Hand-Handhold Coupling: Effect of Handle Shape, Orientation, and Friction on Breakaway Strength Justin G. Young, Charles Woolley, Thomas J. Armstrong, and James A. Ashton-Miller, University of Michigan, Ann Arbor 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 over- head 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 horizon- tal 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) com- ponents. 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. Address correspondence to Justin G. Young, 1205 Beal Ave., Industrial and Operations Engineering, University of Michigan, Ann Arbor, MI 48109; jgy@umich.edu. HUMAN FACTORS, Vol. 51, No. 5, October 2009, pp. 705-717. DOI: 10.1177/0018720809355969. Copyright © 2009, Human Factors and Ergonomics Society. INTRODUCTION Motivation Falls are a major cause of injury and mortal- ity in the working-age population. The Bureau of Labor Statistics reports that 827 fatalities resulted from falls in U.S. workplaces in 2006, with 77 deaths associated with falls from non- moving vehicles, 132 from ladders, and 21 involving steps or stairs (BLS, 2007). An aver- age of 136,118 nonfatal injuries associated with falls from ladders are treated in U.S. emergency rooms each year, with a 50% increase in the number of injuries from 1990 to 2006 (D’Souza, Smith, & Trifiletti, 2007). Background The hand is commonly used to help sup- port the body by gripping handles and other objects in the workplace. There are many situ- ations in which a loss of hand-handhold cou- pling can result in a fall to the same or a lower level. Examples include climbing into or out of heavy equipment (tractors, semitrucks), climb- ing on ladders, hanging onto moving vehicles (garbage truck personnel), and using safety rails (stairways, scaffolding, bathroom grab rails) (Barnett & Poczynck, 2000; Bottoms, 1983). In many of these situations, if the individual were to slip or fall, his or her weight would be trans- ferred suddenly from the feet to the hands, and the strength of the couple between the hand and the handhold being grasped would determine whether the individual will support his or her body weight or lose grip of the handhold and be injured. The hand is also the interface that allows workers to hold and use work objects, such as