REACH-TO-GRASP MOVEMENTS DURING OBSTACLE AVOIDANCE

The transport and grip components are two controlled components of a p rehensile movement. These components are coordinated so that objects o f varying size and shape resting in diverse locations can be grasped e asily. It has been suggested that the timing between these two compone nts is a specified parameter, although the origin of such timing is un known. 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 betwee n the starting position and the target object location. Subjects were asked to reach over a Plexiglas barrier and grasp a 2-cm dowel. To rea ch the object, the subject had to elevate the hand. At issue in this e xperiment is whether changes in hand path trajectory caused by obstacl e avoidance produce corresponding changes in the kinematics of grip ap erture. The findings showed that reaching in the presence of an obstac le resulted in the prolongation of most transport component time param eters except peak acceleration and a few amplitude parameters. Changes in the transport component also produced systematic prolongation in a ll time parameters of grip kinematics, including grip closure time. Te mporal prolongation was also reflected in a significant decrease in gr ip opening and closing velocity; only relative time-to-peak closing ve locity was maintained. Closure distance and maximum grip aperture were smaller for the obstacle conditions. Together with the observed small er variability for the distance to peak aperture, these findings sugge st that spatial localization of the hand aperture is an important preh ensile movement control feature. Parameterization processes for the gr ip component are closely linked to those of the transport component, a nd their organization appears to be interdependent.