Muscle responses to simulated torque reactions of hand-held power tools

The aim of this work was to investigate physiological responses to torque r eaction forces produced by hand-held power tools used to tighten threaded f asteners. Such tools are used repetitively by workers in many industries an d are often associated with upper limb musculoskeletal complaints. The tool s considered for stimulation in this study had straight handles and require d from 100 to 400 ms to tighten fasteners to a peak torque of 1.0 to 2.5 Nm and from 50 to 150 ms for the torque to decay to zero. A tool stimulator w as constructed to apply a programmed torque profile to a handle similar to that of a straight in-line power screwdriver. Wrist flexor and extensor sur face EMGs and handle position were recorded as subjects held handles subjec ted to controlled torque loads that tended to flex the wrist. It was found that: (1) very high EMG values occurred even though torques were of short d uration (50 to 600 ms) and the peak torques were low (7 - 28% of maximum st rength); (2) high EMGs in anticipation of torque are directly related to to rque build-up rate and peak torque; (3) high peak flexor and extensor EMGs during and following torque onset are related to torque build-up rate and p eak torque; (4) minimum time of peak EMGs of 72-87 ms following the onset o f torques with 50 ms build-up suggests the contribution of an extensor musc le stretch reflex component; delayed peak for longer build-ups suggests a c entral control of muscle force in response to torque; (5) angular excursion s of handles increase with decreasing torque build-up time and increasing t orque magnitude causes increasing eccentric work; (6) the results show that the slow torque build-up times (450 ms) correspond to minimum peak EMGs; a nd (7) accumulated EMGs increase with increasing torque and torque build-up times. Further studies are needed to evaluate fatigue and musculoskeletal injuries associated with prolonged periods of tool use.