Hand-Handhold Coupling: Effect of Handle Shape, Orientation, and Friction on Breakaway Strength

OBJECTIVE The aim was to determine the maximum force that can be exerted on an object before it is pulled or slips from the grasp of the hand ("breakaway strength") for fixed overhead handholds of varying orientation, shape, and friction. BACKGROUND Many studies have quantified hand strength by having participants squeeze, pull on, or create torque on an object or handle, but few studies have measured breakaway strength directly. METHOD In two experiments, hand strength was measured as both overhead breakaway strength for handholds typical of fixed industrial ladders and as maximum isometric grip strength measured using a common Jamar grip dynamometer. RESULTS Breakaway strength was greatest for a fixed horizontal cylinder ("high friction"; 668 +/- 40 N and 691 +/- 132 N for Experiments 1 and 2, respectively), then for a horizontal cylinder that simulated low surface friction ("low friction"; 552 +/- 104 N), then for a vertical cylinder (435 +/- 27 N), and finally, for a vertical rectangular-shaped rail (337 +/- 24 N). Participants are capable of supporting only their own body weight with one hand when grasping the fixed horizontal cylinder. Breakaway strength for both the high- and low-friction horizontal cylinders was significantly greater than isometric grip strength (1.58 +/- 0.25 and 1.26 +/- 0.19 times, respectively). CONCLUSION Results support the hypothesis that hand-handhold coupling is composed of active (isometric or eccentric finger flexion) and passive (frictional) components. Traditional isometric grip strength alone does not predict the strength of a couple between a hand and a handhold well. APPLICATION This research shows that handhold shape, orientation, and friction are important in the safe design of grab rails or ladders.