Load compensation tasks evoke tremor in cerebellar patients: the possible role of long latency stretch reflexes

'Tremor' is one of the clinical signs of cerebellar dysfunction. Its nature remains subject to debate, one hypothesis being that of a predominant role of peripheral afferences in its genesis. This study was designed to study whether load compensating tasks, evoking sudden stretch, and thus stimulation of peripheral afferences induced tremor in cerebellar patients. We study the kinematics and EMG pattern of a load compensating task which consists of maintaining a constant elbow position despite the onset and cessation of a 2 Nm torque loading the elbow flexors in eight cerebellar patients and six controls. Angular position and velocity, and EMG of the biceps and triceps are recorded at a sampling rate of 1 kHz. In normal subjects, trajectories are simple with little overshoot of the aimed position. EMG analysis shows a long latency stretch response (LLSR) which initiates a phasic and then tonic voluntary activity. In cerebellar patients, the two prominent cinematic features are hypermetria and tremor. The stretch response is of the same latency, but the EMG pattern is modified with bursts of activity related to the tremor. These results show severe perturbations of load compensating tasks in cerebellar patients. We discuss the possible role of the exaggeration of LLSR in both hypermetria and tremor.