muscle fatigue monitor featuring , in one aspect , detecting a myoelectric signal and providing an electrical output ; providing a first signal corresponding to a portion of said output having frequency components above a first cutoff frequency , and a second signal corresponding to a portion of said output having frequency components below a second cutoff frequency ; providing a control signal corresponding to the difference in magnitude between said first and second signals ; varying said cutoff frequencies in accordance with the value of said control signal ; and displaying the variation of said cutoff frequencies as a function of time .