Deficits in phasic muscle force generation explain insufficient compensation for interaction torque in cerebellar patients

A simple paradigm was used to investigate how patients with cerebellar lesions cope with the need to correct for joint interactions during a multi-joint movement. Normal subjects and patients with cerebellar degeneration performed fast unconstrained elbow flexions with the instruction to voluntarily fixate the shoulder joint. Angular kinematics and inverse dynamics analyses were performed. A susceptibility index quantified how strong-concomitant shoulder-motion depended on interactions from the elbow. Amplitudes of involuntary shoulder movements increased with elbow movement speed and were generally larger in patients. Susceptibility indices were smaller in patients, indicating a more variable compensatory response, however, increased with elbow movement speed. We conclude that patients were significantly less able to 'tune' their postural stabilizing response to match interaction torques. However, the velocity dependence of this effect points to a deficit in generating normal levels of phasic torque.