The hypothesis that acidosis may decrease muscle excitability and cause fatigue is widely discussed in the literature. It was shown in vitro that acidification decreases the propagation velocity of muscle AP. The reduction of muscle AP propagation velocity (CV) is often discussed as the major cause of the alterations in the EMG spectrum during muscle contraction, since both effects have been found to show a close correlation. Nevertheless, the results of Nielsen et al. (2001) demonstrated that lactic acidosis can play a protective role for muscle excitability and force in vitro. We examined the effects of exercise-induced changes in pH on muscle electrical activity during voluntary exercise with different protocols. Our data showed that the relation between CV, pH and EMG spectrum was substantionally different in dependence on exercise duration and intencity as well as on blood flow. In contrast to Nielsen et al., acidification did not recover the amplitude of the muscle compound AP during high intensity exercise in our experiments. We have found that changes in pH and in CV only weekly related with alteration of the EMG power spectrum. It can be concluded that 1)during voluntary exercise the effects of changes in pH on the muscle excitability and CV may be counterbalanced by other effects (e.g. increased TU or ion redistribution) and 2) changes in AP propagation velocity are not the only cause and under certain conditons are not the main cause of changes in the EMG power spectrum.