The experiment reported examined: (a) the role of the geometrical body scaled informational invariant for the transition of human grip configurations; (b) whether the same invariant can be scaled considering also the force applied during the grasp phase; and (c) how the temporal duration of the grasp and displacement phases of prehension are scaled to the object properties of size and mass. Adult subjects performed a series of trials in reaching, grasping, and displacing spheres that varied in size and mass. The grip transitions were described by the body scaled relation:where Ls and Ms are, respectively, the diameter and the mass of the spheres grasped and Lh and Mh are the length and the mass of the hand. The impulse during the grasp phase was linearly related with the mass of the spheres within each density. The temporal durations of the grasp and displacement components were scaled coherently to the object properties. These findings provide support to the hypothesis that the grasp and displacement components of prehension are organized coherently within a single action.
P. Cesari is with the Department of Sport and Movement Science at the University of Verona, 37131 Verona, Italy. K.M. Newell is with the Department of Kinesiology at The Pennsylvania State University, University Park, PA 16802-3408.