The ATP-PC energy system is the system by which the body fuels 10 to 20 seconds of intense exercise by using stored ATP, the high-energy molecule that fuels muscles, and then through phosphocreatine, which is quickly converted to ATP to further fuel muscle contractions. This system is also sometimes called the ATP-CP energy system, because phosphocreatine is also known as creatine phosphate.
A small amount of ATP and phosphocreatine is stored in the muscle cells, so the ATP-PC energy system isn't related to food or beverage intake directly before or during exercise. The ATP-PC energy system also doesn't require the presence of oxygen, so it's said to be anaerobic. Alactic exercise utilizes the ATP-PC energy system to increase its efficiency. This type of exercise includes 10-second bursts of high intensity followed by 30-second recovery periods.
After both the stored ATP and the stored phosphocreatine in the muscle cells are used up, the body needs to produce more ATP to continue fueling the muscles. First, the body produces ATP directly from carbohydrates through glycolysis. This produces lactic acid as a waste product. After about 10 minutes, the lactic acid builds up to levels that cause pain and fatigue. During longer exercise periods the body relies on aerobic metabolism, which produces ATP from carbohydrates, fats and proteins with the help of oxygen from the circulatory system.