A study was undertaken to investigate whether the amplitudes of reflex EMG components (M1, M2 and M3) induced by sudden muscle stretching in the wrist flexor, are modified according to the reaction movement during wrist flexion, and how the aspects of reflex EMG components are related to premotor time under conditions of stretch stimulus (SS-PMT) and light stimulus (LS-PMT) . Fifteen healthy men, ranging in age from 22 to 28yr, participated in the study. A DC torque motor was used to evoke the reflex EMG activities on the wrist flexor. Analysis of the surface electromyogram recorded from the wrist flexor showed that short and long latency reflex components appeared in response to muscle stretch. In almost subjects, the amplitude of the M2 component was higher during reaction task than during no reaction task. The subjects were classified into two groups (I, II) according to the presence or absence of reflex components and their EMG amplitudes. SS- and LS-PMTs in group I were significantly shorter than those in group II. The amplitude of M2 in group I was significantly higher than in group II. In group I the M3 component was not identified, since the M2 component was followed by a voluntary EMG burst.
These results showed that the amplitudes of long latency reflex components increased during the reaction task in the flexion direction, and suggest that long latency components contribute to the initiation of voluntary movement in subjects with a shorter premotor time.

A study was conducted to investigate the relationship between long latency activities evoked by a sudden muscle stretch of the right wrist flexor and a peformance during a target match task accompanying wrist movement. The index of performance was the target match time (TMT) from onset of a handle displacement untill the handle position output was matched with a traget line. The standard deviation of TMT (TMT-S.D.) was calculated. Sixteen healthy men, ranging in age from 20 to 27 yr., participated in the study. A DC torque motor was used to generate an angular displacement at the right wrist joint. Analysis of the surface electromyogram recorded from the wrist flexor showed that the short and long latency reflex components (Ml, M 2 and M3) were evoked by muscle stretching. In the target match task, the amplitude of the M 2 and M 3 components for all subjects were significantly smaller than the M 1 component (p<0.01) . In some subjects the M 3 component was not identifiable, since its amplitude was very small in the target match movement. There were significant correlations between the M 2 amplitude and performances (TMT : r=0.857, p<0.01 and TMT-S. D. ; r=0.651, p<0.01) during the target match task. In shoat, the smaller the M 2 amplitude was, the shorter the TMT became and the smaller the TMT-S. D, became. These results suggest that the change in the M 2 component is related to motor control for the target match task.

The relationships were examined between the relative force level and exertion strategy during rapid isometric contractions. Thirteen subjects exerted isometric pinchings as fast as possible at various force levels. Force curves and surface EMGs from the first dorsal interosseous muscle were obtained during the contractions. While the single-peaked force curves remained analogous up to a certain force level, they changed to multi-peaked ones at higher force levels. The critical force level, which could be determined in all subjects as relative force level to MVC, was positively correlated with the %MVC EMG magnitude for unit time, obtained below the critical level. EMG durations were significantly shorter blow than above the critical level. These results suggest that different exertion strategies are utilized in rapid contractions at different force levels, and that individual differences in the critical force level are related to difference in the manner of motor unit activation.