Changes in short and long latency stretch responses during the transition from posture to movement

Experiments were performed in 18 normal subjects to estimate the time course of changes in the gains of pathways mediating short- and long-latency responses to muscle stretch during the transition from a maintained posture against a steady load to a rapid ballistic movement. Subjects were instructed to rapidly flex or extend their forearm in response to a tone from an initial position of 90 degree of elbow flexion. Torque pulses stretching the biceps muscle were applied to the forearm at 8 different times before and after the signal to initiate the movement, and the gains of short- and long-latency pathways were estimated from averages of rectified biceps EMG activity for 20 trials at each time interval between the onsets of the tone and torque pulse. The findings demonstrate that changes in the magnitude of long-latency responses (M2, M3) occur during the period between the onset of the auditory signal and the voluntary motor response. However, the magnitude of the short-latency response (M1) remains unchanged until after the onset of voluntary motor activity. The differences in the timing of short- and long-latency stretch responses suggests that activity in long-latency pathways may play an important preparatory role in facilitating the transition from posture to movement.