Kinematics of initiating a two-joint arm movement in patients with cerebellar ataxia.

OBJECTIVE To characterize kinematically any systematic aberration in multi-joint movements in cerebellar ataxia. METHODS Nine patients with cerebellar degeneration and nine normal subjects, mobile only at the shoulder and elbow of the right arm, were required to produce left-to-right cross-body linear hand trajectories on the horizontal surface of a digitizing tablet. Nonlinearity indicated failure of precise coordination of the two joints. A wide range of hand speeds was studied. Data analysis was restricted primarily to the first 130 ms of movement. RESULTS As hand velocities increased, normal subjects and, especially, patients produced misdirected, curved paths. Normal subjects had significant curvature when peak speeds exceeded 100 cm/s and a trend toward significant bi-directional angular deviation at velocities greater than 300 cm/s. In patients, peak path curvature was significantly greater than normal at peak velocities of 50 to 200 cm/s. By 3.3 cm, their paths deviated significantly outward at all but the slowest speeds. Overall, patients' maximal hand velocities and shoulder angular velocities, as well as maximal angular accelerations at both joints, were significantly lower than normal. CONCLUSIONS The patients' trajectory aberrations were attributed to a deficient rate of rotation at the shoulder relative to that at the elbow. Relative to task requirements, their rate of torque development was apparently deficient at both joints. but to a greater degree at the shoulder. Joint torque-rate impairment may contribute to the ataxia in both multi- and single-joint movements of patients with cerebellar disorders. A similar, but smaller impairment may produce milder nonlinearity in high-velocity movements of normal subjects.