Psychophysically determined work durations for limiting shoulder girdle fatigue from elevated manual work

Abstract An early symptom of many soft tissue disorders is excessive muscle fatigue during manual work. This study adopted a psychophysical approach to determine work durations for limiting shoulder-girdle fatigue. In a series of four experiments, each subject monitored his/her level of arm/shoulder discomfort while performing a task requiring repetitive, elevated arm movements. Over the course of a short (3–6 hr) work day, the subject terminated each trial, initiating a 1-min rest period, when the discomfort reached a pre-specified level on Borg's 10-point category-ratio scale. Average trial durations decreased minimally or remained stable over the work day. Nevertheless, increases in task loading variables, including repetition rate, required force, tool weight and reach height, each led to significant decreases in work duration. (Average trial durations associated with varying levels of work demand ranged from 29 to 160 sec.) Rate and force of movement had the largest effects on work duration. The effects of reach height and tool weight were small by comparison. Thus, the former two factors are relatively more crucial in the development of shoulder and arm fatigue. Unexpectedly, males tended to engage in longer work trials than females, despite controlling for upper body strength. The possible role of gender-related experimental demand characteristics is discussed.