Myoelectric alterations after voluntary induced high - and low - frequency fatigue.

The aim of the study was to find whether voluntary induced high- and low-frequency peripheral fatigue exhibit specific alteration in surface EMG signal (SEMG) during evoked and maximum voluntary contractions. Ten male students of physical education performed 60 s long stretch-shortening cycle (SSC) exercise with maximal intensity and 30 s long concentric (CON) exercise with maximal intensity. To verify voluntary induced peripheral fatigue, knee torques during low- (T20) and high-frequency electrical stimulation (T100) of relaxed vastus lateralis muscle (VL) were obtained. Contractile properties of the VL were measured with passive twitch and maximal voluntary knee extension test (MVC). Changes in M-waves and SEMG during MVC test were used to evaluate the differences in myoelectrical signals. T100/T20 ratio decreased by 10.9 ± 8.4 % (p < 0.01) after the SSC exercise and increased by 35.9 ± 17.5 % (p < 0.001) after the CON exercise. Significant SEMG changes were observed only after the CON exercise where peak to peak time of the M-waves increased by 9.2 ± 13.3 % (p < 0.06), SEMG amplitude during MVC increased by 32.9 ± 21.6 % (p < 0.001) and SEMG power spectrum median frequency decreased by 11.0 ± 10.5 % (p < 0.05). It is concluded that high frequency fatigue wasn't reflected in SEMG, however the SEMG changes after the CON seemed to reflect metabolic changes due to acidosis. Key pointsThe SSC exercise induced high-frequency fatigue which was not reflected in any SEMG change.The CON exercise induced dominantly low-frequency fatigue where only SEMG during MVC changedMuscle fibre membrane excitability was not changed due to low- and high-frequency fatigue but mainly reflected metabolic changes.Changes in muscle compound action potential did not follow those changes seen after electrically elicited HF and LF fatigue.