Frequency analysis of the surface electromyogram during sustained isometric contractions.

Four male and four female volunteers served as subjects in these experiments to assess the frequency components of the surface EMG during and following brief (3 s) and sustained isometric contractions of the handgrip muscles. Two types of fatiguing contractions were performed. Contractions were either maintained to fatigue at a constant tension of up to 100% of their strength or were maintained as a sustained maximal effort in the unfatigues or previously fatigued muscle. The frequency components of the surface EMG were assessed by calculating the power spectra of 1.5 s samples of the EMG from a fundamental frequency of 4 Hz through the first 128 harmonics by Fourier analysis; the centre frequencies of the resultant power spectra were then used as an index of the mean frequency of the EMG. The results of these experiments showed that the centre frequency was independent of the tension exerted by the muscle during brief isometric contractions but decreased linearly with time throughout the duration of fatiguing isometric contractions at tensions between 25 and 100% MVC. During sustained maximal effort, the frequency initially decreased linearly with time. However, once the target tension could no longer be maintained, the centre frequency remained constant throughout the remainder of the contraction. The frequency was found to recover within 1 min following exercise at all tensions examined.