Prehension synergies: Trial-to-trial variability and principle of superposition during static prehension in three dimensions

We performed three-dimensional analysis of the conjoint changes of digit forces during prehension (prehension synergies) and tested applicability of the principle of superposition to three-dimensional tasks. Subjects performed 25 trials at statically holding a handle instrumented with six-component force/ moment sensors under seven external torque conditions; -0.70, -0.47, -0.23, 0.00, 0.23, 0.47, and 0.70 Nm about a horizontal axis in the plane passing through the centers of all five digit force sensors (the grasp plane). The total weight of the system was always 10.24 N. The trial-to-trial variability of the forces produced by the thumb and the virtual finger (an imagined finger producing the same mechanical effects as all 4 finger forces and moments combined) increased in all three dimensions with the external torque magnitude. The sets of force and moment variables associated with the moment production about the vertical axis in the grasp plane and the axis orthogonal to the grasp plane consisted of two noncorrelated subsets each; one subset of variables was related to the control of grasping forces (grasp control) and the other sassociated with the control of the orientation of the hand-held object (torque control). The variables associated with the moment production about the horizontal axis in the grasp plane did not include the grip force (the normal thumb and virtual finger forces) and showed more complex noncorrelated subsets. We conclude that the principle of superposition is valid for the prehension in three dimensions. The observed high correlations among forces and moments associated with the control of object orientation could be explained by chain effects, the sequences of cause-effect relations necessitated by mechanical constraints.