EFFECTS OF PREMOTION SILENT PERIOD ON SINGLE MOTOR UNIT FIRING AT INITIATION OF A RAPID CONTRACTION

We compared the single motor unit (SMU) activity between movements wit h a premotion silent period (PMSP) and without PMSP in EMG. Fourteen S MUs in the gastrocnemius muscle and 6 SMUs in the soleus muscle were r ecorded from 5 volunteers during isometric plantar flexion. Tonically firing SMUs failed to fire just before the onset of a rapid contractio n with PMSP more frequently than without PMSP. SMUs tended to fire wit hin 10 msec (the gastrocnemius SMUs) or 20 msec (the soleus SMUs) from the onset of the phasic EMG discharge when PMSP occurred. In a rapid contraction without PMSP, the initial firing of SMUs occurred with lon ger latency than that in a rapid contraction with PMSP. The latency of the initial SMU firing in a rapid contraction related to the precedin g time of the last SMU firing during a sustained contraction to the on set of the phasic EMG discharge. When the preceding firing was long en ough, the latency distributed around 10 msec. On the other hand, for s horter preceding times,the latency lengthened with shortening of the p receding time. It is suggested that the PMSP makes the preceding time long and increases the susceptibility of motor units to the descending command at the initiation of a rapid contraction.