RAPID WRIST MOVEMENTS IN PATIENTS WITH ESSENTIAL TREMOR - THE CRITICAL ROLE OF THE 2ND AGONIST BURST

Ballistic wrist flexion movements towards 15, 30 and 60 degrees visual targets were studied in a group of 17 patients with hereditary essent ial tremor. Compared with age-matched normal subjects (n = 16), there were three main kinematic differences: patients overshot the target a little more; the kinematic profile of their movements was more 'asymme tric' due to higher peak decelerations; and their movements initiated tremor Ballistic movements performed by patients with essential tremor were associated with a triphasic pattern of agonist-antagonist-agonis t muscle activity similar to that of normal subjects. The duration and size of each EMG burst was normal. The onset latency of the antagonis t EMG burst was also normal, but the onset of the second agonist EMC b urst was delayed The delay in the onset of the second agonist EMG acti vity resulted in unopposed action of the antagonist muscle in the seco nd half of each movement. As a result, deceleration occurred too rapid ly as the hand returned past the target leading to a series of damped oscillations around the point of aim. The onset latency of the second agonist EMG burst correlated significantly with the tremor period; the longer the period the later the burst. We speculate that the delay in the second agonist burst reflects an abnormality in the timing of ant icipatory muscle activity in essential tremor and that this may involv e cerebellar mechanisms.