Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control

Abstract

Background 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. Methods 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. Results 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). Conclusions 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. Trial registration N/A. Electronic supplementary material The online version of this article (10.1186/s12984-019-0523-y) contains supplementary material, which is available to authorized users.

Effects of targeted assistance and perturbations on the relationship between pelvis motion and step width in people with chronic stroke

“…Perhaps for the participants with near-normal paretic ρdisp values, assistive forces interfered with the baseline gait pattern perceived as stable, and thus were resisted. Similar counter-intuitive effects of assistance have been previously observed in PwCS, with artificial plantarflexor power reducing anterior propulsive forces, 26 and knee flexion assistance for swing phase ground clearance increasing leg circumduction. 27 Manipulating post-stroke step width Beneficial effects may require adjustment of the force-field control paradigm, such that some participants do not perceive the assistance as perturbations that should be resisted.…”

Validity of Measurement for Trailing Limb Angle and Propulsion Force during Gait Using a Magnetic Inertial Measurement Unit

“…e anterior component of GRF (AGRF) during late stance is an important element of propulsion force. Impulse of AGRF in late stance is usually used to estimate the propulsion force and increases velocity of center of mass during late stance [7][8][9]. Previous studies report that propulsion force correlates with hip extension angle [10][11][12] as well as hip exion and ankle plantar exion moments [13].…”

Isolating the Energetic and Mechanical Consequences of Imposed Reductions in Ankle and Knee Flexion during Gait.

“…; height: 1.75±0.13 m; mass: 75.5±15.7 kg) walking at 0.8 m s -1 on an instrumented split-belt treadmill (Bertec, Columbus, OH, USA). We selected this speed because it is within the range of speeds reported for persons poststroke [28,42], allowed ambulation with bracing restricting both the ankle and knee of simultaneously, and was su ciently fast to allow for the detection of potential metabolic differences between conditions. Participants completed ve conditions, each lasting seven minutes, including: (1) control [control]: no brace worn, (2) unlocked [unlocked]: knee brace worn but unlocked, and three locked conditions: (3) unilaterally locked ankle [locked-ank], (4) unilaterally locked knee [locked-knee], and (5) unilaterally locked ankle + knee [locked-a+k].…”

“…Therefore, a common objective of post-stroke gait interventions is to alter the underlying mechanics and reduce additional work that may be associated with metabolic penalties [26][27][28]. However, the independent roles of ankle and knee dysfunction on compensatory mechanics and energetic cost are di cult to discern because ankle and knee motion are interrelated.…”