The molecule that provides energy for the contraction of exercising muscles is called ATP (adenosine triphosphate). ATP is the immediate source of energy for all body functions.
It supplies the energy needed to breathe, circulate blood, eliminate body wastes, and maintain body temperature. It also supplies the energy for muscle contraction, whether the muscles are needed to do homework, walk to class, or compete in a track meet.
ATP for muscle contraction is produced inside the muscle cells, primarily from the breakdown of glucose and fatty acids. During vigorous exercise, ATP is produced and consumed rapidly by the muscles. If exercise is more leisurely, ATP is produced and consumed more slowly.
ATP can be produced in different ways. One way is called aerobic metabolism. This type of metabolism requires oxygen. Aerobic exercise, such as jogging, biking, or swimming, relies on aerobic metabolism and therefore requires oxygen.
Aerobic metabolism produces ATP slowly, but it is very efficient: It produces a great deal of ATP from each molecule of glucose. Aerobic metabolism also can produce ATP from fatty acids. People store so much energy as body fat that there is always fat available as an energy source.
ATP can also be produced in the absence of oxygen at the cell, which occurs (among other times) during high-intensity exercise, such as sprinting. This anaerobic metabolism produces ATP very quickly.
But anaerobic metabolism produces fewer ATP molecules from each glucose molecule than aerobic metabolism does, and it cannot use fatty acids.
Therefore, relying on anaerobic metabolism uses up fuel, that is glucose, very rapidly. When glucose stores are depleted, it is not possible to continue exercising at the same intensity level, and the person feels fatigue.
People with a greater ability to deliver oxygen to the muscles (and for the muscles to use this oxygen to generate ATP by aerobic metabolism) can perform more intense exercise before feeling fatigue.
A person's ability to deliver oxygen to muscles and use aerobic metabolism to generate energy is referred to as his or her aerobic capacity. It also is referred to as maximal oxygen consumption, or VO2max. Someone with a greater aerobic capacity can exercise at a higher intensity for a longer time.
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