Reach-to-grasp movements during obstacle avoidance.

The transport and grip components are two controlled components of a prehensile movement. These components are coordinated so that objects of varying size and shape resting in diverse locations can be grasped easily. It has been suggested that the timing between these two components is a specified parameter, although the origin of such timing is unknown. The present study examines the interdependency of the reach and grasp components when the transport component is modified by placing an obstacle of varying height (9 cm and 11 cm) in the hand path between the starting position and the target object location. Subjects were asked to reach over a Plexiglas barrier and grasp a 2-cm dowel. To reach the object, the subject had to elevate the hand. At issue in this experiment is whether changes in hand path trajectory caused by obstacle avoidance produce corresponding changes in the kinematics of grip aperture. The findings showed that reaching in the presence of an obstacle resulted in the prolongation of most transport component time parameters except peak acceleration and a few amplitude parameters. Changes in the transport component also produced systematic prolongation in all time parameters of grip kinematics, including grip closure time. Temporal prolongation was also reflected in a significant decrease in grip opening and closing velocity; only relative time-to-peak closing velocity was maintained. Closure distance and maximum grip aperture were smaller for the obstacle conditions. Together with the observed smaller variability for the distance to peak aperture, these findings suggest that spatial localization of the hand aperture is an important prehensile movement control feature. Parameterization processes for the grip component are closely linked to those of the transport component, and their organization appears to be interdependent.