@article{liu_naseri_lee_hu_lewek_huang_2023, title={A simplified model for whole-body angular momentum calculation}, volume={111}, ISSN={["1873-4030"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85144824437&partnerID=MN8TOARS}, DOI={10.1016/j.medengphy.2022.103944}, abstractNote={The capability to monitor gait stability during everyday life could provide key information to guide clinical intervention to patients with lower limb disabilities. Whole body angular momentum (Lbody) is a convenient stability indicator for wearable motion capture systems. However, Lbody is costly to estimate, because it requires monitoring all major body segment using expensive sensor elements. In this study, we developed a simplified rigid body model by merging connected body segments to reduce the number of body segments, which need to be monitored. We demonstrated that the Lbody could be estimated by a seven-segment model accurately for both people with and without lower extremity amputation.}, journal={MEDICAL ENGINEERING & PHYSICS}, author={Liu, Ming and Naseri, Amirreza and Lee, I-Chieh and Hu, Xiaogang and Lewek, Michael D. and Huang, He}, year={2023}, month={Jan} }
 @article{fylstra_lee_li_lewek_huang_2022, title={Human-prosthesis cooperation: combining adaptive prosthesis control with visual feedback guided gait}, volume={19}, ISSN={["1743-0003"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85144315570&partnerID=MN8TOARS}, DOI={10.1186/s12984-022-01118-z}, abstractNote={Personalizing prosthesis control is often structured as human-in-the-loop optimization. However, gait performance is influenced by both human control and intelligent prosthesis control. Hence, we need to consider both human and prosthesis control, and their cooperation, to achieve desired gait patterns. In this study, we developed a novel paradigm that engages human gait control via user-fed visual feedback (FB) of stance time to cooperate with automatic prosthesis control tuning. Three initial questions were studied: (1) does user control of gait timing (via visual FB) help the prosthesis tuning algorithm to converge faster? (2) in turn, does the prosthesis control influence the user's ability to reach and maintain the target stance time defined by the feedback? and (3) does the prosthesis control parameters tuned with extended stance time on prosthesis side allow the user to maintain this potentially beneficial behavior even after feedback is removed (short- and long-term retention)?A reinforcement learning algorithm was used to achieve prosthesis control to meet normative knee kinematics in walking. A visual FB system cued the user to control prosthesis-side stance time to facilitate the prosthesis tuning goal. Seven individuals without amputation (AB) and four individuals with transfemoral amputation (TFA) walked with a powered knee prosthesis on a treadmill. Participants completed prosthesis auto-tuning with three visual feedback conditions: no FB, self-selected stance time FB (SS FB), and increased stance time FB (Inc FB). The retention of FB effects was studied by comparing the gait performance across three different prosthesis controls, tuned with different visual FB.(1) Human control of gait timing reduced the tuning duration in individuals without amputation, but not for individuals with TFA. (2) The change of prosthesis control did not influence users' ability to reach and maintain the visual FB goal. (3) All participants increased their prosthesis-side stance time with the feedback and maintain it right after feedback was removed. However, in the post-test, the prosthesis control parameters tuned with visual FB only supported a few participants with longer stance time and better stance time symmetry.The study provides novel insights on human-prosthesis interaction when cooperating in walking, which may guide the future successful adoption of this paradigm in prosthesis control personalization or human-in-the-loop optimization to improve the prosthesis user's gait performance.}, number={1}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={Fylstra, Bretta L. and Lee, I-Chieh and Li, Minhan and Lewek, Michael D. and Huang, He}, year={2022}, month={Dec} }
 @article{pimentel_feldman_lewek_franz_2022, title={Quantifying mechanical and metabolic interdependence between speed and propulsive force during walking}, volume={4}, ISSN={["2624-9367"]}, DOI={10.3389/fspor.2022.942498}, abstractNote={Walking speed is a useful surrogate for health status across the population. Walking speed appears to be governed in part by interlimb coordination between propulsive (FP) and braking (FB) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, FP generated during push-off has received significantly more attention as a contributor to walking performance. Our goal was to first establish empirical relations between FP and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any link between FP and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or FP across a range of condition intensities. Walking with larger and smaller FP led to instinctively faster and slower walking speeds, respectively, with ~80% of variance in walking speed explained by FP. We also found that comparable changes in either FP or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining ~53% of the variance in net metabolic power and ~14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between FP and walking speed, building confidence that interventions designed to increase FP will translate to improved walking speed. Repeating this protocol in other populations may identify other relations that could inform the time course of gait decline due to age and disease.}, journal={FRONTIERS IN SPORTS AND ACTIVE LIVING}, author={Pimentel, Richard E. and Feldman, Jordan N. and Lewek, Michael D. and Franz, Jason R.}, year={2022}, month={Sep} }
