In human muscle cells, the fermentation process produces lactic acid and adenosine triphosphate. Lactic acid fermentation occurs usually during physical activity, when less oxygen reaches the cells, so the cells adjust by producing energy that doesn't require oxygen in the process.
Glucose, made up of sugar molecules, is an important energy source for humans. When the body needs energy, this glucose breaks down in a process called glycolysis and results in pyruvate molecules and NADH molecules. When there is a lack of oxygen, lactic acid fermentation uses the pyruvate molecules that have accumulated during glycolysis and the NADH molecules to produce energy for human muscle cells. Lactic acid and ATP, or adenosine triphosphate, are the byproducts of this process. While ATP is a compound that provides energy, the lactic acid is a waste product.
Because lactic acid can harm the muscles, once the physical activity declines, a person experiences rapid breathing. This immediately adds oxygen to the lactic acid to convert it back to harmless pyruvate molecules. A sign of a lack of oxygen for glycolysis and high lactic acid in the muscles is muscle pain and fatigue. As the high-energy exercise continues, the lactic acid builds up, and the pH drops.