Invariant ankle moment patterns when walking with and without a robotic ankle exoskeleton

Abstract

To guide development of robotic lower limb exoskeletons, it is necessary to understand how humans adapt to powered assistance. The purposes of this study were to quantify joint moments while healthy subjects adapted to a robotic ankle exoskeleton and to determine if the period of motor adaptation is dependent on the magnitude of robotic assistance. The pneumatically-powered ankle exoskeleton provided plantar flexor torque controlled by the wearer's soleus electromyography (EMG). Eleven naïve individuals completed two 30-min sessions walking on a split-belt instrumented treadmill at 1.25 m/s while wearing the ankle exoskeleton. After two sessions of practice, subjects reduced their soleus EMG activation by ~36% and walked with total ankle moment patterns similar to their unassisted gait (r 2 = 0.98±0.02, THSD, p>0.05). They had substantially different ankle kinematic patterns compared to their unassisted gait (r 2 = 0.79±0.12, THSD, p<0.05). Not all of the subjects reached a steady state gait pattern within the two sessions, in contrast to a previous study using a weaker robotic ankle exoskeleton . Our results strongly suggest that humans aim for similar joint moment patterns when walking with robotic assistance rather than similar kinematic patterns. In addition, greater robotic assistance provided during initial use results in a longer adaptation process than lesser robotic assistance.

Development and Evaluation of a Powered Artificial Gastrocnemius for Transtibial Amputee Gait

“…The knee moment impulse reduction may stem from an attempt by the participants to prevent the total knee moment from changing in the presence of the external disturbance. This result is consistent with other studies, as humans have been observed to maintain invariant joint moments at both the ankle [25] and hip joints [26] when an external torque from an exoskeleton is applied. This finding has a potential consequence: if the applied joint moment from the AG exceeded the affected knee moment from the ZERO condition, the participant might attempt to maintain total joint kinetics by providing an active extension moment with their quadriceps, so to prevent the total knee flexion moment from increasing.…”

Locomotor adaptation to a soleus EMG-controlled antagonistic exoskeleton

“…We found that humans significantly reduced soleus recruitment when walking with a powered exoskeleton that acutely increased the gain between either soleus Kao et al 2010a) or gastrocnemius (Kinnaird and Ferris 2009) activation and the resulting plantar flexor torque. This resulted in total (human ϩ exoskeleton) ankle moments that were comparable to unassisted walking (Kao et al 2010a).…”

“…We have previously employed robotic exoskeletons to investigate human adaptation to acute disruptions of the neuromuscular map of two plantar flexor muscles: soleus and gastrocnemius (Cain et al 2007;Gordon and Ferris 2007;Kao et al 2010a;Kinnaird and Ferris 2009). These two muscles typically display similar activation profiles during gait (Arsenault et al 1986).…”

Design and evaluation of a new exoskeleton for gait rehabilitation

“…In order to guide the development of lower limb exoskeletons, some studies (Kao et al, 2010) had, as purpose, the understanding of how humans adapt to powered assistance.…”