Quantitative needle electromyography during sustained maximal effort

We describe changes in time domain as well as in frequency domain of the electromyographic (EMG) signal during sustained maximal effort. The anterior tibial muscle in five subjects and the brachial biceps muscle in eight subjects were examined with needle electrodes. The forces exerted decreased exponentially with time to ∼50% of the initial values after 1 min of sustained maximal effort. The mean and median power frequency declined exponentially with time. The decrease in number of spikes (turns, zero crossings) and amplitude (mean amplitude, integrated electrical activity) of the electrical activity in fatigued muscle, as compared with initial values can be explained in part by a dropout of some motor units and a decrease in firing rate. During the first minute, a decrease in number of turns and an increase in amplitude occurred in fatigued muscle as compared with values at the same force level in fresh muscle, probably owing to a decrease in conduction velocity along the fatigued muscle fiber and synchronous firing. This was consolidated by a linear relationship between the decrease in number of turns and the decrease in mean power frequency. After the first minute, a continued increase in amplitude only suggested contractile element fatigue. This order of physiologic changes was the same as that previously observed at sustained submaximal force.