 @article{lee_liu_lewek_hu_filer_huang_2022, title={Toward Safe Wearer-Prosthesis Interaction: Evaluation of Gait Stability and Human Compensation Strategy Under Faults in Robotic Transfemoral Prostheses}, volume={30}, ISSN={["1558-0210"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85139401676&partnerID=MN8TOARS}, DOI={10.1109/TNSRE.2022.3208778}, abstractNote={Although advanced wearable robots can assist human wearers, their internal faults (i.e., sensors or control errors) also pose a challenge. To ensure safe wearer-robot interactions, how internal errors by the prosthesis limb affect the stability of the user-prosthesis system, and how users react and compensate for the instability elicited by internal errors are imperative. The goals of this study were to 1) systematically investigate the biomechanics of a wearer-robot system reacting to internal errors induced by a powered knee prosthesis (PKP), and 2) quantify the error tolerable bound that does not affect the user’s gait stability. Eight able-bodied participants and two unilateral transfemoral amputees walked on a pathway wearing a PKP, as the controller randomly switched the control parameters to disturbance parameters to mimic the errors caused by locomotion mode misrecognition. The size of prosthesis control errors was systematically varied to determine the error tolerable bound that disrupted gait stability. The effect of the error was quantified based on the 1) mechanical change described by the angular impulse applied by the PKP, and 2) overall gait instability quantified using human perception, angular momentum, and compensatory stepping. The results showed that the error tolerable bound is dependent on the gait phase and the direction of torque change. Two balance recovery strategies were also observed to allow participants to successful respond to the induced errors. The outcomes of this study may assist the future design of an auto-tuning algorithm, volitionally-controlled powered prosthetic legs, and training of gait stability.}, journal={IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING}, author={Lee, I-Chieh and Liu, Ming and Lewek, Michael D. and Hu, Xiaogang and Filer, William G. and Huang, He}, year={2022}, pages={2773–2782} }
 @article{conway_crudup_lewek_franz_2021, title={Effects of Horizontal Impeding Force Gait Training on Older Adult Push-Off Intensity}, volume={53}, ISSN={["1530-0315"]}, DOI={10.1249/MSS.0000000000002500}, abstractNote={Aging and many gait pathologies are often characterized by deficits in push-off intensity (i.e., propulsive ground reaction forces and peak ankle moment and power output) during walking. Unfortunately, conventional interventions such as progressive resistance training, designed to enhance calf muscle mechanical output, generally fail to translate strength gains to functional improvements in habitual push-off intensity.Horizontal impeding forces applied to the body's center of mass systematically augment the mechanical output required from muscle-tendon units spanning the ankle during the push-off phase of walking, which could convey long-term benefits via training. Therefore, the purpose of this study was to investigate the preliminary efficacy of a 6-wk horizontal impeding force training paradigm on improving habitual push-off intensity in 11 healthy but not physically active older adults (age = 76 ± 4 yr, 6 females and 5 males).We found that older adults significantly (P < 0.05) increased measures of isometric strength by 18%, maximum walking speed by 10%, and 6-min walk test distance by 9% as a result of horizontal impeding force training. As a more clinically significant contribution of this work, we found that those subjects also increased habitual peak ankle moment and peak ankle power during push off after training by a significant 10% and 15%, respectively (P ≤ 0.036).We conclude that the use of horizontal impeding forces in older adults improves their maximum muscular and walking capacities while encouraging access to newfound strength gains, thereby improving habitual push-off intensity during walking.}, number={3}, journal={MEDICINE AND SCIENCE IN SPORTS AND EXERCISE}, author={Conway, Katie A. and Crudup, Keyaira L. and Lewek, Michael D. and Franz, Jason R.}, year={2021}, month={Mar}, pages={574–580} }
