Effect of Shoulder Flexion Angle and Repetition Frequency on Upper-limb Muscle Activity and Maximum Endurance Time

Objective: The purpose of this study was to evaluate the effect of shoulder flexion angle and task type on upper-limb muscle activity and maximum endurance time during a simulated drilling task.<br/>Background: Repetitive upper-limb tasks are commonly observed in manufacturing, construction and logistics industries. Awkward shoulder postures and high repetition of tasks are well known contributors to high prevalence rate of work-related musculoskeletal disorders (WMSDs). Although both shoulder posture and task repetition rate are recognized as main ergonomic risk factors, few studies have systematically examined their combined effect on muscle activity and maximum endurance time.<br/>Method: Seven healthy males participated in this experiment. Participants performed 12 tasks combining four shoulder flexion angles (45°, 90°, 135° and 180°) and three task types (static holding, 6 reps/min and 12 reps/min). Surface electromyography (sEMG) was recorded from four upper-limb muscles (upper trapezius, anterior deltoid, middle deltoid, biceps brachii). Maximum endurance time (MET) was defined as the duration which participants could maintain the task until they voluntarily stopped due to fatigue or discomfort. A two-way ANOVA was used to examine the effects of shoulder angle and task type.<br/>Results: Both shoulder flexion angle and task type had significant effects on muscle activity and MET (all p < 0.05). Muscle activity was highest during high-frequency task and at 180° of shoulder flexion. Under static and low-frequency tasks, the increase in muscle activity associated with greater shoulder flexion angle was relatively modest; however, during high-frequency tasks, the increase in muscle activity with increasing shoulder flexion angle became more pronounced.<br/>Conclusion: This study demonstrates that high-frequency repetitive tasks induced faster fatigue compared to static holding, and the combination of high shoulder angles and high repetition rates constituted a high-risk exposure pattern. These findings highlight the importance of minimizing overhead work and controlling task repetition rates to prevent WMSDs.<br/>Application: This study provides quantitative evidence on the combined effects of task type and shoulder posture, and is expected to serve as a scientific basis for prevention of work-related musculoskeletal disorders in the field of occupational safety and health.