The sensitivity of shoulder muscle fatigue to vertical hand location during complex manual force exertions

Abstract

Occupational shoulder overexposures, including increased muscle activity and associated localized muscle fatigue, typically scale with manual exertion height during static or intermittent overhead work. It is unknown how this response is affected during complex (i.e. multi-directional and varying magnitude) manual force exertions, and how this work profile may attenuate task endurance. The purpose of this study was to evaluate the influence of manual exertion height on shoulder muscle fatigue responses during a multi-directional manual exertion protocol. Fourteen participants completed a shoulder fatiguing protocol by exerting manual forces (push up, down, left, and right) at three vertical hand location heights (low, shoulder and overhead). Decline in strength, myoelectric indicators of fatigue, ratings of perceived discomfort and endurance time were used to track the progression of fatigue at the different height conditions, and differential effects due to direction of exertion were also examined. The overhead and shoulder height conditions resulted in significantly increased muscle activation, discomfort and strength decline in comparison to the low condition. Fatigue responses between overhead and shoulder level work were generally similar, as no differences existed in endurance time, RPD and mean power frequency for these conditions, but the overhead condition resulted in a substantial 18% reduction in downwards strength. The downwards exertions seemed to elicit the greatest fatigue response, despite the upwards direction requiring the highest global muscle activity. These results add evidence that repetitive overhead work should be avoided when possible, but provides novel insight on how direction of force application can be further considered in design recommendations.