Relationship between the degree of inhibited stretch reflex activities of the wrist flexor and reaction time during quick extension movements

Abstract

It has been reported that stretch reflex responses, including the long latency component, are modulated by motor preparation for the direction and type of movement. In the present study, human subjects were required to make a reaction movement in the direction of the wrist extension following a muscle stretch to the wrist flexor, and we investigated the relationship between the modulation of reflex activities of the wrist flexor and the length of reaction time (premotor time) of the wrist extensor. Twenty-five healthy males, ranging in age from 20 to 28, participated in the experiments. A DC torque motor was used to evoke the reflex EMG responses on the flexor. Averaging the rectified EMG, recorded with the surface electrodes over the flexor, showed short and long latency reflexes (M1 and M2 components) in response to the muscle stretch. For all subjects, the amplitudes of the reflex components during the extension reaction movement decreased, compared to those amplitudes in the non-reaction tasks. The decrease in the M2 component, which is considered a transcortical reflex, was significantly larger than the decrease in the M1 component, which is a spinal reflex. Moreover, there were correlations between reaction time to muscle stretch and the degree of decrease in reflex activities with the extension reaction ( r=0.652 for M1, r=0.813 for M2, P<0.01). It became clear that the subjects with shorter reaction times inhibited their reflex activities of the flexor, particularly the M2 component which prevents the extension movement, to a greater degree than the subjects with longer reaction times. Therefore, our results suggest that the degree of M2 modulation directly reflects the individual motor control required to perform quick movements.