Static and dynamic myoelectric measures of shoulder muscle fatigue during intermittent dynamic exertions of low to moderate intensity

Despite extensive research on muscular fatigue during prolonged static effo rts, there have been relatively few studies of more complex tasks (dynamic and intermittent). A laboratory study of overhead work tasks was conducted to investigate whether electromyographic (EMG) measures can potentially ser ve as indicators of fatigue, particularly for ergonomic tasks analysis. Six teen participants performed the tasks until they either developed substanti al discomfort or reached a 3-h limit. EMG signals were obtained at interval s throughout the experiment from four shoulder muscles, both statically (du ring fixed-level test contractions) and dynamically (during, task performan ce). Both EMG root mean square (RMS) amplitude and spectral content (mean a nd median power frequencies) were examined and compared in terms of their v ariability and sensitivity. In addition, a new fatigue index was developed to allow for the estimation of substantial fatigue onset. Variability was f ound to differ significantly between muscles and EMG measures, and was gene rally lowest for mean power frequencies obtained during static test contrac tions. Sensitivity was typically greatest for RMS versus spectral measures, and slightly higher for median than mean power frequencies. The results su ggest that fatigue during dynamic tasks, while a complex phenomenon, can be monitored and quantified using EMG.