The equation for lactic acid fermentation after glycolysis is, pyruvic acid + NADH --> lactic acid + NAD+. In the absence of oxygen, NADH that is synthesized in the glycolysis reacts with the pyruvic acid to form lactic acid and NAD+, in the presence of the enzyme, lactate dehydrogenase.
The fermentation process involves the transfer of a hydrogen atom from NADH to the pyruvate molecule, converting C=O bond into the C-O bond. The resulting molecule is, lactate, which further forms the lactic acid. Muscle fatigue is stimulated due to the formation of lactic acid, which normally is made after strenuous exercise, in the absence of oxygen. Presence of Oxygen, drives the utilization of NADH towards the electron transport chain.