Multi-digit Position and Force Coordination in Three- and Four-Digit Grasping

In this paper, we investigate the redundancy problem by examining how humans control grasping of a hand-held object using three-and four-digits in response to external perturbations. Our results revealed a similar variability in digits' initial placements when grasping with three-and four-digits. Moreover, the distribution of digit normal forces were modulated depending on the number of digits used, their locations, and the type of the external perturbations. Our results suggest that the redundancy problem was addressed by the central nervous system in a similar fashion on a trial-to-trial basis in terms of digits' initial placements, but differently in terms of digits normal force distribution that was controlled online depending on the number of digits actively involved in the grasp.