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During rapid, high-intensity exercise, your muscles use large amounts of energy in the form of a compound called adenosine triphosphate, or ATP. To allow you to keep working out, ATP stores must be replenished quickly. Muscles utilize both anaerobic and aerobic metabolism to produce ATP, but during fast-paced, strenuous exercise, anaerobic metabolism predominates.
Your cells store energy in the form of ATP. When muscles contract, they break down ATP in a reaction that provides energy. However, muscle cells only store enough ATP to fuel a few seconds of maximal contraction. To keep working, they must replenish their ATP supply. They can do this for a short period of time by breaking down another high-energy compound called creatine phosphate. Because stores of creatine phosphate are also limited, this energy system, called the phosphagen system, can only sustain maximal muscle output for about 10 seconds. The phosphagen system is the primary energy source during very short, rapid bursts of activity, such as a sprint.
To sustain exercise for more than 10 seconds, muscles must break down fuel sources such as carbohydrates and fats to provide the energy to re-synthesize ATP. Carbohydrate metabolism is faster than fat metabolism, so carbs provide a high percentage of the energy used during very high-intensity workouts. Because carbs can be metabolized anaerobically, without oxygen, they become a vital energy source when oxygen supply to your muscles can't keep up with demand.
The breakdown of carbs to provide energy without oxygen is called anaerobic glycolysis. This process releases energy very rapidly. Glucose, or blood sugar, and stored carbs in the form of glycogen in your muscle cells are broken down through a series of steps to form a compound called pyruvate. This process yields two to three molecules of ATP for each molecule of glucose. If no oxygen is available, pyruvate is converted to lactic acid, which can accumulate in your tissues and blood during rapid exercise.
If oxygen is available, pyruvate can be further broken down aerobically to produce as many as 30 additional molecules of ATP, making aerobic metabolism, although slower, much more efficient than anaerobic metabolism. Fats can also be broken down aerobically to produce large quantities of ATP. Although during very rapid exercise, anaerobic metabolism predominates, aerobic metabolism also contributes to muscles cells' ATP production. After a vigorous workout, your muscles restock ATP supplies aerobically, accounting, in part, for post-exercise oxygen consumption.