However, this takes energy. When this happens the ATP needs to be regenerated or your body will cease to function. Glycogen is stored primarily in the liver, but a small amount is stored in muscle tissue. Glucose is present in the blood. These are the sugars used for glycolysis.
Lactic acid is a by-product of glycolysis.
However, lactic acid can itself be converted into ATP in the presence of oxygen, so this waste product can also be a fuel. This is called the anaerobic threshold. After about two minutes above the anaerobic threshold lactic acid will begin to build up. The first and most obvious result of too much lactic acid in the muscles is an intense, burning pain. However, muscle performance does not necessarily degrade at this point.