A speed-adaptive myoelectric ankle exoskeleton to improve post-stroke walking performance
Taylor JMD, Emily M McCain, Tracy N Giest, and Gregory S Sawicki
1. University of Queensland, St Lucia, QLD, Australia
2. North Carolina State University, Raleigh, NC, USA
3. Georgia Institute of Technology, Atlanta, Georgia, USA
Limited ankle `push-off’ power is a hallmark of post-stroke gait, leading to high inter-limb asymmetry, reduced walking speeds, and elevated metabolic demands . Powered exoskeletons offer a promising approach to restore mechanical deficits by applying torque at the paretic ankle during push-off. Previous devices  have been effective at improving paretic ankle plantarflexion moment, but were unable to increase ankle power output or reduce the metabolic costs of paretic gait. This devices’ performance was limited because it was unable to respond to reductions in plantarflexor muscle activity that occur with added assistance. Here, we developed and tested a powered ankle exoskeleton – driven with the user’s soleus electromyographic (EMG) signal and ground reaction force profile – that modulates propulsive assistance with speed and ensures assistance levels do not decline with reduced muscle activity. We tested the device during a speed-endurance test in 6 stroke survivors walking on an instrumented treadmill with and without an ankle exoskeleton on their paretic limb. We collected kinematics, kinetics, EMG, and metabolic cost. The exoskeleton increased assistive torque as walking speed increased and muscle activity decreased, verifying its efficacy. At all speeds, net paretic ankle power was significantly higher with the exoskeleton compared to normal walking. Despite this improvement in ankle function, only 2 of the 6 subjects experienced a reduction in metabolic cost while wearing the device. This study suggests that assistive devices have potential to restore paretic ankle function in post-stoke individuals, however future work into understanding proximal joint-level effects of added assistance is necessary.
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