Shifts in EMG spectral power during fatiguing dynamic contractions

INTRODUCTION: We examined the etiology of the electromyographic (EMG) spectral shift during dynamic fatigue. METHODS: Nineteen subjects (mean ± SD age = 22.4 ± 1.6 years) performed 50 consecutive maximal concentric isokinetic contractions of dominant leg extensors. Surface EMG signals were detected from the vastus lateralis, rectus femoris, and vastus medialis during each contraction, processed with a wavelet analysis, and the resulting spectra were decomposed with a nonparametric spectral decomposition procedure. RESULTS: The results indicated that the decreases in EMG frequency during the 50 contractions were generally due to reductions in high-frequency power and increases in low-frequency power. In addition, the spectral shifts were most pronounced for the rectus femoris, followed by the vastus lateralis, and then the vastus medialis. CONCLUSIONS: The spectral decomposition procedure is much more sensitive for tracking dynamic fatigue than is EMG mean frequency or median frequency.