The mechanism of electromyographic silent periods preceding a ballistic voluntary plantar flexion]

The present study was designed to investigate the neurophysiological mechanism of electromyographic silent periods(SP) preceding a ballistic voluntary movement. Ten male subjects were asked to respond to the flashing light by performing a plantar flexion as strongly and quickly as possible. The myoelectric signals from agonist (m. gastrocnemius, caput laterale: LG, m. soleus: SOL) and antagonist (m. tibialis anterior: TA) were simultaneously recorded together with the isometric contraction force signal. The excitability of the alpha motoneuron pools by means of the H reflex analysis was also determined at various phases of the movement. The results obtained are as follows: 1) In five out of 10 subjects, the electromyographic silent period was consistently observed (SP Group), while the rest of the subjects did not show such changes (NSP Group). 2) The maximum rate of tension raise (dF/dt) in SP Group (32.3 +/- 24.9 N/ms) was significantly (p less than .05) greater than NSP Group (15.1 +/- 12.0 N/ms). 3) In SP Group, the averaged H-wave mean amplitude decreased about 40ms prior to the appearance of SP. From these results, it can be concluded that SP in this study was attributable to the supraspinal influences so as to allow a specific motor program to be executed for selective recruitment of phasic (high threshold) motor units.