Development of a biomechanical model of hand coupling for axial torque and push exertions on cylindrical handles.

Abstract

Sufficient axial torque and push must be applied with the hands to perform many activities of work, daily living and recreation. Slippage between the hands and work object can result in failure to complete the task and hand injuries. Also, repeated forceful exertions can result in fatigue, damage to the body, and cumulative trauma disorders. This dissertation aims to develop and evaluate biomechanical models that describe axial torque and push on a cylindrical handle in relation to contact force distribution, friction, handle size, and the force application direction. The following new findings were demonstrated using the biomechanical models and empirical data: (1) Applying axial torque in the direction the fingertips point (inward) on a high friction handle can improve maximum torque by 45% for a person with given strength, or reduce required muscle activities for completing a given torque exertion task. It is because skin friction produced by twisting an object towards the fingertips causes flexion of the fingertips and increases grip force and torque. (2) For the same reason, axial push tasks should allow people to apply some inward torque to increase grip force and thus axial push force. (3) Grip force can be four times greater by having the fingertip and thumb tip work together against the palm. (4) High grip force required due to low friction or heavy object weight, can reduce wrist extension strength and twisting capability in the outward direction (away from fingertips). For twisting in the opposite direction, low grip force resulted from fragility of the object can reduce wrist flexion strength and capability of twisting in the inward direction. In addition, a simple method for measuring hand static friction coefficients proposed in this dissertation can be easily used in the field in a timely manner by ergonomics practitioners or product designers to design more efficient and safer work objects. These findings in this dissertation can be implemented into the design of work objects to reduce required muscle activities for tasks that involve axial torque and push exertion, and thus reduce injuries from hand slippage and the risk of fatigue and musculoskeletal disorders.