IEEE IEEE. 2011
  • How Crouch Gait Can Dynamically Induce Stiff-Knee Gait ANNALS OF BIOMEDICAL ENGINEERING van der Krogt, M. M., Bregman, D. J., Wisse, M., Doorenbosch, C. M., Harlaar, J., Collins, S. H. 2010; 38 (4): 1593–1606


    Children with cerebral palsy frequently experience foot dragging and tripping during walking due to a lack of adequate knee flexion in swing (stiff-knee gait). Stiff-knee gait is often accompanied by an overly flexed knee during stance (crouch gait). Studies on stiff-knee gait have mostly focused on excessive knee muscle activity during (pre)swing, but the passive dynamics of the limbs may also have an important effect. To examine the effects of a crouched posture on swing knee flexion, we developed a forward-dynamic model of human walking with a passive swing knee, capable of stable cyclic walking for a range of stance knee crouch angles. As crouch angle during stance was increased, the knee naturally flexed much less during swing, resulting in a 'stiff-knee' gait pattern and reduced foot clearance. Reduced swing knee flexion was primarily due to altered gravitational moments around the joints during initial swing. We also considered the effects of increased push-off strength and swing hip flexion torque, which both increased swing knee flexion, but the effect of crouch angle was dominant. These findings demonstrate that decreased knee flexion during swing can occur purely as the dynamical result of crouch, rather than from altered muscle function or pathoneurological control alone.

    View details for DOI 10.1007/s10439-010-9952-2

    View details for Web of Science ID 000276046600028

    View details for PubMedID 20162360

    View details for PubMedCentralID PMC3233366

  • Recycling Energy to Restore Impaired Ankle Function during Human Walking PLOS ONE Collins, S. H., Kuo, A. D. 2010; 5 (2): e9307


    Humans normally dissipate significant energy during walking, largely at the transitions between steps. The ankle then acts to restore energy during push-off, which may be the reason that ankle impairment nearly always leads to poorer walking economy. The replacement of lost energy is necessary for steady gait, in which mechanical energy is constant on average, external dissipation is negligible, and no net work is performed over a stride. However, dissipation and replacement by muscles might not be necessary if energy were instead captured and reused by an assistive device.We developed a microprocessor-controlled artificial foot that captures some of the energy that is normally dissipated by the leg and "recycles" it as positive ankle work. In tests on subjects walking with an artificially-impaired ankle, a conventional prosthesis reduced ankle push-off work and increased net metabolic energy expenditure by 23% compared to normal walking. Energy recycling restored ankle push-off to normal and reduced the net metabolic energy penalty to 14%.These results suggest that reduced ankle push-off contributes to the increased metabolic energy expenditure accompanying ankle impairments, and demonstrate that energy recycling can be used to reduce such cost.

    View details for DOI 10.1371/journal.pone.0009307

    View details for Web of Science ID 000274590600031

    View details for PubMedID 20174659

    View details for PubMedCentralID PMC2822861

  • Dynamic arm swinging in human walking PROCEEDINGS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES Collins, S. H., Adamczyk, P. G., Kuo, A. D. 2009; 276 (1673): 3679–88


    Humans tend to swing their arms when they walk, a curious behaviour since the arms play no obvious role in bipedal gait. It might be costly to use muscles to swing the arms, and it is unclear whether potential benefits elsewhere in the body would justify such costs. To examine these costs and benefits, we developed a passive dynamic walking model with free-swinging arms. Even with no torques driving the arms or legs, the model produced walking gaits with arm swinging similar to humans. Passive gaits with arm phasing opposite to normal were also found, but these induced a much greater reaction moment from the ground, which could require muscular effort in humans. We therefore hypothesized that the reduction of this moment may explain the physiological benefit of arm swinging. Experimental measurements of humans (n = 10) showed that normal arm swinging required minimal shoulder torque, while volitionally holding the arms still required 12 per cent more metabolic energy. Among measures of gait mechanics, vertical ground reaction moment was most affected by arm swinging and increased by 63 per cent without it. Walking with opposite-to-normal arm phasing required minimal shoulder effort but magnified the ground reaction moment, causing metabolic rate to increase by 26 per cent. Passive dynamics appear to make arm swinging easy, while indirect benefits from reduced vertical moments make it worthwhile overall.

    View details for DOI 10.1098/rspb.2009.0664

    View details for Web of Science ID 000270172200014

    View details for PubMedID 19640879

    View details for PubMedCentralID PMC2817299

  • A simple method for calibrating force plates and force treadmills using an instrumented pole GAIT & POSTURE Collins, S. H., Adamczyk, P. G., Ferris, D. P., Kuo, A. D. 2009; 29 (1): 59–64


