Tangential load sharing among fingers during prehension

The goal of the study was to examine the force sharing among the fingers during static prehension under systematic loading and postural changes. A custom-built handle was constructed that allowed for bi-directional loading (upward and downward) with different load magnitudes (250, 750 and 1250 g). Five- and three-digit grasps were tested. The fingers were spaced 2, 3 or 6 cm apart. The handle was oriented vertically such that the tangential forces acted parallel to the applied load. There were no differences in tangential sharing patterns between males and females. The factors that did affect the sharing pattern (ranked from the smallest to the largest effect) were: TRIAL (i.e., inter-trial variability), LOAD MAGNITUDE, SUBJECT, HAND POSTURE and LOAD DIRECTION. Normal force sharing accounted for much but not all of the variability in tangential sharing. The results suggest that loading direction should be considered when designing tools that require functional tangential forces for successful task completion (e.g., screwdrivers, jar lids, etc.). A hypothesis to account for the directional changes based on the passive properties of the fingers is proposed.