PREMOVEMENT ELECTROMYOGRAPHIC SILENT PERIOD AND ALPHA-MOTONEURON EXCITABILITY

The purpose of this study was to investigate the possible neurophysiological mechanism of EMG signal silence (premovement silence (PMS)) preceding a ballistic movement in 10 healthy volunteers. The subjects were asked to respond to a flashing light signal by performing a plantar flexion in the ankle as strongly and quickly as possible. The EMG signals from agonists lateral gastrocnemius (LG) and soleus (SO), and antagonist tibialis anterior (TA) were simultaneously recorded together with the force signal. The excitability of spinal alpha-motoneuron pools was also determined at various phases of the movement execution by means of H-reflex analysis. Our results indicated that: (a) the maximal rate of force development (dF/dt) was significantly greater in the group of subjects demonstrating clear PMS than that without PMS; and (b) the significant decrease in H-reflex amplitude was observed approximately 40 ms prior to the appearance of PMS of both LG and SO muscles, which preceded the force development by about 50-60 ms. These data suggest that PMS is associated with an enhancement of rapid force development due possibly to optimal motor unit synchrony. The decreased H-reflex amplitude (decreased monosynaptic reflex gain) in the absence of parallel decrease in the surface EMG recording (maintained alpha-motoneuron activity) prior to PMS and complete disappearance of selected motor unit activities during PMS also suggest that the presynaptic inhibition and/or disfacilitation are mainly involved for PMS during a ballistic movement.