Abstract. The article underscores the impact of recent scientific achievements for the understanding of exercise biochemistry and of muscular fatigue. Firstly as well the anaerobic alactic and lactic as the aerobic stages of the muscular energetic are mentioned. In accordance with Hill’s classical theory intracellular acidosis due to accumulation of lactic acid leads to muscular fatigue. Studies performed in the last two decades challenged a new approach. The acidification related to lactic acid and other umoral factors has in fact in some conditions beneficial effects on the performance of fatigued muscles. This hypothesis sustained on various experimental models must be verified in aerobic exercise and on humans. Are also under scrutiny the complex mechanisms of general, mental and local, muscular fatigue, with deeper exploration of the role of central and autonomic nervous systems.

Key words: lactic acid, intracellular acidosis, muscular and mental fatigue, role of the nervous system.