Modulation of stretch reflex by anticipation of the stimulus through visual information.

A stretch stimulus was applied to the right elbow flexor by the free gravitational fall of a weight. The subject was instructed to flex the elbow joint to lift the weight quickly (resist task) or to extend the elbow joint to overtake the falling weight (assist task) as the voluntary reaction to the stretch stimulus. In the resist task, when the subject could predict the time of stretch stimulus onset by watching through the mirror the behavior of the experimenter going to give the stretch stimulus, the integrated EMG (I-EMG) of M2 (50-80 ms after the stimulus onset) and M 3 (80-100 ms) components of stretch reflex were significantly enhanced compared with when the experimenter's movements were invisible to the subject, while no differences were observed in the background discharge before the stretch stimulus onset (BGA) and the short latency reflex (M 1). In the assist task, the M 2 and M 3 were depressed when the visual information about the stimulus presentation was available, while the BGA and M 1 were unchanged. From these results, it is suggested that (1) the preparatory set should be classified into two categories, i.e., task-related "topographical set" and timing-related "chronographic set", and (2) visual information about the process of stimulus presentation can modulate the reflex activity of stretched muscle allowing the required task to be executed efficiently by accurately anticipating the stretch stimulus onset.