Decreased contribution from afferent feedback to the soleus muscle during walking in patients with spastic stroke.

We investigated the contribution of afferent feedback to the soleus (SOL) muscle activity during the stance phase of walking in patients with spastic stroke. A total of 24 patients with hemiparetic spastic stroke and age-matched healthy volunteers participated in the study. A robotic actuator attached to the foot and leg was used to apply 3 types of ankle perturbations during treadmill walking. First, fast dorsiflexion perturbations were applied to elicit stretch reflexes in the SOL muscle. The SOL short-latency stretch reflex was facilitated in the patients (1.4 +/- 0.3) compared with the healthy volunteers (1.0 +/- 0.3, P = .05). Second, fast plantar flexion perturbations were applied during the stance phase to unload the plantar flexor muscles, thus, removing the afferent input from these muscles to the SOL motoneurons. These perturbations produced a distinct decrease in SOL activity that was significantly smaller in the patients (-30 +/- 3%) compared with the control subjects (-43 +/- 4%, P = .03). Third, slow-velocity, small-amplitude ankle trajectory modifications mimicking small deviations in the walking surface were applied to evaluate the afferent-mediated amplitude modulation of the locomotor SOL electromyogram (EMG). In the healthy volunteers these perturbations generated gradual increments and decrements on the SOL EMG; however, in the patients the SOL EMG modulation was significantly depressed (P = .04). Moreover, this depression was related to the spasticity level measured by the Ashworth score. These results indicate that although the stretch reflex response is facilitated during spastic gait, the contribution of afferent feedback to the ongoing locomotor SOL activity is depressed in patients with spastic stroke.