Short-cycle overhead work and shoulder girdle muscle fatigue

Abstract

The objective of this research was to determine shoulder girdle fatigue for different combinations of weight of workpieces, weight of hand-tools, shoulder postures, arm up time and arm down time that are commonly used in automotive assembly operations. Both objective [surface electromyography (sEMG)] and subjective measures (ratings of perceived exertion, (RPE), fatigue and pain) were used to assess stress, fatigue and pain in the shoulder girdle. Ten healthy young female subjects participated in a laboratory experiment that consisted of a simulation of common automotive assembly job tasks. The simulation consisted of four tasks in a 1-min job-cycle. Each cycle was repeated 50 times. The four tasks were varied with different predetermined combinations of two weights ( W 1 signifying a workpiece and W 2 signifying the hand-tool weight), three exertion times and three shoulder postures. W 1 was either 1.36 or 2.73 kg (3 and 6 lb), and W 2 was 0.45, 0.91 or 1.82 kg (1, 2 and 4 lb). Exertion time was with the arm up for 2 seconds and down for 2 s (2–2) for ten exertions per minute, arm 3 s up and 3 s down (3–3) for seven exertions/min, or arm 5 s up and 3 s down (5–3) for five exertions/min. Each cycle finished with an 8–10 s rest phase to complete a 1-min cycle time. The posture angles were shoulder flexion of 60°, 90° and 120° combined with an included elbow angle of 90°, 120° and 150°, respectively. Experimental combinations ( n=54) were randomly selected. Response variables were recorded in the first minute and every 5 min thereafter for 50 min. The response variables included sEMG, RPE, fatigue and pain in the shoulder girdle. All subjects were able to perform all combinations with the lowest weights; however, in the more extreme postures, a few subjects prematurely terminated the experiments due to fatigue and/or pain. RPEs, as compared to sEMG data, appeared more sensitive and consistent. An analysis of variance showed that all four variables (workpiece weight ( W 1), tool weight ( W 2), arm up and down time, exertion time and shoulder posture) were statistically significant ( p⩽0.01), although the tool weight and workpiece weight were most predictive of capabilities. As expected, the RPE, fatigue and pain increased with an increase in the weights of the workpiece ( W 1) and hand tool ( W 2). Guidelines for acceptable workloads are provided. Relevance to Industry Combinations of weights, exertion time and shoulder posture that cause excessive fatigue are important for designing overhead work and hand-tools. The data reported provide a set of such data for female subjects.