Muscles have three systems for supplying the ATP they need for contraction:

The capacity of these three systems and the rates at which they can produce ATP determine both work capacity and endurance of a muscle (Figure 47.13).

Question

The slopes of the curves are influenced by the time course of the mechanisms supplying the ATP supporting the activity. Because the first 10–20 seconds of high-level muscle work are supported by preformed ATP and CP, no further production of ATP and CP is involved. The production of ATP by glycolysis can produce ATP rapidly but not in large amounts, so the speed of performance goes down in the first minute of intense exercise even though the production of ATP by oxidative metabolism is gradually increasing. Oxidative metabolism is a more efficient and sustainable producer of ATP, but it involves many more biochemical reactions than glycolysis, and cannot proceed as rapidly. Also, it is limited by the delivery of O₂ to the mitochondria, so the speed of sustained performance goes down further.

ATP is present in muscles in very small amounts. However, muscle fibers also contain a storage compound called creatine phosphate (CP). This molecule stores energy in a phosphate bond that it can transfer to ADP. The total energy available in all the muscles of your body in the form of ATP and CP—the immediate energy system—is only about 10 kilocalories. When at rest, you metabolize a kilocalorie of energy in less than a minute. Even though the energy available from ATP and CP is limited, it is available immediately, and it enables fast-twitch fibers to generate a lot of force quickly. During burst activity, the immediate system is exhausted in seconds.

The glycolytic system activates within a few seconds to replace the ATP depleted at the onset of muscle activity. The glycolytic enzymes are located in the cytoplasm of the muscle fiber, and therefore the ATP they generate is rapidly available to the myosin filaments. However, as noted in Chapter 9, glycolysis alone is an inefficient way to produce ATP, and it leads to the accumulation of lactic acid, which slows the process. Thus the glycolytic system and the immediate system together provide the energy needed by active muscles for less than a minute (see Figure 47.13). Fortunately, oxidative metabolism becomes fully active in about a minute, producing relatively huge amounts of ATP because it can completely metabolize carbohydrates and fats to CO₂ and water. However, it requires many reactions, and it takes place in the mitochondria, so O₂ and substrate must get into the mitochondria, and the formed ATP must get from the mitochondria to the myosin filaments in the muscle. These processes are not instantaneous, so the rate at which oxidative metabolism can make ATP available to do work is slower than the rate at which the other two systems can supply ATP.

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The *fuel resources available to the muscles influences how long someone can sustain a high level of aerobic exercise. From the circulating blood, muscle receives glucose and free fatty acids that it metabolizes to generate ATP. At high levels of aerobic exercise, however, most of the fuel used by muscles to produce ATP comes from the reserve of glycogen stored in the muscle itself. Depletion of muscle glycogen contributes to fatigue.

The rate at which muscle glycogen is replenished depends on diet. Glycogen is quickly replenished with a high-carbohydrate diet, but slowly replenished with a high-fat diet. A method used to maximize muscle glycogen is called carbo-loading. About 1 week before an athletic event, athletes typically exercise for 3–5 days at a level that depletes muscle glycogen. Then, 2–3 days before the event, they taper down their level of training and eat a diet rich in complex carbohydrates. The result can be glycogen supercompensation, in which the restoration of muscle glycogen stores “overshoots” and reaches above-normal levels.