Torque build-up during power hand tool operation constitutes a relatively short period of time, but its influence on muscle exertion is most significant due to repeated exposure to relatively high reaction forces. This study considers the dynamic nature of the human power hand tool operator as a single degree of freedom mechanical system. The hand is therefore represented as mass, spring and damping elements. These are dependent upon the posture used and the individual operator. An apparatus was used to obtain these elements by measuring the vibration frequency and amplitude change after an initial disturbance. Twenty-five subjects participated in the experiment. A repeated measures full factorial experiment tested the effects of gender, horizontal distance (30,60, and 90 cm), and vertical distance (55, 93, 142, and 190 cm) on the three passive elements in the model for a pistol grip handle. The result shows that stiffness and mass moment of inertia changed by 20.6 percent and 44.5 percent respectively with vertical location (p<.01), and 23.6 percent and 41.2 percent respectively with horizontal location (p<.01). Mass moment of inertia and damping changed by 31.1 percent and 38.5 percent respectively with gender (p<.01). The handle displacement and hand force during torque build-up was calculated based on this model. The model was validated by having five subjects operate an actual power hand tool and by measuring tool displacement using an Opto Trak motion analysis system. The model predictions were correlated with these measurements (R=.88).
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