The aim of the present study was to gain insight into the control that hemi
paretic subjects have over fast, unimanual aiming movements. Twelve hemipar
etic 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 (spas
tic/nondominant) and preferred (nonspastic/dominant) arm. Although the patt
ern of kinematics of hemiparetic subjects generally corresponded with that
reported in earlier reaching and grasping studies, the commonly observed pr
olonged movement time of the nonpreferred arm as compared to the preferred
arm was absent. The spatial variability of the lateral hand displacements t
oward stationary targets was highest in the spastic arm of the hemiparetic
subjects, indicating diminished motion stability. Even though hemiparetic s
ubjects were expected to be unable to adjust their movements flexibly to th
e position and the velocity of a moving target, they used an initial estima
te of where moving targets would be hit in the same way as the healthy subj
ects did, i.e., they started aiming toward a position in front of the targe
t. 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 he
miparetic subjects. The results are related to possible deficits in the reg
ulation of cocontraction underlying movement stability.