Fatigue and endurance limits during intermittent overhead work

Shoulder problems are prevalent in industrial work, particularly when tasks require the hands to be used at or above shoulder level. Although extensiv e research has been conducted on prolonged static exertions, and several gu idelines for such efforts exist, there is insufficient information for ergo nomic evaluation of tasks that are intermittent and/or dynamic. A laborator y simulation was conducted of overhead assembly work that was both intermit tent and dynamic, and which varied the duty cycle (work/rest ratio), arm re ach, and hand orientation of a tapping task. Results consisted of endurance times and also the times of fatigue onset as indicated by perceived discom fort and declines in muscle strength. Females exhibited longer (22%) endura nce times, delayed reports of discomfort, and slower declines in strength. Significant influences of duty cycle were found on both endurance and fatig ue times, yet arm reach and hand orientation did not have consistent effect s. Distributions of endurance and fatigue times are presented as criteria f or preliminary evaluation of overhead work. Endurance times could be predic ted with only moderate accuracy from earlier indicators of fatigue onset. E xisting guidelines, albeit developed for static tasks, appeared unsuitable for the simulated overhead assembly efforts examined. Furthermore, such gui delines may fail to capture the substantial interindividual variability obs erved in this experiment.