    We propose a new method for calibrating force plates to reduce errors in center of pressure locations, forces, and moments. These errors may be caused by imperfect mounting of force plates to the ground or by installation of a treadmill atop a force plate, which may introduce distorting loads. The method, termed the Post-Installation Least-Squares (PILS) calibration, combines features of several previous methods into a simple procedure. It requires a motion capture system and an instrumented pole for applying reference loads. Reference loads may be manually applied to the force plate in arbitrary locations and directions. The instrumented pole measures applied load magnitudes through a single-axis load cell, and load directions through motion capture markers. Reference data and imperfect force plate signals are then combined to form a linear calibration matrix that simultaneously minimizes mean square errors in all forces and moments. We applied the procedure to standard laboratory force plates, as well as a custom-built, split-belt force treadmill. We also collected an independent set of verification data for testing. The proposed calibration procedure was found to reduce force errors by over 20%, and moment errors by over 60%. Center of pressure errors were also reduced by 63% for standard force plates and 91% for the force treadmill. The instrumented pole is advantageous because it allows for fast and arbitrary load application without needing a precise fixture for aligning loads. The linear calibration matrix is simpler than nonlinear correction equations and more compatible with standard data acquisition software, yet it yields error reductions comparable to more complex methods.

    View details for DOI 10.1016/j.gaitpost.2008.06.010

    View details for Web of Science ID 000262592200011

    View details for PubMedID 18755590

    View details for PubMedCentralID PMC2665306

  • Ankle fixation need not increase the energetic cost of human walking GAIT & POSTURE Vanderpool, M. T., Collins, S. H., Kuo, A. D. 2008; 28 (3): 427–33


    We tested whether the metabolic energy cost of walking with the ankles immobilized can be comparable to normal walking. Immobilization of any lower extremity joint usually causes greater energy expenditure. Fixation of the ankle might be expected to eliminate the work it normally performs, to detrimental effect. But fixation using lightweight boots with curved rocker bottoms can also bring some benefits, so that the overall energetic effect might be quite small. We measured oxygen consumption, kinematics, and ground reaction forces in six (N=6) able-bodied human volunteers walking at 1.25 m/s in three conditions: normal walking in street shoes, walking with ankles immobilized by walking boots, and normally with ankles free but also weighted to match the mass of the walking boots. We estimated metabolic energy expenditure, joint work, and overall work performed on the body center of mass as a function of ankle fixation. Ankle fixation with walking boots caused the total rate of energy expenditure for walking to increase by 4.1% compared to normal (P=0.003), but differed by an insignificant amount (0.4% less, P=0.78) compared to walking with equivalent ankle weight. Compared to normal walking, ankle fixation can reduce ankle torque and work during the stance phase, most notably during late stance. This apparently makes up for the loss of ability to push-off as normal. With a suitably lightweight apparatus and curved rocker bottom surface, loss of ankle motion need not increase energy expenditure for walking.

    View details for DOI 10.1016/j.gaitpost.2008.01.016

    View details for Web of Science ID 000259627200012

    View details for PubMedID 18359634

    View details for PubMedCentralID PMC2703459

  • The advantages of a rolling foot in human walking JOURNAL OF EXPERIMENTAL BIOLOGY Adamczyk, P. G., Collins, S. H., Kuo, A. D. 2006; 209 (20): 3953–63


    The plantigrade human foot rolls over the ground during each walking step, roughly analogous to a wheel. The center of pressure progresses on the ground like a wheel of radius 0.3 L (leg length). We examined the effect of varying foot curvature on the mechanics and energetics of walking. We controlled curvature by attaching rigid arc shapes of various radii to the bottoms of rigid boots restricting ankle motion. We measured mechanical work performed on the center of mass (COM), and net metabolic rate, in human subjects (N=10) walking with seven arc radii from 0.02-0.40 m. Simple models of dynamic walking predict that redirection of COM velocity requires step-to-step transition work, decreasing quadratically with arc radius. Metabolic cost would be expected to change in proportion to mechanical work. We measured the average rate of negative work performed on the COM, and found that it followed the trend well (r2=0.95), with 2.37 times as much work for small radii as for large. Net metabolic rate (subtracting quiet standing) also decreased with increasing arc radius to a minimum at 0.3 L, with a slight increase thereafter. Maximum net metabolic rate was 6.25 W kg(-1) (for small-radius arc feet), about 59% greater than the minimum rate of 3.93 W kg(-1), which in turn was about 45% greater than the rate in normal walking. Metabolic rate was fit reasonably well (r2=0.86) by a quadratic curve, but exceeded that expected from COM work for extreme arc sizes. Other factors appear to increase metabolic cost for walking on very small and very large arc feet. These factors may include effort expended to stabilize the joints (especially the knee) or to maintain balance. Rolling feet with curvature 0.3 L appear energetically advantageous for plantigrade walking, partially due to decreased work for step-to-step transitions.

    View details for DOI 10.1242/jeb.02455

    View details for Web of Science ID 000242132100005

    View details for PubMedID 17023589

  • A bipedal walking robot with efficient and human-like gait Collins, S. H., Ruina, A., IEEE IEEE. 2005: 1983–88
  • The RoboKnee: An exoskeleton for enhancing strength and endurance during walking Pratt, J. E., Krupp, B. T., Morse, C. J., Collins, S. H., IEEE IEEE. 2004: 2430–35
  • A three-dimensional passive-dynamic walking robot with two legs and knees INTERNATIONAL JOURNAL OF ROBOTICS RESEARCH Collins, S. H., Wisse, M., Ruina, A. 2001; 20 (7): 607–15