The adenosine triphosphate phosphocreatine system, ATP-CP for short, makes a constant supply of energy for 15-30 seconds. Once the available ATP is used for energy, the body slips into another energy mode to make ATP called glycolysis. This process uses glucose to make ATP and is responsible for the familiar burning sensation felt during training caused by lactic, which is this processes byproduct. Creatine is a supplement commonly used because its ingestion is reported to supply the cells with enough raw material to help the body produce more ATP, thus delaying glycolysis and "the burn" for a few extra seconds. High explosive sports such as sprinting, power lifting, football, and baseball rely heavily on the ATP-PC system. ATP is term the powerhouse of the cell and is also aids the body in nerve transmission, digestion, tissue building, and circulation.

ATP production is either anaerobic or aerobic. During weightlifting, which is 100% anaerobic, ATP is produce without the help of oxygen. During extended periods of exercise such as marathon, which is 100% aerobic, ATP is produced with the help of oxygen. Activities such as soccer or football, which require short bursts of energy, are a combination of aerobic and anaerobic. Boxing is an example of a combine aerobic and anaerobic exercise. The punch is an anaerobic exercise while the repeated bouncing and constant motion is aerobic.

The length of an exercise or activity will ultimately determine which system is at use, for example, a 500 yard dash will use the ATP-PC system to generate ATP, then glycolysis, and finally aerobic metabolism to generate the needed ATP to sustain movement.

ANAEROBIC THRESHOLD
At the onset of an activity, the body will rapidly increase oxygen consumption until the level of demand is met. This can take anywhere from 1-3 minutes, depending on the condition of the individual. Just from casual observation,many have sen those who were out of shape, "suck wind" at the onset of exercise while we also see those who can carry ona conversation and seem to breathe normally while jogging at a moderate pace. These adaptations are dependent on the aerobic conditioning of the individual, commonly coined as begin "in shape".

After exercise, the body's demand for oxygen is still elevate. Respiration is increased after exercise for a considerable amount of time relative to the intensity of the of the exercise and the individuals aerobic conditioning.

Because the body shifts into glycolysis to produce ATP after the ATP-PCr system, if the individual continues to train, there will be an accumulation of lactate. Once this lactate reaches the systemically(overall) in the bloodstream, an aerobic threshold is said to have been met. If levels in the blood get too high, one may get nauseated or vomit to the acidity of the blood.

Generally untrained individuals will have a very low anaerobic threshold, while a highly trained individual, such as an endurance runner, will be able to sustain activity despite high levels of lactic acid. This is due to a very high anaerobic threshold. It is important to remember that this adaptation takes time to develop, so be patient with yourself and others during exercise. Generally is not a matter of will, but physiological limiting factors that cause people new to exercise to suddenly stop. Patience is key when dealing with the body.