Exp Brain Res (1998) 119:493±503  Springer-Verlag 1998 RESEARCH ARTICLE Helge Topka ´ Jürgen Konczak ´ Klaus Schneider Andreas Boose ´ Johannes Dichgans Multijoint arm movements in cerebellar ataxia: Abnormal control of movement dynamics Received: 28 October 1996 / Accepted: 30 September 1997 H. Topka ( ) ) ´ J. Konczak ´ A. Boose ´ J. Dichgans Department of Neurology, University of Tübingen, Hoppe-Seyler-Strasse 3, D-72076 Tübingen, Germany e-mail: topka@uni-tuebingen.de, Fax: +49-7071-29-6507 K. Schneider Institute of Sport Science, Hochschule der Bundeswehr, Neubiberg, Germany Abstract In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from defi- ciencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic inter- action 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 muscu- lar torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assess- ment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Un- restrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dy- namics approach was employed to compute net joint tor- ques, muscular 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 move- ments. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients deficit in gen- erating appropriate levels of muscular force were promi- nent during two different phases of the pointing move- ment. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simulta- neous shoulder joint flexion generated an extensor influ- ence upon the elbow joint. When attempting to terminate the movement, gravitational 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 muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generat- ing sufficient levels of phasic muscular torques signifi- cantly contributes to the patients difficulties in control- ling the mechanical consequences of dynamic interaction forces during multijoint movements. Key words Cerebellar ataxia ´ Limb movements ´ Dynamics ´ Human Introduction Previous studies on multijoint movements in cerebellar limb ataxia have shown characteristic kinematic abnor- malities that are associated with cerebellar function. Pa- tients tend to move slower than normal subjects and ex- hibit reduced peak hand acceleration and deceleration (Bastian et al. 1996; Massaquoi and Hallett 1996). Mo- tions about the shoulder and elbow joints are hypermetric compared with normal subjects and, as a consequence, the curvature of the hand path increases (Goodkin et al. 1993; Massaquoi and Hallett 1996). These kinematic abnormal- ities are only subtle when moving at slow or moderate ve- locities but are marked when subjects attempt fast move- ments (Massaquoi and Hallett 1996; Topka et al. 1994). To understand the nature of the tasks that have to be resolved by the central nervous system when performing accurate multijoint movements, it is helpful to consider the mechanical characteristics of the human arm and to investigate the dynamics of limb movements. Mechani- cally, the human arm represents a multisegmented limb of linked bodies. In order to perform accurate movements, task-adequate intersegmental coordination is required. To achieve coordination among limb segments, muscle tor- ques at each joint have to be adjusted to account for the