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Electromyographic Manifestations of Muscular Fatigue

Electromyographic Manifestations of Muscular Fatigue. Toshio Moritani, Akira Nagata and Masuo Muro. Introduction. IEMG increases progressively as a function of time during sustained muscular contraction with a constant force output.

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Electromyographic Manifestations of Muscular Fatigue

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  1. Electromyographic Manifestations of Muscular Fatigue Toshio Moritani, Akira Nagata and Masuo Muro

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. EMG Slope as a Function of MVC

  7. 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

  8. 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.

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