Development of muscle fatigue assessed by using superposition of evoked and volitional myoelectric potentials

We studied muscle fatigue development using evoked myoelectrical potentials superimposed on volitional ones. The instantaneous frequency of superim posed M-waves and mean power frequencies of volitional electromyography (EMG) declined during sustained contraction, indicating that fatigue progressed. We divided fatigue into 3 phases, with 20 frames in each fatigue phase, corresponding to one-third of the total sample. The instantaneous frequency of superimposed M-waves and mean power frequencies of volitional EMG were correlated during early intensive isometric voluntary contractions and became increasingly uncorrelated as contraction proceeded. The coefficient between the mean power frequency and instantaneous frequency correlation was also greater at the first peak than at the second peak of the superimposed M-wave, indicating that the motor unit action potential was distorted. Distortion in the motor unit action potential shape depends on elongation of the depolarization zone of muscle fiber, because the superimposed M-wave is a peripheral indicator elicited by electrical stimulation. These results suggest that muscle fatigue develops based on the reduction of the conduction velocity of muscle fiber and on the elongation of the depolarization zone of muscle fiber.