A system is described which monitors spectral changes of the surface EMG. It is shown analytically that a relationship exists between the zero crossing rate (ZCR) and the mean, Fa, and median, Fm, frequencies of the surface EMG. Under the existing conditions of this relationship a system can be built which can estimate and monitor the Fa or Fm of the surface EMG from the measured ZCR. It is shown experimentally on the biceps, extensor digitorum and first dorsal interosseus muscles that an excellent agreement exists between the time averages of Fm and ZCR in normal muscles under a variety of muscle length tension and fatigue conditions. A simple portable microcomputer unit which performs the ZCR calculations and which can be used as a muscle fatigue monitor is described.