Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation

Abstract

The focus of this research was to investigate how maximum torque and muscle forces were affected by pronation and supination, i.e., inward and outward rotation of the forearm in a series of screwdriver tests with 6 varied handles. Consecutively, maximum torque for pronation and supination was determined, submaximum isometric levels of torque were demanded, and, finally, an equal dynamic screwing work for all subjects was simulated. Physiological cost of performance was simultaneously measured by registrations of electromyographic activities (EA) from 4 muscles, which were expected to be involved intensively in screwing tasks. Significant and essential differences between maximum torque values produced by pronation and supination of the right and the left arm of the mainly right-handed subjects were found. For clockwise work, as it is necessary e.g., for driving in screws, inward rotations (pronations) of the nondominant hand are at least as strong as outward rotations of the dominant hand. Differences of about 8% favour of pronations were found. Yet, for counter clockwise work involved e.g., in removing a tightened screw, inward rotations of the dominant hand yielded a much more stronger torque strength than outward rotations of the nondominant hand. Differences of more than 50% right-handed subjects were measured. Also, EA values of the 4 muscles monitored on the right arm differed significantly. Systematically operational and physiological differences due to the varied screwdriver grips, as results of investigations which were not the main objective of the study, corresponded well with the findings of prior studies.