 @article{herrero_franz_lewek_2021, title={Gradually learning to increase gait propulsion in young unimpaired adults}, volume={75}, ISSN={["1872-7646"]}, DOI={10.1016/j.humov.2020.102745}, abstractNote={Distorted visual feedback (DVF) may employ both implicit and explicit approaches to enhance motor learning. Our purpose was to test the effect of DVF of gait propulsion on the capacity to alter propulsive forces, and to determine the biomechanical determinants of propulsion. Seventeen young unimpaired individuals walked for three minutes of baseline (no feedback), then completed three randomly ordered, 10-minute Learning conditions: Real, 10DVF, and 20DVF. During the DVF conditions, we gradually decreased the feedback value without the participants' knowledge. For all Learning conditions, participants were instructed to maintain the propulsive force between two targets representing ±1 standard deviation as obtained from baseline. A one-minute retention trial without any feedback was performed immediately after Learning. Participants increased propulsive forces and trailing limb angle in both DVF conditions that persisted through retention; however, no change in ankle plantarflexion moment was noted. These findings offer promise of translation to clinical populations with propulsion deficits and require combined implicit and explicit learning components. • Peak propulsion can be modified using distorted visual feedback. • Distorted visual feedback may use both implicit and strategic learning approaches. • Biomechanically, altered propulsion appears due to increased trailing limb angle.}, journal={HUMAN MOVEMENT SCIENCE}, author={Herrero, Luciana and Franz, Jason R. and Lewek, Michael D.}, year={2021}, month={Feb} }
 @article{mccain_libera_berno_sawicki_saul_lewek_2021, title={Isolating the energetic and mechanical consequences of imposed reductions in ankle and knee flexion during gait}, volume={18}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-021-00812-8}, abstractNote={Abstract Background Weakness of ankle and knee musculature following injury or disorder results in reduced joint motion associated with metabolically expensive gait compensations to enable limb support and advancement. However, neuromechanical coupling between the ankle and knee make it difficult to discern independent roles of these restrictions in joint motion on compensatory mechanics and metabolic penalties. Methods We sought to determine relative impacts of ankle and knee impairment on compensatory gait strategies and energetic outcomes using an unimpaired cohort (N = 15) with imposed unilateral joint range of motion restrictions as a surrogate for reduced motion resulting from gait pathology. Participants walked on a dual-belt instrumented treadmill at 0.8 m s −1 using a 3D printed ankle stay and a knee brace to systematically limit ankle motion ( restricted-ank) , knee motion ( restricted-knee ), and ankle and knee motion ( restricted-a + k) simultaneously. In addition, participants walked without any ankle or knee bracing ( control ) and with knee bracing worn but unrestricted ( braced). Results When ankle motion was restricted ( restricted-ank, restricted-a + k ) we observed decreased peak propulsion relative to the braced condition on the restricted limb. Reduced knee motion ( restricted-knee, restricted-a + k ) increased restricted limb circumduction relative to the restricted-ank condition through ipsilateral hip hiking. Interestingly, restricted limb average positive hip power increased in the restricted-ank condition but decreased in the restricted-a + k and restricted-knee conditions, suggesting that locking the knee impeded hip compensation. As expected, reduced ankle motion, either without ( restricted-ank ) or in addition to knee restriction ( restricted-a + k ) yielded significant increase in net metabolic rate when compared with the braced condition. Furthermore, the relative increase in metabolic cost was significantly larger with restricted-a + k when compared to restricted-knee condition. Conclusions Our methods allowed for the reproduction of asymmetric gait characteristics including reduced propulsive symmetry and increased circumduction. The metabolic consequences bolster the potential energetic benefit of targeting ankle function during rehabilitation. Trial registration N/A.}, number={1}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={McCain, Emily M. and Libera, Theresa L. and Berno, Matthew E. and Sawicki, Gregory S. and Saul, Katherine R. and Lewek, Michael D.}, year={2021}, month={Feb} }
 @article{mccain_berno_libera_lewek_sawicki_saul_2021, title={Reduced joint motion supersedes asymmetry in explaining increased metabolic demand during walking with mechanical restriction}, volume={126}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2021.110621}, abstractNote={Recent research has highlighted the complex interactions among chronic injury- or disease-induced joint limitations, walking asymmetry, and increased metabolic cost. Determining the specific metabolic impacts of asymmetry or joint impairment in clinical populations is difficult because of concurrent neurological and physiological changes. This work investigates the metabolic impact of gait asymmetry and joint restriction by unilaterally (asymmetric) and bilaterally (symmetric) restricting ankle, knee, and combined ankle and knee ranges of motion in unimpaired individuals. We calculated propulsive asymmetry, temporal asymmetry, and step-length asymmetry for an average gait cycle; metabolic rate; average positive center of mass power using the individual limbs method; and muscle effort using lower limb electromyography measurements weighted by corresponding physiological cross-sectional areas. Unilateral restriction caused propulsive and temporal asymmetry but less metabolically expensive gait than bilateral restriction. Changes in asymmetry did not correlate with changes in metabolic cost. Interestingly, bilateral restriction increased average positive center of mass power compared to unilateral restriction. Further, increased average positive center of mass power correlated with increased energy costs, suggesting asymmetric step-to-step transitions did not drive metabolic changes. The number of restricted joints reduces available degrees of freedom and may have a larger metabolic impact than gait asymmetry, as this correlated significantly with increases in metabolic rate for 7/9 participants. These results emphasize symmetry is not by definition metabolically optimal, indicate that the mechanics underlying symmetry are meaningful, and suggest that available degrees of freedom should be considered in designing future interventions.