Effects during isometric voluntary contractions of elbow flexion muscles on EMG-reaction time (EMG-RT)

Abstract

The present experiments were performed to investigate changes of EMG reaction time with upperarm muscles. The reaction time in electromyogram of the elbow flexor (EMG-Reaction Time) was measured during quick isometric contractions with maintaining muscle force (the fraction of maximal voluntary contraction, MVC). Subjects, responding to audio stimulus (AS) and turning on electricity on skin stmulus (SS) presented about 2 sec to 4 sec after a verbal warning, and then responding his biceps brachii muscle as fast as possible in following contractive conditions : 0%MVC, 20%MVC, 40%MVC, 60%MVC and 80%MVC. Mean EMG-RTS were calculated from 10 to 20 trials. Compared to 60%MVC condition, EMG-RTS of 0% MVC were longer in both stimulus conditions. EMG-RTs and fractions of %MVC were correlated significant inverse between both AS and SS conditions. On the other hand, RT of SS condition was shown to be particularly shorter than that of AS condition with increasing % MVC. The EMG-RT should be influenced by the increase of muscle activities according to % MVC. Therfore, these results suggest that the motor cortex and/or other central location and the motoneuron pools may be excited due to enhancement of peripheral somatosentory activities.