Response synergies over a single leg when it is perturbed during the complex rhythmic movement of pedalling

Previous studies report that perturbing the posture of humans evokes specific patterns of muscular synergies in the legs. This study investigated the pattern of muscular responses of a whole limb when it was rapidly perturbed in the phase of extending during stationary pedalling. Subjects were instructed to resist. Accordingly, we anticipated increased extensor activity at knee and ankle to overcome the perturbation. This did not occur in the initial responses, appearing at latencies of 85-132 ms (mean = 104 ms). In contrast, there was facilitation in tibialis anterior, and the knee extensors vastus medialis and rectus femoris, together with profound inhibition of the ankle extensors soleus and lateral gastrocnemius. The anticipated extensor response across the limb appeared in the subsequent pattern of electromyogram (EMG) activity, with latencies ranging from 121 to 195 ms (mean = 168 ms), together with a large increase in propulsive force on the pedal. The difference in EMG patterns and latencies between initial and subsequent synergies was used to separate the responses into an earlier 'prevolitional' and a later 'volitional' component.