Effects of Exertion Variability on the FDI Muscle Fatigue during Intermittent Contractions

Many current workplace tasks involve prolonged and/or repetitive physical exertion, which can result in localized muscle fatigue (LMF). LMF has been shown to reduce a worker’s performance and increase the chance of work-related musculoskeletal disorders (WMSDs). Exertion variation is an approach known to impart a generally positive impact to reduce LMF. In this study, our main goal was to observe how exertion variability under specific laboratory working conditions affects the fatigue and recovery of the first dorsal interosseous (FDI) muscle. Twelve participants performed finger abduction task during four different experimental conditions: 1) baseline intermittent contractions (20% MVC); 2) active rest with low exertion (5% MVC); 3) short increases of muscle contraction to 25% MVC; and 4) brief large contractions during rest (60% MVC). Measures of discomfort (subjective), force fluctuations, reduction in the maximum voluntary contraction (MVC), root mean square (RMS) and mean power frequency (MPF) of the electromyography (EMG) were collected. After performing the fatiguing task for one hour, the participants’ overall %MVC and MPF were reduced by 19.3% and 9.1%, respectively. Additionally, RMS, Ratings of perceived discomfort (RPD) and COV values increased by 28.7%, 49.5%, and 18.7%, respectively, indicating the presence of FDI fatigue. Even when subjected to slightly higher workloads during short increases of muscle contraction to 25% MVC, and brief large contractions during rest (60% MVC), no significant differences were seen in the %MVC and RMS values of these conditions compared to the baseline intermittent contraction (20% MVC). While these conditions present larger physical demand, the existence of non-significantly different levels of LMF could indicate positive effects of exertion variability. Meanwhile, the participants’ %MVC increased by 13.7% during the 30-minute recovery session, and the rate of recovery was not affected by the experimental conditions. Results of our study could be useful in developing a range of interventions in occupational settings that involve repetitive low to moderate exertion levels