EFFECTS OF ISOMETRIC FORCE ADJUSTMENTS ON PREMOTION SILENT PERIOD APPEARANCE

Abstract

A silent period in the surface EMG preceding the onset of phasic EMG activity has been observed when a ballistic movement was made on a weak sustained contraction. The purpose of this study was to investigate the premotion silent period (PMSP) appearance patterns during isometric force adjustments of a rapid elbow extension. The five subjects were instructed to perform the ballistic elbow extension movements for matching their force with a target. For MVC task, they were asked to extend their elbow as quickly and as hard as possible responding to the light signal. Thereafter, they extend their elbow as quickly as possible and align the force with the target (40, 60, 80% MVC) responding to the light. Rapid elbow extensions were exerted in a weak isometric elbow extension (< 20% MVC) responding to the imperative signal. The reaction movements were repeated 20 times with each task. Surface EMG was recorded from the triceps (lateral and long heads) and biceps brachii muscles. The PMSP frequency (r = 0.96), the peak dT/dt (r = 0.92), the rate of torque rise (r = 0.94) were all highly correlated with the intensities of muscle contraction. The PMSP duration and the time to peak torque, however, were not proportional to peak torque. When subjects perform tasks that require control of ballistic force, they didnÕt adjust the time to peak torque, but the peak dT/dt and the rate of torque rise. These force control characteristics of rapid contraction derived from highly stereotyped EMG pattern in agonist and antagonist muscles. This pattern is programmed in a motor system. A recruitment of additional motor units of larger size plays an important role in a ballistic contraction. The previous study reported that the PMSP makes the preceding time longer and increases the susceptibility of the motor units to the descending command at a initiation of the rapid contraction. In this study, the PMSP appeared frequently as the reaction force increased. It is proposed that the PMSP appearance mechanism is affected by the central nervous system controlling force intensity.