Documenting the Temporal Pattern of Fatigue Development

OCCUPATIONAL APPLICATIONS Fatigue is linked to task performance and the development of musculoskeletal disorders. By understanding how fatigue progresses over time, fatigue may be a useful indicator of injury risk and a workplace evaluation tool. Selecting fatigue measures is challenging, though, especially for occupational applications where activities are largely uncontrolled and multiple body regions are monitored. We used a battery of central and peripheral fatigue measures to observe the effects of task intensity, which may play a role in determining contributions from central or peripheral sites. We also observed the effects of muscle contraction type, which influences the interplay between central and peripheral fatigue. And finally, we observed the effect of body region. We conclude that both central (e.g., perceived fatigue) and peripheral (e.g., muscle activity) measures should be selected to consider task effects. However, further work to better understand the choice and interpretation of measures is needed to improve occupational fatigue monitoring. TECHNICAL ABSTRACT Background: Neuromuscular fatigue is linked to work quality and productivity and is possibly a precursor to musculoskeletal disorders. However, to have merit as a prognostic indicator of injury risk and performance, the temporal development of fatigue should be better understood. Fatigue is task dependent and involves a complex interaction between central and peripheral mechanisms, which complicates the measurement and interpretation of its development. Purpose: This study investigated whether a set of central (e.g., rating of perceived fatigue) and peripheral (e.g., muscle mechanomyography) neuromuscular measures can document fatigue development in different task conditions. Methods: Sixteen participants performed four tasks that differed in the type of contraction, intensity, and body region. The test battery consisted of seven measures reflecting impairments to central and/or peripheral sites. Eleven participants performed an 8-hour light precision micro-pipetting task to evaluate an identical test battery. Temporal responsiveness of measurements was determined from mixed-model, repeated measures analysis of the time and pattern of fatigue development from principal component analysis. Results: During intermittent handgrip activity, central fatigue increased followed by peripheral fatigue. Sustained isometric handgrip led to a similar response, with increasing tremor and mechanical muscle activity after 30 minutes of exercise. Higher intensity, intermittent handgrip resulted in substantial increases in peripheral fatigue. Intermittent handgrip activity yielded five principal components, whereas sustained handgrip and shoulder conditions respectively yielded three and six components. Four components categorized by contraction type and body region were identified over the micro-pipetting task. Conclusions: A battery of neuromuscular indices can identify predominant fatigue mechanisms and the pattern of fatigue development. We found that task intensity might be a factor regarding the domain that contributes to neuromuscular fatigue, while the muscular contractile condition might determine the interplay between central and peripheral sites. During a pipetting task, fatigue development was localized to specific body regions.