}, journal={JOURNAL OF BIOMECHANICS}, author={McCain, Emily M. and Berno, Matthew E. and Libera, Theresa L. and Lewek, Michael D. and Sawicki, Gregory S. and Saul, Katherine R.}, year={2021}, month={Sep} }
 @article{pieper_lewek_franz_2020, title={Can shank acceleration provide a clinically feasible surrogate for individual limb propulsion during walking?}, volume={98}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2019.109449}, abstractNote={Aging and many pathologies that affect gait are associated with reduced ankle power output and thus trailing limb propulsion during walking. However, quantifying trailing limb propulsion requires sophisticated measurement equipment at significant expense that fundamentally limits clinical translation for diagnostics or gait rehabilitation. As a component of joint power, our purpose was to determine if shank acceleration estimated via accelerometers during push-off can serve as a clinically feasible surrogate for ankle power output and peak anterior ground reaction forces (GRF) during walking. As hypothesized, we found that young adults modulated walking speed via changes in peak anterior GRF and peak ankle power output that correlated with proportional changes in shank acceleration during push-off, both at the individual subject (R2 ≥ 0.80, p < 0.01) and group average (R2 ≥ 0.74, p < 0.01) levels. In addition, we found that unilateral deficits in trailing limb propulsion induced via a leg bracing elicited unilateral and relatively proportional reductions in peak anterior GRF, peak ankle power, and peak shank acceleration. These unilateral leg bracing effects on peak shank acceleration correlated with those in peak ankle power (braced leg: R2 = 0.43, p = 0.028) but those effects in both peak shank acceleration and peak ankle power were disassociated from those in peak anterior GRF. In conclusion, our findings in young adults provide an early benchmark for the development of affordable and clinically feasible alternatives for assessing and monitoring trailing limb propulsion during walking.}, journal={JOURNAL OF BIOMECHANICS}, author={Pieper, Noah L. and Lewek, Michael D. and Franz, Jason R.}, year={2020}, month={Jan} }
 @misc{awad_lewek_kesar_franz_bowden_2020, title={These legs were made for propulsion: advancing the diagnosis and treatment of post-stroke propulsion deficits}, volume={17}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-020-00747-6}, abstractNote={Abstract Advances in medical diagnosis and treatment have facilitated the emergence of precision medicine. In contrast, locomotor rehabilitation for individuals with acquired neuromotor injuries remains limited by the dearth of (i) diagnostic approaches that can identify the specific neuromuscular, biomechanical, and clinical deficits underlying impaired locomotion and (ii) evidence-based, targeted treatments. In particular, impaired propulsion by the paretic limb is a major contributor to walking-related disability after stroke; however, few interventions have been able to target deficits in propulsion effectively and in a manner that reduces walking disability. Indeed, the weakness and impaired control that is characteristic of post-stroke hemiparesis leads to heterogeneous deficits that impair paretic propulsion and contribute to a slow, metabolically-expensive, and unstable gait. Current rehabilitation paradigms emphasize the rapid attainment of walking independence, not the restoration of normal propulsion function. Although walking independence is an important goal for stroke survivors, independence achieved via compensatory strategies may prevent the recovery of propulsion needed for the fast, economical, and stable gait that is characteristic of healthy bipedal locomotion. We posit that post-stroke rehabilitation should aim to promote independent walking, in part, through the acquisition of enhanced propulsion. In this expert review, we present the biomechanical and functional consequences of post-stroke propulsion deficits, review advances in our understanding of the nature of post-stroke propulsion impairment, and discuss emerging diagnostic and treatment approaches that have the potential to facilitate new rehabilitation paradigms targeting propulsion restoration.}, number={1}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={Awad, Louis N. and Lewek, Michael D. and Kesar, Trisha M. and Franz, Jason R. and Bowden, Mark G.}, year={2020}, month={Oct} }
