Reliability of postural response as a function of muscular synergisms: Effect of supraspinal lesions

The characteristics of muscle discharge were evaluated for a non-choice reaction time postural task. Five normal and four hemiplegic subjects were required to match the height of a target trace representing 50% of total body weight by swaying on a force plate in a predetermined direction. The number of default response for hemiplegics (trials with no muscle discharge) was significantly higher than normal and the nature of muscle discharge was tonic rather than phasic. As the number of trial repetitions increased, postural reaction time showed a tendency to decrease for normals and increase for hemiplegics. We have proposed that the reliability and phase quality of muscle discharge following target appearance was directly proportional to the degree of muscle preprogramming. In this regard, patients with supraspinal lesions could not fully integrate automatic components of postural sway and instead relied on conscious intervention to maintain balance.