Evaluation of the amplitude and frequency components of the surface EMG as an index of muscle fatigue

The frequency components and amplude of the surface electromyogram (EMG) were measured during both 3-s (tensions of 5-100% of the maximum voluntary contraction (MVC)) and fatiguing contractions at 25, 40 and 70% MVC in the handgrip, biceps, adductor pollicis and quadriceps muscles in six male subjects. For the handgrip and biceps muscles, the experiments were repeated at three different muscle lengths: the length at which the muscle was able to exert its maximum isometric strength, and the muscle length above and below that length which corresponded to a length at which the muscle could exert 80% of it maximum. The frequency components of the EMG were the same during brief fatiguing isometric contractions in any of the muscles examined here as long as the muscles contracted near their optimal length. Shortening the muscle length prior to contraction caused an increase in the power in the low frequencies of the EMG power spectra while stretching a muscle had the opposite effect during isometric contractions. The amplitude of the EMG during brief and fatiguing contractions were similar for all muscles except the biceps. The biceps showed a non-linear relation between the amplitude of the EMG and tension during brief isometric contractions and to a lesser extent during fatiguing contractions.