Comparison of two types of silent period observed just before a rapid voluntary movement : counter movement and non-counter movement

Abstract

It has been observed that the premotion silent period appears just before a rapid voluntary movement. This phenomenon is believed to be caused by one of the inhibitory discharge from the central nervous system. On the other hand, the silent period is also observed when muscles are relaxed by counter movement. The present study was designed to compare some physical performances of both silent periods. Twenty healthy males aged 18-24 years were subjected to a series of experiment. They were asked to maintain the posture of standing with flexed knee joint at about 50 degrees on the force plate. In the first condition, they were requested to extend their knee joint responding to a flashing lamp as quickly as possible (Non-counter movement). In the second condition, they were asked to extend knee joint as quickly as possible with counter movement to a flashing lamp. The EMG activities of M. rectus femoris, M. vastus lateralis, M. vastus medialis and M. biceps femoris were recorded using bipolar surface electrodes. Force curve from a force plate and electrogoniogram were recorded simultaneously. The following results were obtained from this experiment. 1) The rate of appearance in complete silent period was higher on the counter movement (M. rectus femoris: 37.8%, M. vastus lateralis: 81.6%, M. vastus medialis: 59.7%) than on the non-counter movement (28.2%, 57.6%,46.5%, respectively). 2) The latency of silent period was significantly later on the counter movement (M. vastus lateralis:132.8 msec, M. vastus medialis: 135.5 msec) than on the non-counter movement (119.4 msec, 116.4 msec,respectively). 3) The duration of silent period was significantly longer on the counter movement (M. vastus lateralis: 100.0 msec, M. vastus medialis: 96.6 msec) than on the non-counter movement (40.9 msec, 41.9 msec, respectively). 4) No significant difference in the rate of tension rise was found between the counter movement (3.7kg/msec) and the non-counter movement (2.7 kg/msec). 5) The peak value of the force curve was significantly greater on the counter movement (173.0 kg)than on the non-counter movement (141.2 kg). It is suggested from these results that the silent period of the counter movement delayed the reaction time (latency and duration) and increased the muscular strength in comparison with the premotion silent period.