Changes in the H-reflexes of ankle extensor and flexor muscles at the initiation of a stepping movement in humans.

The underlying neural mechanisms of the silencing of electromyographic (EMG) activity observed between antagonistic muscles of the human ankle joint preceding a fast stepping movement were examined. Twelve subjects were asked to perform a unilateral stepping movement under a reaction time condition while standing upright. A silent phase (SP) which was composed of the time difference between the offset of tonic soleus (Sol) activity and the onset of phasic tibialis anterior (TA) activity was observed consistently in all subjects. At the time interval corresponding to the SP, inhibition of the Sol Hoffmann reflex (H-reflex) and facilitation of the TA H-reflex took place. In nine of the 12 subjects, disynaptic reciprocal Ia inhibition (Ia inhibition) from TA Ia fibers to the Sol motoneuron (MN) pool increased before the TA EMG onset. We also observed a clear distinction between the changes in the time course of the Ia inhibition and that of long latency reciprocal (D1) inhibition in six of the 12 subjects. However, when the subjects performed the same motor task with their backs supported against a wall wherein the tonic Sol activation disappeared, no changes in the H-reflex occurred. These results suggest that these changes, occurring at the segmental level, might be controlled by the supraspinal motor center in a feed-forward manner during anticipatory postural adjustment. The functional significance would be to counterbalance the disturbance from an intentional forthcoming movement in order to maintain an adequate posture.