Biomechanics of Hand/Handhold Coupling and Factors Affecting the Capacity to Hang On.

Abstract

A loss of hand/object coupling can lead to injury. Of particular importance are situations where the hand is used to support the body, as slippage of the hand from a handhold can lead to a fall. The general aim of this dissertation is to create knowledge that can explain the strength of the couple between the hand and a grasped handhold. Specifically, methods to measure the hand’s functional capacity to create force on a grasped handhold were developed and the effects of handhold properties on the capacity to hang on were evaluated.

This dissertation presents and discusses the following new findings: (1) hand/handhold coupling is comprised of active (muscle) and passive (friction) components, (2) surface friction increases the amount of force needed to pull an overhead handhold from the grasp of the hand (“breakaway strength”) by 25% compared to simulated zero-friction conditions. (3) isometric grip strength significantly under- or over-predicts the ability to hang onto handholds, (4) smaller handholds than would be predicted by grip strength literature increase capacity 7-17% for horizontal handholds, (5) capacity is reduced by 16-32% for handhold cross-sectional shapes with corners as compared to cylinders, (6) subjects can only support their bodyweight for horizontal handholds with one hand, (7) wearing gloves may increase or decrease capacity depending on glove properties, (8) friction influences the normal force distribution over a handle surface during pull tasks and consequently affects required biomechanical loading of finger flexor muscles.

These findings can be used as the basis for the development of biomechanical models that predict how much force can be exerted on the object before it slips free or is pulled from the grasp of the hand. They can also be used to recommend design of handholds on ladders, fixed equipment, stairwells, and other safety critical items to increase the ability to support the body with the hand. Similarly, they can be used to design handles that reduce required hand force for tools and work equipment, reducing risk of slippage, injury, localized fatigue and work-related musculoskeletal disorders.