How does exercise duration affect the fuels the body uses

Which fuel powers muscles depends on how long a person has been exercising. During the first few seconds, the body can use the small amounts of ATP and creatine phosphate stored in muscles. After that, additional ATP must be made.

Energy Right Now: AT P and Creatine Phosphate

During the first few steps of a morning jog, the heart and lungs have not had time to step up oxygen delivery to the muscles. Still, ATP is needed immediately to fuel muscle contraction. During the first 10 to 15 seconds of exercise, the muscles use energy from stored ATP and creatine phosphate, another high-energy compound stored in the muscle.

Oxygen is not needed to use these fuels. In a resting muscle, there is enough stored ATP to fuel muscles for about three seconds. As the ATP in muscle is used, enzymes transfer phosphate from creatine phosphate to adenosine diphosphate (ADP) to form more ATP. The amount of creatine phosphate stored in the muscle at any time is also small.

It will fuel muscle activity for an additional 8 to 10 seconds. Short, high-intensity exercise, such as a 100-meter dash, a 25-meter swim, or lifting a heavy weight, can be fueled almost exclusively by energy from stored ATP and creatine phosphate.

These sources are also important in any activity requiring brief bursts of maximal effort, such as driving for a layup in a basketball game, thrusting upward during a pole vault, or launching a shot put. However, sustaining exercise beyond an immediate burst - and recovering from an all-out effort - require additional ATP. This ATP is generated from the metabolism of carbohydrates, protein, and fat.

Fueling Short-Term Exercise: Anaerobic Metabolism

After 10 to 15 seconds of exercise, the stores of ATP and creatine phosphate in muscles will be gone. However, the heart and lungs still need more time to increase oxygen delivery to the muscles. At this point, muscle cells use anaerobic metabolism, which does not require oxygen, to make ATP. The reactions of anaerobic metabolism take place in the cytoplasm of the cell.

Anaerobic metabolism can use only glucose as fuel. In a process called glycolysist, the 6-carbon glucose is broken into two 3-carbon molecules of pyruvate and generates two molecules of ATP, which are available to power muscle contraction. The byproducts of anaerobic ATP production - pyruvate and high-energy electrons - combine to form a molecule called lactic acid.

The lactic acid can be transported out of the muscle for use in other tissues. The liver can convert lactic acid back to glucose. However, if the amount of lactic acid produced exceeds the amount that can be used, it begins to build up in the muscle and the blood. This buildup of lactic acid is associated with fatigue, but is no longer believed to be an important cause of fatigue.

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