Task-Dependent Associations Between Motor Variability and Perceived Exertion in Field-Studies of Watchmaking and Automotive Assembly

Repetitive tasks increase the risk of musculoskeletal disorders due to the constrained movement patterns and limited opportunities for load redistribution. While motor variability (MV) may influence both perceived exertion and injury risk, its role in real-world industrial settings remains limitedly studied. This field study examined the associations between MV and perceived exertion in experienced watchmakers and automotive assembly workers performing representative repetitive tasks under natural working conditions. Full-body kinematics were recorded using wearable sensors, and MV was quantified using magnitude-, structure-, and coordination-based metrics. Regression modeling showed that MV was strongly associated (adjusted R 2 ≈ .96) with perceived low back exertion in both occupational groups. Coordination among trunk, neck, and shoulder segments were identified as particularly important, while the specific MV dimensions linked to perceived exertion differed by task. These findings suggest that the association between MV and perceived exertion is task-specific and should be evaluated through multidimensional assessment approach.