The influence of hand-arm posture on the biodynamic responses under z(h)-axis vibration is investigated in terms of driving point mechanical impedance and absorbed power under various combinations of hand grip and push forces, handle sizes and excitation levels. Laboratory measurements of the biodynamic responses were performed on seven healthy male subjects exposed to two levels of broadband random vibration in the 8-1000 Hz frequency range using three instrumented cylindrical handles of different diameter (30, 40 and 50 mm), and different grip (10, 30 and 50 N) and push (25, 50 and 75 N) forces. The experiments involved grasping the handle with two different postures, consisting in flexed forearm with elbow angle of 90 degrees and extended forearm with elbow angle of 180 degrees, with wrist being in the neutral position for both postures. The results revealed remarkable effects of the hand-arm posture on both the mechanical impedance and the absorbed power characteristics. The low-frequency apparent mass magnitude of the hand-arm with the extended forearm posture was observed to be approximately three times than that with the flexed forearm posture. Furthermore, 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. This posture revealed considerably higher coupling with the vibrating handle and a damper-like behavior of the hand-arm system in the very low-frequency range. This posture also resulted in considerably higher power absorption than the flexed forearm posture for the majority of the combinations of hand forces considered. Depending on the handle size, excitation level, and hand forces, the total power absorbed by the hand-arm with an extended hand-arm could be up to 96% higher than that with the flexed elbow. From the results, it is evident that the hand-arm posture strongly affects the biodynamic response, when exposed to z(h)-axis vibration. Relevance to industry Operators assume considerable variations in the hand-arm posture while operating hand-held power tools. Such variations coupled with variations in the hand forces imparted on the tool handle could cause considerably different biodynamic responses of the hand-arm system. The current International Standard describes the range of idealized biodynamic response in terms of the driving-point mechanical impedance under a fixed posture involving 90 degrees elbow angle (ISO-10068, 1998). The standard on the assessment of antivibration gloves also requires the same posture (ISO- 10819, 1996). This study presents the effect of hand-arm posture on the biodynamic responses in terms of both driving-point mechanical impedance and power absorption by the hand-arm system. The influences of hand forces and handle sizes on both measures are also presented. The results of the study clearly suggest high significance of the hand-arm posture in view of the biodynamic responses. The results of the study attained on the driving-point mechanical impedance are expected to provide the vital knowledge and data for enhancing the current standard and its applications. The results attained on the influence of posture on the absorbed power characteristics of the hand-arm system are further expected to enhance our knowledge on the assessment of vibration exposure. (c) 2005 Elsevier B.V. All rights reserved.
