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The amplitude of the surface EMG signal may provide a more accurate reflection of motor unit activity during sustained fatiguing contractions than spectral parameters which are more commonly used to estimate muscle fatigue. In this paper, theoretical relationships between surface EMG amplitude measures and mean motor unit firing rates and muscle fiber conduction velocity (MFCV) are established. It is proposed that using these relationships, under conditions where motor unit recruitment and synchronization can be assumed to be negligible, such as at high force levels or in smaller muscles, it may be possible to obtain an estimate of relative changes in motor unit firing rates during a sustained isometric contraction. Using EMG amplitude and MFCV data gathered from the brachioradialis muscle during 80% maximum voluntary contraction, relative changes in mean motor unit firing rates were estimated in this manner. MFCV and the estimated firing rate changes were then incorporated into a model of the surface EMG signal. Simulated EMG data was generated individually for each subject and EMG amplitude and spectral parameters calculated from the simulated and experimental data were found to compare well.