RELATIONSHIP BETWEEN THE DEGREE OF INHIBITED STRETCH REFLEX ACTIVITIES OF THE WRIST FLEXOR AND REACTION-TIME DURING QUICK EXTENSION MOVEMENTS

It has been reported that stretch reflex responses, including the long latency component, are modulated by motor preparation for the directi on and type of movement. In the present study. human subjects were req uired to make a reaction movement in the direction of the wrist extens ion following a muscle stretch to the wrist flexor, and we investigate d the relationship between the modulation of reflex activities of the wrist flexor and the length of reaction time (premotor time) of the wr ist 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, r ecorded 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 du ring the extension reaction movement decreased, compared to those ampl itudes in the non-reaction tasks. The decrease in the M2 component, wh ich is considered a transcortical reflex, was significantly larger tha n the decrease in the M1 component, which is a spinal reflex. Moreover , there were correlations,ns between reaction time to muscle stretch a nd the degree of decrease in reflex activities with the extension reac tion (r = 0.652 for M1, r = 0.813 fur M2, P < 0.01). It became clear t hat the subjects with shorter reaction times inhibited their reflex ac tivities of the flexor, particularly the MZ component which prevents t he extension movement, to a greater degree than the subjects with long er reaction times. Therefore, our results suggest that the degree of M 2 modulation directly reflects the individual motor control required t o perform quick movements. (C) 1997 Elsevier Science Ireland Ltd.