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.
Top athletes can train their muscles to work effectively with elevated lactic acid levels.
Eventually, however, the lactic acid will build up to the point where the blood will become more acidic. This interferes with the chemical reactions that cause the muscles to contract, and performance will quickly degrade until the body starts working aerobically. Glycogen dominates for about the first 30 minutes of aerobic exercise and then fat is brought into play. The fat reserves are generally much greater. One pound of fat contains enough energy to run over 30 miles.