Emg Signs of Fatigue in Anterior and Posterior Deltoid Muscles: Questioning the Role of Rms During Fatigue

Abstract

When a skeletal muscle fails to maintain a required force of contraction, it is fatigued. While the time-dependent shift in mean power frequency (MPF) of electromyographic (EMG) signals to lower frequencies is a well-established indicator of fatigue, the change in root mean squared (RMS) values of EMG signal has not been interpreted consistently in literature. This research investigates localized muscle fatigue in anterior and posterior deltoid muscles during isometric, isotonic, sustained contractions by examining patterns of fatigue through spectral and amplitude analysis of EMG recordings in terms of MPF and RMS, respectively. The EMG signals were obtained simultaneously from the anterior and posterior deltoid muscles of 10 healthy subjects by placing silver-silver chloride surface electrodes in bipolar configuration over each muscle on the dominant arm of the subject. Subjects held a weight of 2.5 pounds in one hand while maintaining that arm in a constant forward flexion position at 90, where the anterior deltoid is the agonist muscle and the posterior deltoid is the antagonist muscle. After a rest period of five minutes, the arm was then held in a constant backward extension position at 90abduction and extended as much as possible. MPF and RMS were calculated for every 4% of endurance time and averaged between subjects. The shift in MPF to lower frequencies was a consistent observation while, the patterns of RMS varied between the positions and subjects, and exhibited an overall decrease. The latter result challenges the common belief that RMS increases during fatigue and lead to the hypothesis that the pattern of RMS during fatigue is not a reliable indicator of fatigue.