 @article{brandt_riddick_stallrich_lewek_huang_2019, title={Effects of extended powered knee prosthesis stance time via visual feedback on gait symmetry of individuals with unilateral amputation: a preliminary study}, volume={16}, ISSN={["1743-0003"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85072172371&partnerID=MN8TOARS}, DOI={10.1186/s12984-019-0583-z}, abstractNote={Abstract Background Establishing gait symmetry is a major aim of amputee rehabilitation and may be more attainable with powered prostheses. Though, based on previous work, we postulate that users transfer a previously-learned motor pattern across devices, limiting the functionality of more advanced prostheses. The objective of this study was to preliminarily investigate the effect of increased stance time via visual feedback on amputees’ gait symmetry using powered and passive knee prostheses. Methods Five individuals with transfemoral amputation or knee disarticulation walked at their self-selected speed on a treadmill. Visual feedback was used to promote an increase in the amputated-limb stance time. Individuals were fit with a commercially-available powered prosthesis by a certified prosthetist and practiced walking during a prior visit. The same protocol was completed with a passive knee and powered knee prosthesis on separate days. We used repeated-measures, two-way ANOVA (alpha = 0.05) to test for significant effects of the feedback and device factors. Our main outcome measures were stance time asymmetry, peak anterior-posterior ground reaction forces, and peak anterior propulsion asymmetry. Results Increasing the amputated-limb stance time via visual feedback significantly improved the stance time symmetry ( p = 0.012) and peak propulsion symmetry ( p = 0.036) of individuals walking with both prostheses. With the powered knee prosthesis, the highest feedback target elicited 36% improvement in stance time symmetry, 22% increase in prosthesis-side peak propulsion, and 47% improvement in peak propulsion symmetry compared to a no feedback condition. The changes with feedback were not different with the passive prosthesis, and the main effects of device/ prosthesis type were not statistically different. However, subject by device interactions were significant, indicating individuals did not respond consistently with each device (e.g. prosthesis-side propulsion remained comparable to or was greater with the powered versus passive prosthesis for different subjects). Overall, prosthesis-side peak propulsion averaged across conditions was 31% greater with the powered prosthesis and peak propulsion asymmetry improved by 48% with the powered prosthesis. Conclusions Increasing prosthesis-side stance time via visual feedback favorably improved individuals’ temporal and propulsive symmetry. The powered prosthesis commonly enabled greater propulsion, but individuals adapted to each device with varying behavior, requiring further investigation.}, number={1}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={Brandt, Andrea and Riddick, William and Stallrich, Jonathan and Lewek, Michael and Huang, He Helen}, year={2019}, month={Sep} }
 @article{mccain_dick_giest_nuckols_lewek_saul_sawicki_2019, title={Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control}, volume={16}, ISSN={["1743-0003"]}, DOI={10.1186/s12984-019-0523-y}, abstractNote={Ankle exoskeletons offer a promising opportunity to offset mechanical deficits after stroke by applying the needed torque at the paretic ankle. Because joint torque is related to gait speed, it is important to consider the user’s gait speed when determining the magnitude of assistive joint torque. We developed and tested a novel exoskeleton controller for delivering propulsive assistance which modulates exoskeleton torque magnitude based on both soleus muscle activity and walking speed. The purpose of this research is to assess the impact of the resulting exoskeleton assistance on post-stroke walking performance across a range of walking speeds. Six participants with stroke walked with and without assistance applied to a powered ankle exoskeleton on the paretic limb. Walking speed started at 60% of their comfortable overground speed and was increased each minute (n00, n01, n02, etc.). We measured lower limb joint and limb powers, metabolic cost of transport, paretic and non-paretic limb propulsion, and trailing limb angle. Exoskeleton assistance increased with walking speed, verifying the speed-adaptive nature of the controller. Both paretic ankle joint power and total limb power increased significantly with exoskeleton assistance at six walking speeds (n00, n01, n02, n03, n04, n05). Despite these joint- and limb-level benefits associated with exoskeleton assistance, no subject averaged metabolic benefits were evident when compared to the unassisted condition. Both paretic trailing limb angle and integrated anterior paretic ground reaction forces were reduced with assistance applied as compared to no assistance at four speeds (n00, n01, n02, n03). Our results suggest that despite appropriate scaling of ankle assistance by the exoskeleton controller, suboptimal limb posture limited the conversion of exoskeleton assistance into forward propulsion. Future studies could include biofeedback or verbal cues to guide users into limb configurations that encourage the conversion of mechanical power at the ankle to forward propulsion. N/A.}, journal={JOURNAL OF NEUROENGINEERING AND REHABILITATION}, author={McCain, Emily M. and Dick, Taylor J. M. and Giest, Tracy N. and Nuckols, Richard W. and Lewek, Michael D. and Saul, Katherine R. and Sawicki, Gregory S.}, year={2019}, month={May} }