Effects of Finger Combination and Center of Mass for Digit Force Control During Multi-Finger Grasping

This study aimed to examine the digit force control during multi-finger grasping with different finger combination and different object's center of mass (COM). Thirty healthy subjects were instructed to grasp a water cup to complete a lifting and descending with three types of gesture: (1) two digits by the thumb and index finger, (2) three digits by the thumb, index and middle fingers, and (3) four digits by the thumb, index, middle and ring fingers. A load was placed randomly at one of the following positions: middle, distal, proximal, ulnar and radial positions, at the bottom of the water cup, rendering torques at different directions. The digit forces were recorded using a group of six-component force/torque transducers. The mean (MN), standard deviation (SD) and the mean power frequency (MPF) were used as parameters to indicate the force levels and changes during grasping. Results showed significant differences in MN and SD of the grip force (p <; 0.05) when the subject were grasping the cup with the load at different locations by only their thumb and index finger. No significant differences were observed in the MN nor SD of digit forces among the different load conditions (p>0.05) by either the three-digit or the four-digit combinations. There were greater changes as the load was positioned at the distal than at other sites of the base, with respect to the condition when the load was positioned in middle, which may suggest a more robust control of digit forces against the perturbations of the ulnar side (U) and radial side (R) torques than those of the distal (D) torques. Moreover, the mean of grip force of thumb was significantly higher than that of other fingers in all conditions (p <; 0.05), meanwhile lower MPF of grip force of thumb during three-digit grasping and four-digit grasping were found in the SD at all different load conditions (p <; 0.05), which may reveal greater stability of thumb and greater flexibility of other fingers. These results showed the different finger combinations during multi-finger grasping may have inconsistent robustness dealing with different COM. This study provides a novel approach to quantify the capacity of digit force control during multi-digit grasping for a functional performance.