Abstract
The power spectra of bipolar surface EMG during sustained isometric contractions of the biceps brachii were obtained by the autocorrelation and its Fourier transformation technique. The power of the frequency components below about 75 Hz increased with time. The ratio of the power below about 40 Hz to the total power increased. Two phases were suggested in a sustained contraction. In the first phase, the total power increased slightly, and relative increase in the power below 40 Hz took place. In the second phase, the total power increased more steeply in parallel with the power around 15 Hz, and the peak frequency shifted to about 15 Hz near exhaustion in most cases. A decrease of discharge frequency with recruitment may explain the first phase, and grouping of discharges in the recruitment may explain the second phase. No corresponding peak was observed between the power spectra of EMG and those of the upper arm tremor recorded simultaneously.
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