Abstract
The amplitudes of stretch reflex components are known to be modified according to voluntary movement. Specially, the change in long-latency reflex (M2 component) is bigger than that in spinal stretch reflex during voluntary activity. Therefore it has been considered that the M2 component is related to motor control, and it has been assumed that the change M2 component in motor control changes for the purpose of movement. However, it has not been, studied experimentally whether degree of change in M2 component causes functional differences in muscle tension output. The present study was conducted to investigate what effect the M2 component during reaction movement of wrist flexion or extension had on those movements, and to clarify the degree to which modulation of long-latency activity played a functional role in movement performance. Seventeen healthy men, ranging in age from 20 to 28 participated in the study. A DC torque motor was used to generate sudden angular displacements in the extension direction at the right wrist joint. Analysis of the averaged surface electromyogram recorded from the wrist flexor muscle showed that short- and long-latency reflexes (M1 and M2 components) were evoked by muscle stretching. The results are summarized as follows: 1) The M2 amplitude from flexor muscle was increased during reaction movement in the flexion direction, and decreased in the extension direction conversely. In short, the long-latency reflex activity was task-dependent. 2) In flexion reaction task, the bigger the M2 amplitude was, the smaller the handle movement which was drived by torque motor in the extension direction became. Consequently, there was negative correlation between the increase of M2 amplitude and the shortening of flexion motor time in movement performance. 3) In extension reaction task, smaller M2 amplitude led a faster handle speed in the extension direction. Consequently, there was positive correlation between the decrease of M2 amplitude and the shortening of extension motor time in movement performance. These results reveal that a change of the M2 component is closely related to motor control with supraspinal pathways, and movement performance is subject to the degree of modulation of long-latency activity. This seems to suggest that a change in the long-latency reflex has a functional effect on the voluntary muscle tension output in the initiation of movement.
