Fatigue-associated changes in the electromyogram of the human first dorsal interosseous muscle

Muscle fatigue is a clinically important symptom, often analyzed using electromyography (EMG). We analyzed fatigue reactions of the first dorsal interosseous muscle (FDI) during a maintained contraction at half-maximal force ((1/2)-MVC test). EMGs were recorded with large surface electrodes and, simultaneously, with intramuscular fine-wire electrodes. Compound muscle action potentials (M waves) were evoked by electrical ulnar nerve stimulation. During the first half of the test, an almost direct proportionality was found between the variations in voluntary rectified and smoothed EMG (rsEMG) and in M-wave area as recorded with surface electrodes. This indicated that much of the variation in voluntary EMG reflected changes in the spike-generating properties of the muscle fibers. The changes in the fatigue-associated rsEMG were often quantitatively markedly different for the "wide-angle" recording from the surface and the more local intramuscular recording. This suggests that fatigue-associated EMG-responses of the FDI have a markedly heterogeneous intramuscular distribution.