Short-cycle overhead work and shoulder girdle muscle fatigue

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 (W1 signifying a workpiece and W2 signifying the hand-tool weight), three exertion times and three shoulder postures. W1 was either 1.36 or 2.73 kg (3 and 6 lb), and W2 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 (W1), tool weight (W2), 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 (W1) and hand tool (W2). Guidelines for acceptable workloads are provided.Relevance to IndustryCombinations 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.