Muscle fatigue and endurance during repetitive intermittent static efforts: development of prediction models.

Localized muscle fatigue has received growing attention as a potential design variable and exposure metric in research towards prevention of musculoskeletal disorders in the workplace. While fatigue during sustained static work has been investigated extensively, effects during tasks comprising work-rest cycles are less clear. Work-rest models for static intermittent work have been presented in several reports, but the applicability is often limited to specific conditions. A study was conducted that facilitated a description of the relationships between static intermittent efforts and muscle endurance and fatigue. Exercises consisted of 1 h (maximum) of repetitive static arm abductions, involving a range of muscle contraction levels (10-30% maximum voluntary exertion), duty cycles (0.2-0.8) and cycle times (20-180 s). A between-subject central composite experimental design was used and 15 different exercise conditions were examined with six participants (three females and three males) for each. Along with endurance times, temporal changes related to fatigue were monitored using muscle strength, ratings of discomfort and electromyography (EMG) obtained from the middle-deltoid muscle during the contraction phase of the work cycles. The results of this study showed the influence of contraction level and duty cycle on the majority of fatigue measures used, while cycle time tended to affect EMG spectral measures. Using a response surface methodology, several fatigue prediction models and contour plots were developed that can be employed as an aid for design and evaluation of light repetitive static tasks. Good correspondence was generally found between discomfort rating and other measures of fatigue, suggesting the usefulness of this measure for rapid assessments of local fatigue in the workplace.