Abstract: BACKGROUND: Upper limb work-related musculoskeletal disorders warrant attention due to their adverse impacts on an individual’s health and socioeconomic well-being, with muscle fatigue being a primary contributing factor.
Objective: To investigate the interaction effects of work posture and load combinations on muscle fatigue characteristics during static upper limb tasks using joint analysis of electromyography  spectrum and amplitude, aiming to provide multidimensional biomechanical evidence for preventing upper limb work-related musculoskeletal disorders.
Methods: Fourteen healthy subjects performed static tasks of shoulder flexion and abduction at 20°, 40°, and 60° angles with loads set at 10%, 30%, and 50% of their one-repetition maximum. Surface electromyography signals and Borg CR-10 scale data were simultaneously recorded. Muscle fatigue was quantified using joint analysis of electromyography spectrum and amplitude-derived metrics, including root mean square slope and median frequency slope. Paired samples t-tests were used to analyze the proportion of fatigue states before and after fatigue. Analysis of variance was used to assess the effects of postures, loads, and their interaction on fatigue state proportions.
Results and Conclusion: (1) The posture-load combination significantly influenced task tolerance (P < 0.05), with the 60°-50% one-repetition maximum combination yielding the shortest maximum endurance time (P < 0.001). (2) Changes in muscle fatigue states before and after fatigue under specific combinations: fatigue state proportions in the supraspinatus and upper trapezius significantly decreased under the 20° 10% one-repetition maximum combination (P < 0.05). Fatigue state proportions in the upper trapezius showed an extremely significant difference under the 40° 10% one-repetition maximum combination (P < 0.01). (3) The effects of postures and loads on muscle fatigue were independent of each other. At 20°flexion, the proportion of fatigue in the anterior deltoid muscle showed significant differences (P < 0.05) when comparing loads of 10% one-repetition maximum versus 30% one-repetition maximum, and 10% one-repetition maximum versus 50% one-repetition maximum (P < 0.05); however, under the load condition of 10% one-repetition maximum, the fatigue proportions between the anterior and the anterior and middle deltoids showed significant differences (P < 0.05) only between the 20° and 60° angles, specifically during the pre-fatigue and full-range phases (P < 0.05).(4) No significant interaction effects between postures and loads were detected (P > 0.05). These finding indicate that high-load and high-angle tasks markedly accelerate muscle fatigue; postures and loads independently influence fatigue progression without observable synergistic effects. Joint analysis of electromyography spectrum and amplitude shows limited sensitivity to muscle fatigue characteristics in capturing posture-load interactions.

Key words: musculoskeletal disorders, upper limb fatigue, work posture, external load, interactive effects, maximum endurance time, joint analysis of electromyography spectrum and amplitude