Soundmyogram and EMG cross-spectrum during exhausting isometric contractions in humans

During sustained isometric contractions, the firing rate (FR) of motor units (MUs) is influenced by the development of fatigue. Both the electromyogram (EMG) and the muscular sound detected on the muscle surface are compound signals in which the MU FR information is contained. The aim of this work was to retrieve the dominant MU firing frequency by means of EMG and soundmyogram (SMG) cross-spectrum (CS) analysis throughout contractions of different intensities. Twelve healthy male medical students performed three exhausting isometric tasks with their elbow flexors at 20, 80, and 100% of the maximal voluntary contraction (MVC). EMG and SMG were picked up from the biceps brachii belly using, respectively, two silver bars and a piezo-electric contact sensor. With respect to the initial values, the SMG, EMG, and CS mean frequencies (MFs) changed as followed: during 20% MVC the MF of SMG did not decrease, while EMG and CS MFs decreased by approximately 30 and 20%, respectively; throughout 80% MVC, only EMG MF decreased exponentially by approximately 40%, while SMG and CS MFs presented a temporary increase followed by a decrement; during 100% MVC, an exponential decrease of the EMG, SMG, and CS MF values was seen. Spectral analysis indicated that the dominant FR affects the SMG and the EMG spectral characteristics at a different rate. Moreover, the CS power distribution was consistent with the MU's dominant firing frequencies at different contraction levels and with its modifications in response to fatigue.