An investigation of symmetry in upper extremity strength during sagittal plane lifting tasks

Abstract

Biomechanical models are one of the tools industry and the National Institute for Occupational Safety and Health (NIOSH) use to aid in the reduction of injuries due to overexertion (NIOSH, 1981). These models have been determined to be reasonably effective in estimating the maximum acceptable weight for a lift (Aghazadeh and Ayoub, 1985). Biomechanical models consider many variables, including posture, gender, strength capabilities, coefficient of static friction, and anthropometry. In two-dimensional static models, symmetric loading is assumed in the upper and lower extremities. This study was designed to evaluate the degree to which loading is symmetric in the upper extremities in an apparently symmetric task. Subjects performed a series of bilateral strength tests: shoulder flexion strength, elbow flexion strength, and grip strength. Four different lifts were performed and analyzed separately: stoop lift, squat lift, upright lift, and free lift. T-tests were conducted to test the statistical significant difference in the exertions on the right side and left side in each posture and in the bilateral strength tests. The assumption of symmetric loading during the stoop lift and squat lift is statistically valid, at the 0.05 level, for this sample of subjects. The assumption of symmetric loading during the upright lift and free lift is not statistically valid. A statistical difference in maximum shoulder flexion strength was found with the right shoulder 9% stronger than the left. However, no statistical difference was found in the maximum elbow flexion strength or the maximum grip strength. Assumptions of symmetric loading in the upper extremities during symmetric tasks appears to be reasonable for the simpler two-dimensional models, however three-dimensional models should address asymmetric loading/strength issues.