Is spectral analysis of the surface electromyographic signal a clinically useful tool for evaluation of skeletal muscle fatigue?

Normal values for initial median frequency (IMF) of the electromyographic (EMG) power-density spectrum must be determined before EMG spectral analysis can be used to evaluate clinically muscle fatigue. This study attempts to establish normal values in four muscles. Thirty-one healthy subjects performed isometric contractions of the biceps brachii, triceps, deltoid, and tibialis anterior muscles at 50% maximum voluntary contraction. Linear regression analysis was used to compute the IMF and the slope of the median frequency as it decayed with fatigue over 45 seconds. The IMF of the tibialis anterior (mean +/- standard deviation, 116 +/- 20 Hz) was significantly higher (p < 0.001) than that of the biceps (90 +/- 18 Hz), triceps (85 +/- 18 Hz), and deltoid (87 +/- 15 Hz). The deltoid and tibialis anterior had the steepest slopes. The IMF of all muscles was greater in men than in women, but gender did not affect the slope. This study attempted to establish normal values for IMF and slope in specific muscles. However, the range of normal values is so broad that it may preclude the clinical use of spectral analysis to evaluate muscle fatigue.