MULTIJOINT ARM MOVEMENTS IN CEREBELLAR-ATAXIA - ABNORMAL CONTROL OF MOVEMENT DYNAMICS

In cerebellar ataxia, kinematic aberrations of multijoint movements ar e thought to originate from deficiencies in generating muscular torque s that are adequate to control the mechanical consequences of dynamic interaction forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impai rment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate th e relative contribution of these two mechanisms and, therefore, analyz ed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Unrestrained v ertical arm movements were performed at three different target movemen t velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, m uscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast m ovements. In contrast to normal subjects, patients produced hypermetri c movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of tor que change at elbow and shoulder joints. The patients' deficit in gene rating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simu ltaneous shoulder joint flexion generated an extensor influence upon t he elbow joint. When attempting to terminate the movement, gravitation al and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor mu scular torques occurred simultaneously at both joints. This temporal p attern of muscular torque generation at shoulder and elbow joint was p reserved in patients. Our data suggest that an impairment in generatin g sufficient levels of phasic muscular torques significantly contribut es to the patients' difficulties in controlling the mechanical consequ ences of dynamic interaction forces during multijoint movements.