1 Journal of Applied Biomechanics, 2013, 29, 1-11 © 2013 Human Kinetics, Inc. Steven L. Fischer (Corresponding Author) is with the School of Kinesiology and Health Studies, Queen’s University, Kingston, ON, Canada. Bryan R. Picco, Richard P. Wells, and Clark R. Dickerson are with the Department of Kinesiology, University of Waterloo, Waterloo, ON, Canada. The Roles of Whole Body Balance, Shoe-Floor Friction, and Joint Strength During Maximum Exertions: Searching for the “Weakest Link” Steven L. Fischer, 1 Bryan R. Picco, 2 Richard P. Wells, 2 and Clark R. Dickerson 2 1 Queen’s University; 2 University of Waterloo Exerting manual forces is critical during occupational performance. Therefore, being able to estimate maximum force capacity is particularly useful for determining how these manual exertion demands relate to available capacity. To facilitate this type of prediction requires a complete understanding of how maximum force capacity is governed biomechanically. This research focused on identifying how factors including joint moment strength, balance and shoe-loor friction affected hand force capacity during pulling, pressing downward and pushing medially. To elucidate potential limiting factors, joint moments were calculated and contrasted with reported joint strength capacities, the balancing point within the shoe-loor interface was calculated and expressed relative to the area deined by the shoe-loor interface, and the net applied horizontal forces were compared with the available friction. Each of these variables were calculated as participants exerted forces in a series of conditions designed to systematically control or restrict certain factors from limiting hand force capacity. The results demonstrated that hand force capacity, in all tested directions, was affected by the experimental condi- tions (up to 300%). Concurrently, biomechanical measures reached or surpassed reported criterion thresholds inferring speciic biomechanical limitations. Downward exertions were limited by elbow strength, whereas pulling exertions were often limited by balance along the anterior-posterior axis. No speciic limitations were identiied for medial exertions. Keywords: biomechanics, hand force, ergonomics, force capability Incorporating human force producing capability into job design can be an effective method to match job demands with workers’ functional capacity. Since the nineteenth century, researchers have measured force production in an effort to match individual capability with anticipated performance demands (Sargent, 1897) to optimize performance and minimize injury risk. However, measuring individual worker capabilities to facilitate this matching is time and cost intensive. Alternatively, models designed to predict force producing capability could enable designers to more readily match job demands with prospective worker capabilities. However, there have been few attempts to develop comprehensive predictive models (Grieve, 1979a; 1979b; Kerk et al., 1994), and none designed to incorporate three-dimensional tasks. This may be due to an incomplete understanding about how force capacity is limited. Several factors, both extrinsic and intrinsic to the worker, can limit force producing capability during manual material handling exertions. Limiting factors include whole body balance (Kerk et al., 1998; Holbein & Chafin, 1997), shoe-loor friction (Kroemer, 1974), hand-handle friction (Seo et al., 2010), and individual joint moment strength (Chafin, 1997). Whole body balance, as a factor limiting occupa- tional performance, has been operationally deined within the paradigm of postural stability. Postural stability is achieved when static moment equilibrium is met (Grieve, 1979a; 1979b; Kerk et al., 1994). In this paradigm the hand force and the force of gravity acting on the center of mass (COM) each contribute moments about a balancing point or fulcrum within the area deined by the shoe-loor interface and must balance to achieve static equilibrium. Maximum pulling force for example (Figure 1), could therefore be achieved by shifting the balancing point as far forward as possible while concurrently shifting the COM posteriorly. This strategy serves to increase the moment arm and corresponding moment contribution from the COM, while reducing the moment arm of the applied hand force, allowing the maximum applied hand force to be obtained such that the applied moment does not exceed the available counterbalancing moment of the COM. An Official Journal of ISB www.JAB-Journal.com ORIGINAL RESEARCH