Maximal acceptable torques of highly repetitive screw driving, ulnar deviation, and handgrip tasks for 7-hour...

This study represents a continuation of a series of psychophysical studies on repetitive motions of the wrist and hand conducted at the Liberty Mutual Research Center for Safety and Health. The purpose of the study was to quantify maximum acceptable torques of six motions performed on separate days but within the context of the same experiment. The six motions were screw-driving clockwise with a 31-mm handle, a 40-mm handle, and a 39-mm yoke handle; screw-driving counterclockwise with a 31-mm handle; ulnar deviation with a power grip (similar to knife cutting), and a handgrip task (similar to a pliers task). A psychophysical methodology was used in which the subject adjusted the resistance on the handle, and the experimenter manipulated or controlled all other variables. Ten subjects performed the six tasks at repetition rates of 15, 20, and 25 motions per minute. Subjects performed the tasks for 7 hours per day, 5 days per week, for 4 weeks. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work they performed. Symptoms were recorded by the subjects during the last 5 min of each hour. The results revealed that mean maximum acceptable torques ranged from 0.33 to 0.65 Nm for screw driving, 1.08 to 1.13 Nm for ulnar deviation, and 4.80 to 4.85 Nm for the handgrip task. These values represent 14 to 24% (median of 17%) of maximum isometric torque depending on the frequency and motion. A table of maximum acceptable torques and forces of the six motions is presented for application in the field.