Answer: The energy to fuel strenuous exercise comes from the breakdown of ATP, or adenosine triphosphate. Because your body has a limited supply — about 85 grams — of ATP, the substance must be resynthesized. Lactic acid helps to do this by playing a role in anaerobic glycolysis : the breakdown of carbohydrates without using oxygen.
Glycolysis forms hydronium molecules and pyruvic acid, which becomes lactate; when there is not enough oxygen available, the buildup of hydronium causes the pH of muscle cells to drop below normal levels of 7.1, causing acidity. When pH drops to 6.5, muscle contraction is impaired and nerve endings in the muscle are stimulated, causing pain and burning. You may also become disoriented and nauseated and associate these with lactic acid symptoms.