Abstract-During a sustained muscle contraction, the lpectlUm of the myoelectric lignal is known to undergo compression as a function of time. Previous investigators have shown that the frequency compres sion is related to the decreasing conduction velocity of the muscle fi bers. It is proposed that the frequency compression may be tracked by obtaining a continuous estimate of a characteristic frequency of the spectrum, such as the mean and median, or the ratioof low-frequency components to high-frequency components of the speetrum. A theo retical analysis was performed to investigate the restrictions in estimat ing the three parameters, as well as their sensitivity to the conduction velocity. The ratio parameter was found to be mostsensitive to con duction velocity, but was the least reliable of the three. The median frequency was the least sensitive to noise. Therefore, from a theoretical point of view, the median frequency is the preferred parameter. A technique is described which determines an unbiased consistent esti mate of the median frequency. The technique may be readily imple mented in analog hardware.

Abstract-As a muscular contraction is sustained, the spectrum of the myoelectric signal is compressed into lower frequencies. This spectral compression has been associated with localized muscular fatigue by several investigators. A device is presented that implements a technique to track the spectral compression by calculating the median frequency and two other parameters of the spectrum. The device is referred to as the muscle fatigue monitor (MFM) and is built with analog circuitry so that parameters are calculated and displayed in real-time and on-line. The technique is based on modulated filters which are implemented by using periodically-controlled switches to effectively vary the cutoff frequencies of the filters, The median frequency of a myoelectric signal obtained during a sustained, constant-force, isometric contraction was calculated by the MFM and digital computation. The results obtained by the two techniques were essentiaIIy the same, verifying the opera tion of the MFM.Several other techniques to quantify the spectral compression are discussed and compared to the MFM. While most of the other tech niques do track spectral compression, the features of the MFM make it appropriate and accurate for clinical and industrial applications.

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