Anaerobic energy is energy produced by glycolysis and lactic acid fermentation in the body. Anaerobic means "without air," and it is contrasted with aerobic energy, which is derived from oxygen. Certain types of exercise, called anaerobic exercise, help the body to begin anaerobic metabolism. Anaerobic exercise helps build muscle mass and performance, while aerobic exercise helps strengthen the cardiovascular system and manage weight. Anaerobic exercises are intense, but only performed for a maximum of two minutes, at which point anaerobic energy production is at its peak.
Lactic acid fermentation produces anaerobic energy by converting sugars such as fructose, glucose, and sucrose into cellular energy, creating lactic acid as a byproduct. This process, called glycolysis, creates adenosine triphosphate (ATP), a molecule that transports chemical energy within cells, and pyruvate, an organic acid that becomes lactic acid in the absence of oxygen. Lactic acid helps maintain energy production, but eventually it must be removed by the body, as excess lactic acid contributes to lactic acidosis and muscle soreness.
In lactic acidosis, the pH of the blood drops, causing the blood to become acidic. Lactic acid can be converted back into pyruvate through oxidation in the Krebs cycle, or it can be converted into glucose in the liver via the Cori cycle. Glucose created by the Cori cycle will simply undergo glycolysis again, so it is important for the body to get adequate oxygen to prevent lactic acidosis. Because the body eventually requires oxygen to maintain a healthy metabolism and prevent lactic acid buildup, anaerobic exercise can only be performed in brief spurts.
The level of exercise intensity at which lactic acid begins to accumulate in the bloodstream, marking the limit of anaerobic energy production, and hence of efficient exercise, is known as the lactate threshold (LT) or anaerobic threshold (AT). A person's LT can be increased by regular exercise. It is generally considered to be between 90 to 95 percent of a person's maximum heart rate, while aerobic exercise is performed at about 70 to 80 percent of the maximum heart rate. In interval training, a person exercises above the LT for a short period of time to produce anaerobic energy and build the muscles, then exercises at a lower rate to recover and oxygenate the body, then repeats the process.