RECIPROCAL INHIBITION OF SOLEUS MOTOR OUTPUT IN HUMANS DURING WALKING AND VOLUNTARY TONIC ACTIVITY

The extent to which an active, human motoneuron pool can be inhibited via short-latency inhibitory pathways was studied by stimulating the common peroneal nerve and recording the inhibition of on-going soleus electromyographic (EMG) activity. The responses were compared at the same EMG level during walking and tonic voluntary activity to determine whether the inhibition was task dependent. In both tasks the amount of inhibition (measured as the depression in rectified, filtered, and averaged EMG activity) increased approximately linearly with the amount of motor activity, as determined from the mean EMG level before stimulation (correlation coefficient ≥0.9). No difference in the amount of inhibition was found between the two tasks at the same stimulus and EMG levels. Previously published studies based on the H-reflex method have reported that the amount of inhibition decreases with the amount of motor activity. On the contrary, single-unit studies and the present results suggest that segmental inhibitory reflexes retain their capacity to mediate a rapid reduction of motoneuronal discharge during voluntary activity. This inhibition may be important in regulating the amount of activity early in the stance phase of walking and during the transition from stance to the swing phase. Analytic results are derived in an Appendix that should be of general interest in interpreting the inhibition of motor units from a peristimulus time histogram (PSTH). The linear correlation between inhibition and level of voluntary activity can be explained if newly recruited units are strongly inhibited by the stimulus, whereas previously active motor units are inhibited relatively less, as their firing rate increases with increasing background activity.