In a previous study, an unexpected u-shaped relationship was observed between load level andfatigue/recovery responses. Moderate load levels resulted in lower perceived discomfort, pain,and fatigue, and shorter recovery times compared to either low or high load levels. Thisphenomenon has been reported in other studies, but no article has examined the possiblemechanisms that might explain this u-shaped relationship. In this paper, we re-examined thepreviously published data and found that the phenomenon does not appear to be due to theexperimental artefact; the u-shape may be due to unexpectedly lower fatigue effects at moderateloads, and higher fatigue effects at lower loads. We then conducted a literature review andidentified several possible physiological, perceptual, and biomechanical explanatory mechanisms.No single mechanism explains the entirety of the phenomenon. Further research is needed onthe relationship between work exposures, fatigue, and recovery, and the mechanisms related tothe u-shaped relationship.

Practitioner summary: We examine a previously observed u-shaped relationship between loadlevel and fatigue/recovery, where moderate force resulted in lower perceived fatigue andshorter recovery times. A u-shaped fatigue response suggests that simply minimising load levelsmight not be an optimal approach to reduce the risk of workplace injuries.