Muscle fatigue in relation to EMG during repeated and maintained maximal isometric contractions.

The decrease of muscular force output during two different maximal isometric voluntary contractions of knee extension were studied for 6 male subjects in relation to EMG. EMGs were recorded by bipolar surface electrodes from the m. vastus lateralis and were integrated by a modified Miller's circuit. In repeated contractions at one-second intervals, only slight changes were observed in the integrated EMG despite a marked decrease in force output to about 45% of the initial value. In counterparts of the maintained contraction for 2 min, both force output and integrated EMG decreased with time, and tended to plateau at a level corresponding to 20% and 45% of the initial value, respectively. From the viewpoint of blood supply, it was concluded that in the repeated contractions the decrease in force output was mainly due to the local factors in the muscle, especially in the fast twitch fibres, while in the maintained contraction in decrease in force output was caused not only by the failures in local factors but also central fatigue.