Why should the body produce lactic acid

People strenuous exercise, the need to quickly use a large amount of energy, the first to use the body of free ATP and Calcium lactate  and glucose rapid glycolysis produces pyruvate, a short man strenuous exercise relative lack of oxygen in the body, so pyruvate in glycolysis under hypoxia lactic acid, which can quickly provide a large number of ATP to the muscles for energy when the body glucose depletion, and then break down muscle and liver glycogen, muscle glycogen directly Lactic acid for energy, and to break down glycogen into glucose and then enter blood glucose again become functional; strenuous exercise when the body stops, get the oxygen supply, lactic acid is transported to the previously generated gluconeogenesis in the liver to regenerate glucose.