@article{zhang_tu_si_lewek_huang_2023, title={A Robotic Assistance Personalization Control Approach of Hip Exoskeletons for Gait Symmetry Improvement}, ISSN={["2153-0858"]}, DOI={10.1109/IROS55552.2023.10341440}, abstractNote={Healthy human locomotion functions with good gait symmetry depend on rhythmic coordination of the left and right legs, which can be deteriorated by neurological disorders like stroke and spinal cord injury. Powered exoskeletons are promising devices to improve impaired people's locomotion functions, like gait symmetry. However, given higher uncertainties and the time-varying nature of human-robot interaction, providing personalized robotic assistance from exoskeletons to achieve the best gait symmetry is challenging, especially for people with neurological disorders. In this paper, we propose a hierarchical control framework for a bilateral hip exoskeleton to provide the adaptive optimal hip joint assistance with a control objective of imposing the desired gait symmetry during walking. Three control levels are included in the hierarchical framework, including the high-level control to tune three control parameters based on a policy iteration reinforcement learning approach, the middle-level control to define the desired assistive torque profile based on a delayed output feedback control method, and the low-level control to achieve a good torque trajectory tracking performance. To evaluate the feasibility of the proposed control framework, five healthy young participants are recruited for treadmill walking experiments, where an artificial gait asymmetry is imitated as the hemiparesis post-stroke, and only the ‘paretic’ hip joint is controlled with the proposed framework. The pilot experimental studies demonstrate that the hierarchical control framework for the hip exoskeleton successfully (asymmetry index from 8.8% to − 0.5%) and efficiently (less than 4 minutes) achieved the desired gait symmetry by providing adaptive optimal assistance on the ‘paretic’ hip joint.}, journal={2023 IEEE/RSJ INTERNATIONAL CONFERENCE ON INTELLIGENT ROBOTS AND SYSTEMS (IROS)}, author={Zhang, Qiang and Tu, Xikai and Si, Jennie and Lewek, Michael D. and Huang, He}, year={2023}, pages={6125–6132} }
 @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{duppen_wrona_dayan_lewek_2023, title={Evidence of Implicit and Explicit Motor Learning during Gait Training with Distorted Rhythmic Auditory Cues}, ISSN={["1940-1027"]}, DOI={10.1080/00222895.2023.2231874}, abstractNote={Abstract Gait training with rhythmic auditory cues contains motor learning mechanisms that are weighted more explicitly than implicitly. However, various clinical populations may benefit from a shift to gait training with greater implicit motor learning mechanisms. To investigate the ability to incorporate more implicit-weighted motor learning processes during rhythmic auditory cueing, we attempted to induce error-based recalibration using a subtly varying metronome cue for naïve unimpaired young adults. We assessed the extent of implicit and explicit retention after both an isochronous metronome and subtly varying metronome frequency during treadmill and overground walking. Despite 90% of participants remaining unaware of the changing metronome frequency, participants adjusted their cadence and step length to the subtly changing metronome, both on a treadmill and overground (p < 0.05). However, despite evidence of both implicit and explicit processes involved with each metronome (i.e., isochronous and varying), there were no between-condition differences in implicit or explicit retention for cadence, step length, or gait speed, and thus no increased implicit learning advantage with the addition of error-based recalibration for young, unimpaired adults.}, journal={JOURNAL OF MOTOR BEHAVIOR}, author={Duppen, Chelsea Parker and Wrona, Hailey and Dayan, Eran and Lewek, Michael D.}, year={2023}, month={Jun} }
 @article{mccain_dalman_berno_libera_lewek_sawicki_saul_2023, title={The influence of induced gait asymmetry on joint reaction forces}, volume={153}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2023.111581}, abstractNote={Chronic injury- or disease-induced joint impairments result in asymmetric gait deviations that may precipitate changes in joint loading associated with pain and osteoarthritis. Understanding the impact of gait deviations on joint reaction forces (JRFs) is challenging because of concurrent neurological and/or anatomical changes and because measuring JRFs requires medically invasive instrumented implants. Instead, we investigated the impact of joint motion limitations and induced asymmetry on JRFs by simulating data recorded as 8 unimpaired participants walked with bracing to unilaterally and bilaterally restrict ankle, knee, and simultaneous ankle + knee motion. Personalized models, calculated kinematics, and ground reaction forces (GRFs) were input into a computed muscle control tool to determine lower limb JRFs and simulated muscle activations guided by electromyography-driven timing constraints. Unilateral knee restriction increased GRF peak and loading rate ipsilaterally but peak values decreased contralaterally when compared to walking without joint restriction. GRF peak and loading rate increased with bilateral restriction compared to the contralateral limb of unilaterally restricted conditions. Despite changes in GRFs, JRFs were relatively unchanged due to reduced muscle forces during loading response. Thus, while joint restriction results in increased limb loading, reductions in muscle forces counteract changes in limb loading such that JRFs were relatively unchanged.}, journal={JOURNAL OF BIOMECHANICS}, author={McCain, Emily M. and Dalman, Morgan J. and Berno, Matthew E. and Libera, Theresa L. and Lewek, Michael D. and Sawicki, Gregory S. and Saul, Katherine R.}, year={2023}, month={May} }
 @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={Abstract}, 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 non-disabled 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={ABSTRACT}, 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.}, 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}, 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}, 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}, 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} }