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An electromyographic study of maximum torques and upper extremity muscle activity in simulated screwdriving tasks.

Habes-DJ; Grant-KA
Int J Ind Ergon 1997 Oct; 20(4):339-346
The influences of workstation and hand tool design on maximum torque and upper extremity muscle activity were examined during a simulated screwdriving task. Fifteen male subjects performed a series of isometric maximum and submaximum torque exertions using a standard screwdriver handle. Handle height, distance from body, orientation, and diameter were varied and measurements were made. Surface electromyography (EMG) was used to measure activity in the anterior deltoid, triceps brachii, biceps brachii, extensor digitorum, flexor digitorum superficialis, and flexor pollicis longus muscles. Mean torque strength was significantly higher for handle diameters of 3.7 centimeters (cm) than for handle diameters of 2.9cm. Mean torque strength was significantly higher for screwdrivers operated at elbow height than for screwdrivers operated at shoulder height. When the handle was positioned at the shoulder, the mean torque strength was significantly higher for the vertical orientation than for the horizontal orientation. Torque was not significantly affected by reach distance. For all muscle groups examined, the position and diameter of the handle had significant effects on the EMG/torque ratio. The EMG/torque ratio was higher for handles held at shoulder height than for handles held at elbow height. The EMG/torque ratio decreased with increasing handle diameter. Significant interactive effects between handle height and reach distance and between handle orientation and handle height on the EMG/torque ratio were noted. The condition which minimized the EMG/torque ratio for all muscle groups consisted of using the 3.7cm handle in the vertical orientation at elbow height and full reach distance. For most of the muscles examined, the EMG/torque ratio tended to decrease with increasing exertion level. The authors conclude that workstations and tools can be designed to maximize production while minimizing muscle stress.
Humans; Hand-tools; Equipment-design; Task-performance; Muscle-function; Muscle-tension; Electrophysiological-measurements; Industrial-design; Ergonomics; Body-mechanics
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Journal Article
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NIOSH Division
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International Journal of Industrial Ergonomics