A study of hand-handle interactions and hand-arm biodynamic response to vibration

Hand-arm vibration syndrome (HAVS) is the term often used for the symptoms associated with prolonged occupational exposure to hand-arm vibration arising from hand-held power tools. The human hand and arm response to vibration has been widely investigated in terms of force-motion relationships at the hand-handle interface. Owing to the complex nature of the biological system, the reported data suggest inconsistent contributions due to various intrinsic and extrinsic factors. Furthermore, the injury risk posed by exposure to hand-tool vibration is strongly related to the hand-handle contact force. This dissertation research concerns with the study of hand-handle interactions under static and dynamic grasping of different handles, and characterization of hand-arm biodynamic response to vibration. The hand-handle interactions in static grasping task were characterized for various hand force combinations through measurement of interface pressure distributions using a flexible capacitive pressure sensing grid. The pressure peaks were assessed in view of known pressure discomfort threshold and sustained pressure values. The results suggest that pressure developed in the thenar eminence, when grasping a large size handle, could exceed the discomfort threshold. The hand-forces were defined in terms of independent grip and push forces, and hand-handle coupling and contact forces. The measured data were used to propose regression models for estimating contact force and interface peak pressure from directly measurable grip and push forces as a function of the handle size. The biodynamic responses of the human hand-arm system exposed to xh- and zh-axes vibration were characterized in terms of driving-point mechanical impedance and dissipated power. The experiments were designed to study the influences of various intrinsic and extrinsic factors, namely handle geometry, posture, magnitude and direction of vibration and hand forces. Both biodynamic responses were found to be better correlated with the coupling force below 200 Hz, and with the contact force at higher frequencies under zh-axis of vibration. Apart from the experimental observations, the high significance of majority of the main factors was supported by multi-factor ANOVA. The effects of handle size, and push and grip forces on the biodynamic responses of the human hand-arm exposed to vibration were observed to be more significant for the extended forearm posture than that for the bent-elbow posture. The effects were far more significant for the extended arm posture, which revealed considerably higher coupling with the vibrating handle