Electromyographic Manifestations of Muscular Fatigue Toshio Moritani, Akira Nagata and Masuo Muro 1/3/2019

Introduction IEMG increases progressively as a function of time during sustained muscular contraction with a constant force output. This increase is due to recruitment of motor units to compensate for the loss of contractility due to some degree of impairment of fatigued Mus. EMG fatigue curves (EMG Amplitude vs Time) could provide a measure of MU’s fatigability.

Methods 8 male subjects MVC of elbow flexors and plantar flexors was measured The subjects were then instructed to maintain isometric contractions at 30, 40, 60 and 80% MVC for as long as possible. A 45 minute rest was allowed between contractions Subject also completed 10 trials of 2-3 sec MVC’s to obtain mean power frequency at MVC Ag-Ag CL, 2 mm contact diameter and 6 mm interelectrode distance was used.

Methods Fast Fourier Transform with the Hamming window Mean Power Frequency was calculated from EMG frequency spectrum. Fatigability was estimated by the rate of increase in IEMG as a function of time (IEMG Slope) at a given force output level.

EMG as a Function of Time Despite the equivalent level of muscular effort, the IEMG Slope for the bicep was nine times greater than the soleus.

EMG Slope as a Function of MVC

Effects of Fatigue on Frequency Spectrum MPF for the bicep at 40% MVC declined by 43.7 Hz after fatigue. MPF for the soleus at 40% MVC declined by 7.3 Hz after fatigue. The extent of decline in the MPF was correlated (r = .95) with the MPF obtained during MVC. This correlation suggested that MU’s with higher MPF would fatigue to a greater extent than those with relatively lower MPF

Discussion A muscle group composed of a higher percent of FT fibers would manifest a greater fatigability than that of lower percent of FT fibers. Fatigue causes an increase in EMG amplitude followed by a decrease in EMG amplitude as a function of time. Fatigue causes the frequency spectrum of the EMG signal to shift to lower frequencies.