Kinematic variability of grasp movements as a function of practice and movement speed.

Grasp movements were studied in six female subjects to determine the effects of practice and movement speed on kinematics and movement variability. Subjects performed four-joint pinch movements of the index finger and thumb, with 200 repetitions at each of three durations (100, 200, and 400 ms). As observed previously, movements of high velocity were performed with bell-shaped, single-peaked velocity profiles. In contrast, slower movements (approximately 200, 400 ms) were performed as a series of two to four submovements with multiple peaks in the associated joint angular velocity profiles. With practice, only the slowest movements (400 ms duration) showed significant reductions in variability of joint end-positions. Surprisingly, variability of finger and thumb joint end-positions did not increase with increasing movement speed as has been observed for arm pointing movements. This was apparently due to reductions in positional variability during deceleration of the movement which offset increases in positional variability during acceleration. Neither practice nor movement speed affected variability of the location of fingertip contact on the thumb, which always occurred on the thumb distal pulpar surface.