PREHENSION WITH TRUNK ASSISTED REACHING

For prehensile tasks, where objects are located beyond the normal reac hing space, the trunk is bent forward to assist in the transport of th e wrist to the object. Such task behaviors raise complex motor control issues such as how is the trunk movement incorporated into the motor plan. In this experiment, seated subjects were asked to reach and gras p a small and a large object placed on a table located beyond their ma ximal reach. Forward trunk bending was required to extend the reach di stance. For such reaching movements, the wrist velocity consisted of a bell shape profile similar to those seen when the arm is the sole tra nsport agent. In most trials, the trunk was the first to initiate move ment, although there was no strict pattern of initiation order. The tr ansport data showed that trunk and arm movement components were decoup led at the end of the reach. While the object was being grasped and li fted, the trunk continued moving for approximately 180 ms after the gr asp. Wrist deceleration time expressed in absolute and relative values was sensitive to object size. The time from maximum peak aperture to the end of wrist movement also was significantly longer for grasping t he small compared to the large object. No such relationships were obse rved for the trunk. Temporal coupling was only observed between the gr ip and wrist transport component. Time to maximum aperture was signifi cantly correlated with time to peak wrist deceleration and only rarely with time to trunk deceleration peak. When the trunk participates in the transport of the wrist to an object, these findings suggest that o nly the wrist component is directly related to the achievement of the grasp. While the trunk assisted the arm to reach the object, the kinem atic parameter recorded did not reveal any evidence of direct coupling . The presented data suggests that the planning takes place at the lev el of the hand and that endpoint is the primary variable controlled.