The same basic metabolic systems are present in muscle as in other parts of the body. However, special quantitative measures of the activities of three metabolic systems are exceedingly important in understanding the limits of physical activity. These systems are (1) the phosphocreatine-creatine system, (2) the glycogenlactic acid system, and (3) the aerobic system.

Adenosine Triphosphate. The source of energy actually used to cause muscle contraction is adenosine triphosphate (ATP), which has the following basic formula:

Adenosine-PO3 ~ PO3 ~ PO3-

The bonds attaching the last two phosphate radicals to the molecule, designated by the symbol ~, are high energy phosphate bonds. Each of these bonds stores 7300 calories of energy per mole of ATP under standard conditions (and even slightly more than this under the physical conditions in the body. Therefore, when one phosphate radical is removed, more than 7300 calories of energy are released to energize the muscle contractile process. Then, when the second phosphate radical is removed, still another 7300 calories become available. Removal of the first phosphate converts the ATP into adenosine diphosphate (ADP), and removal of the second converts this ADP into adenosine monophosphate (AMP).

The amount of ATP present in the muscles, even in a well-trained athlete, is sufficient to sustain maximal muscle power for only about 3 seconds, maybe enough for one half of a 50-meter dash. Therefore, except for a few seconds at a time, it is essential that new ATP be formed continuously, even during the performance of short athletic events.

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