Locomotor adaptation to a soleus EMG-controlled antagonistic exoskeleton

Abstract

Locomotor adaptation in humans is not well understood. To provide insight into the neural reorganization that occurs following a significant disruption to one's learned neuromuscular map relating a given motor command to its resulting muscular action, we tied the mechanical action of a robotic exoskeleton to the electromyography (EMG) profile of the soleus muscle during walking. The powered exoskeleton produced an ankle dorsiflexion torque proportional to soleus muscle recruitment thus limiting the soleus' plantar flexion torque capability. We hypothesized that neurologically intact subjects would alter muscle activation patterns in response to the antagonistic exoskeleton by decreasing soleus recruitment. Subjects practiced walking with the exoskeleton for two 30-min sessions. The initial response to the perturbation was to "fight" the resistive exoskeleton by increasing soleus activation. By the end of training, subjects had significantly reduced soleus recruitment resulting in a gait pattern with almost no ankle push-off. In addition, there was a trend for subjects to reduce gastrocnemius recruitment in proportion to the soleus even though only the soleus EMG was used to control the exoskeleton. The results from this study demonstrate the ability of the nervous system to recalibrate locomotor output in response to substantial changes in the mechanical output of the soleus muscle and associated sensory feedback. This study provides further evidence that the human locomotor system of intact individuals is highly flexible and able to adapt to achieve effective locomotion in response to a broad range of neuromuscular perturbations.

Fig. 1.

Electrodes placed on the skin of the subject's leg recorded electromyographic (EMG) signals from the soleus muscle, a plantar flexor. A computer processed the soleus EMG to control air pressure sent to an artificial pneumatic muscle so that there was a proportional relationship between EMG amplitude and air pressure. As air pressure increased, it caused the artificial muscle to develop tension by shortening in length. The exoskeleton effectively created a cocontraction about the ankle joint proportional to soleus muscle activation. White arrows about the ankle joint show the direction of torque created by the exoskeleton and user's soleus muscle. Thus, when the soleus muscle was activated, the exoskeleton produced a dorsiflexor torque limiting the ability of the soleus to produce plantar flexor torque.

Fig. 2.

A: mean and SD of step cycle torque profiles calculated from all 10 subjects. Data for net ankle overground is the net ankle torque subjects created during overground walking without the exoskeleton. B: data for the two powered exoskeleton (Exo.) conditions including the dorsiflexion torque and power produced solely by the exoskeleton during treadmill walking. The exoskeleton is only able to actively produce dorsiflexion torques. When the power created by the exoskeleton is positive, it indicates the exoskeleton is generating energy occurring during periods when the artificial pneumatic muscle is both shortening (during ankle joint dorsiflexion) and producing torque. Negative values indicate the exoskeleton is absorbing energy, which occur when the artificial pneumatic muscle is both lengthening (during ankle joint plantar flexion) and producing torque. Vertical dashed lines indicate stance to swing transitions.

Fig. 3.

Mean and SD of step cycle kinematic profiles for the ankle, knee, and hip joint angles calculated from all 10 subjects during baseline, initial resistance, and final resistance time periods. Vertical dashed lines indicate stance to swing transitions.

Fig. 4.

Mean and SD of maximum ankle plantar flexion, maximum knee extension, and maximum hip extension occurring at specific phases of the gait cycle during the baseline, initial resistance, and final resistance time periods. Data were calculated from all 10 subjects. *Significantly different from baseline.

Fig. 5.

Mean and SD of step cycle EMG profiles calculated from all 10 subjects for the soleus (SOL), medial gastrocnemius (MG), lateral gastrocnemius (LG), and tibialis anterior (TA) during the baseline, initial resistance, and final resistance time periods. EMG values were normalized to the peak value occurring during baseline. Vertical dashed lines indicate stance to swing transitions.

Fig. 6.

Mean and SD of step cycle EMG profiles calculated from all 10 subjects for the rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL), and medial hamstring (MH) during the baseline, initial resistance, and final resistance time periods. EMG values were normalized to the peak value occuring during baseline. Vertical dashed lines indicate stance to swing transitions.

Fig. 7.

Mean and SD of step cycle profiles from individual subjects showing data of the SOL, MG, LG, ankle angle, and exoskeleton dorsiflexor torque at the baseline and final resistance time periods. Subject 1 selected to minimize plantar flexor activity in response to the exoskeleton resistance (A) and Subject 4 selected to maintain muscle activity at levels similar to baseline in response to the exoskeleton resistance (B). Vertical dashed lines indicate stance to swing transitions.

Fig. 8.

Linear regressions showing the relationship between the changes in peak EMG values calculated from soleus-medial gastrocnemius and soleus-lateral gastrocnemius during the final resistive time period. EMG values have been normalized to baseline.