The role of reflexes in the resting tremor of Parkinson's disease.

Forces and electromyograms were recorded from patients with Parkinson's disease during imposed joint movements. Muscles which were stretched by 3 to 5 Hz sinusoidally alternating movements often showed vigorous bursts of EMG activity whose timing established that it was a reflex response to the movement. The same movements provoke no stretch reflex response from normal subjects. When resting tremor was present the driven movements sometimes entrained it, and the bursts of EMG activity then became locked to the imposed movement. On other occasions the tremor activity continued at its own rate; EMG bursts then occurred at times unrelated to the movement, and the irregular force records reflected a conflict between the movement and the muscle activity. Tremor was most consistently entrained when a large mass of muscle was driven through a large movement at a frequency that was close to the usual tremor frequency. Tremor which involved synchronous contractions of muscles at different joints was often resistant to the effects of our imposed movements, but it could sometimes also be entrained by large movements of a single joint. When tremor was entrained by a driving movement, the EMG discharge was indistinguishable from a reflex response, and the limb exerted forces on the machinery which had the timing and magnitude that would be expected of a reflex response. Spontaneous tremor in the same subjects had frequencies which altered in the predicted way with changes of mechanical load. We conclude that peripheral reflexes are more important in parkinsonian tremor than has often been supposed, although afferent activity from the moving limb probably interacts with other potentially oscillatory mechanisms.