Kinematics of fast hemiparetic aiming movements toward stationary and moving targets.

The aim of the present study was to gain insight into the control that hemiparetic subjects have over fast, unimanual aiming movements. Twelve hemiparetic subjects with cerebral palsy and twelve healthy subjects were asked to hit, as quickly as possible, stationary and moving targets projected onto a frontoparallel screen. The task was performed with the nonpreferred (spastic/nondominant) and preferred (nonspastic/dominant) arm. Although the pattern of kinematics of hemiparetic subjects generally corresponded with that reported in earlier reaching and grasping studies, the commonly observed prolonged movement time of the nonpreferred arm as compared to the preferred arm was absent. The spatial variability of the lateral hand displacements toward stationary targets was highest in the spastic arm of the hemiparetic subjects, indicating diminished motion stability. Even though hemiparetic subjects were expected to be unable to adjust their movements flexibly to the position and the velocity of a moving target, they used an initial estimate of where moving targets would be hit in the same way as the healthy subjects did, i.e., they started aiming toward a position in front of the target. In both subject groups, this spatial estimate and the movement time (MT) varied as a function of target velocity, suggesting that the use of target-velocity information in hitting moving targets is unaffected in spastic hemiparetic subjects. The results are related to possible deficits in the regulation of cocontraction underlying movement stability.