Functional relationships between grasp and transport components in a prehension task

Abstract Prehension in adults is a highly developed motor skill that affords the study of how components of a movement are coordinated to produce the near endless variety of acts that serve to acquire objects in near body space. This study used three-dimensional movement analysis to describe the kinematic characteristics and coordination of the transport (the reach) and the manipulation (the grasp) components of prehension. Six subjects reached for and grasped 10 different sized wooden disks. Results indicated that the initial phase of the prehension movement could be considered as structured in advance of the movement in that, over the ten disk sizes, time of limb transport was a constant up to peak deceleration. The time after peak deceleration to object contact, however, increased as disk size decreased. This led to the movement trajectories being uniquely shaped for each disk size and as such did not support a proportional duration based model of motor programming. Other findings indicated that maximum grip aperture was reached progressively sooner as disk size was decreased and that maximum grip aperture was highly related to the size of the to-be-grasped disk. In terms of the coordination between the transport and grasp components, support was not found for a temporal linkage between them nor did their coordination appear to be dependent on a motor program. Rather, from an analysis of the spatial variability of the transport and grasp components, evidence was found for supporting the idea that the coordination between these two components was achieved by a sensorimotor process. Through this process, and given the goal of a prehension movement, it was argued that the two components are linked functionally rather than temporally or spatially. These results are discussed in terms of current sensorimotor models of motor control and prehension.