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A theory concerning the spectral density of surface electromyogram (EMG) in isometric contraction is described and herein, a conduction velocity measure, which was derived from a surface EMG model, is suggested. In order to confirm the validity of this measure, the spectral modification with respect to muscle fatigue is studied experimentally. The EMG signals were obtained from the biceps brachii and the rectus femoris in four subjects. The spectral modification shifted to a lower frequency as fatigue developed. The conduction velocity measure decreased linearly in both the biceps brachii and rectus femoris, but this tendency was more pronounced in the biceps. It is suggested that the spectral shift was concerned with conduction velocity of action potential along the muscle fibres. From these experimental results, we believe that we are justified in claiming that our conduction velocity measure is a useful index to estimate motor function.

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