Force distribution at the hand/handle interface for grip and pull tasks

Abstract

The purpose of this experiment was to investigate how surface pressure distribution at the hand/handle interface changes during gripping and pulling and to enhance understanding of coupling between hands and grasped objects. Two pressure-sensing arrays were wrapped around a 3.18 cm diameter cylindrical handle which was fixed to a six-axis load cell. Six male subjects grasped the fixed overhead instrumented handle and performed two exertion types: downward pull exertions of 30, 60, and 90 percent of their measured grip strength and a maximal isometric grip exertion. Consistent with previous studies, the greatest pressure was exerted on the distal segments of the phalanges and at the base of the thumb and palm during maximum isometric gripping. However, when pulling, pressure on the palm (underside of the handle) was negligible. Most pull pressure was distributed over the fingers unevenly in a bimodal distribution, with the greatest pressures occurring on the proximal side of the hand followed by the fingertips. This supports the hypothesis that friction acts through the soft tissues of the fingers and creates an increased normal force in the direction of proximal segments (i.e. “belt friction”). When the surface of the handle is lumped into 5 equal zones, the observed bimodal trend is lost, with the greatest average pressure observed at the top of the handle corresponding to the pull direction. This discrepancy highlights the importance of resolution when making conclusions about applied hand forces. Future research is needed to investigate pressures on handles of different size and surface friction to develop comprehensive models of hand/object coupling.