Temporal control of the reach and grip components during a prehension task in humans.

The kinematic changes of the manipulation and transport components during a prehensile movement were examined in an experimental condition that required alterations during the manipulation phase. Subjects started with a normal finger posture but were required to briefly open and close their thumb and index finger while transporting the wrist to grasp the object. The changes in the aperture altered most of the kinematic landmarks of the wrist. There was considerable slowing of most kinematic characteristics for the wrist transport, and the velocity profile of the wrist showed two peaks. The time of occurrence of the first peak was poorly correlated with the time of the first maximal finger aperture (before the brief finger touch). In contrast, the second wrist velocity peak was highly correlated with the time to the second maximal finger aperture (just before the grasp of the object). Thus, as the reach progressed and the object to be grasped was approached, the temporal relationship between the wrist and aperture components of the movement became more temporally related.