The coordination of reaching and grasping in spastic hemiparesis

Abstract

The kinematics and intrinsic dynamics of reaching and grasping movements in six subjects with spastic hemiparesis were studied. Movements were performed with both hands together as well as with each hand in isolation, and two target widths were used. As expected, large manual asymmetries existed in unimanual task performance. These asymmetries were more pronounced for grasping than for reaching, which was a consequence of the fact that in grasping the impaired hand stayed longer at zero velocity before lifting the object. This implies difficulties coordinating the more distal movement component (hand and fingers). In addition, the impaired hand attained the movement goal with a (1) decreased angular range of motion of the joints and (2) an increased trunk involvement. With respect to the intrinsic dynamics, intra- and inter-limb coupling was studied by evaluating cross-correlations of position–time functions joint pairs. In effector space the displacement of the wrist and ipsilateral shoulder were correlated, and in joint-angle space the elbow extension–shoulder flexion angle pair and the elbow extension–shoulder elevation angle pair were analysed. It was found that despite tight couplings between the different pairs, the couping strengths for the impaired limb were consistently lower. This was caused by fragmentation of the movement, mainly due to a large shoulder involvement at the start of the movement concurrent with little or no elbow extension. It is concluded that fragmentation of the movement, operationalised by the value of the cross-correlation between joint pairs is an essential variable by which the level of recovery of function can be captured. It was further shown that the unimpaired hand `mimicked' the impaired hand on a number of movement characteristics under bimanual responding. The impaired hand did not change its style of movement organisation among uni- and bimanual movement responding. It is argued that the apparent stability of the impaired hand can only be sustained at the cost of a decreased flexibility. Finally, inter-limb coupling, assessed by means of cross-correlations of the tangential velocity profiles of both hands, was shown to be high during the course of movement.