Shortening reaction in normal and parkinsonian subjects.

During passive extension of the elbow, the triceps shows a shortening reaction (SR), which is manifested by a burst of EMG activity. The SR and the velocity of passive movement (VPM) were measured in parkinsonian patients and control subjects. The mean SR was larger and the mean VPM was smaller in the parkinsonian group. An unexpected finding was that "reinforcement" caused an increase of the SR in normal subjects but not in the patients. The VPM is considered to be a measure of rigidity, and the SR provides a quantitative approach to the pathophysiology of muscle tone.