FEATURE OF EMG PREMOTION SILENT PERIOD FROM THE VIEWPOINT OF A MOTOR CONTROL

Abstract

An electromyographic silent period preceding to a synchronous discharge in a rapid voluntary movement was investigated from the viewpoint of a motor control. While subject stood and held his knees bended stationary against the gravity, he was asked to respond to a signal with a rapid vertical jump as quickly as possible. During the preparation to the movement, motor units activity, which was detected with use of semi-microelectrodes, revealed itself monotonically, and this activity was identified as motor volley from “continuously long interval firing motor units” from analysis of the discharge frequency as described by Grimby and Hannerz. Hence the resulting silence must be due to disappearance of the tonic motor volleys under the conditions of these experiments. The problem of how this adjustment is brought about was speculated as follows. It can be assumed that the central nervous system switches from tonic to phasic excitation related with a rapid change from tonic to phasic voluntary contraction of muscle. As to the occurrence of the silence, the muscle electrical activity for static contraction must cease earlier than onset of the phasic motor volleys. EMG premotion silent periods, those began not coincidentally in a movement were observed among different muscles. They appeared, in many cases, orderly dependent on the onset of the synchronized muscle electrical activity. However, the silent periods without any order were recorded in a movement. Furthermore, it was confirmed that this silent period can occur in a limited muscle or its part in a movement. These results led to an assumption that the silences with the orderly onset of the synchronized EMG appeared in better-coordinated movement than the movement with the silent period without such order and with the silence in a limited muscle or its part. Even more interesting is the assumption that well trained subject can excite tonic and phasic motor units separately and switch their excitation more agilly, because the premotion silent period was observed frequently in trained subjects.