Two methods used by muscles to make ATP are anaerobic. The third method is aerobic. The aerobic process takes place inside mitochondria. A mitochondria's most important job is to make ATP. Glucose and fatty acids (the building blocks of carbohydrate and fat) are its favorite foods. Aerobic metabolism is the most efficient way to produce ATP in a muscle fiber, although it is not the quickest. Its only bi-products are water and carbon dioxide. These substances leave the fiber by the same path as the oxygen enters. Aerobics create more energy than necessary for the production of ATP. This excess energy is released as heat. This helps the body maintain its optimum operating temperature of 98.6 F.

When available, the aerobic pathway always uses fatty acids. The aerobic energy of a resting muscle comes two- thirds from fatty acids and one-third from glucose. This is "glucose sparing" and occurs because the brain and nerve tissue can only burn glucose to make ATP. The muscles save glucose for the nervous system as often as possible.

During rest, the aerobic pathway supplies the minimum need of limited muscle motion. Strenuous exercise starts the two anaerobic pathways going. The first route depends on creatine phosphate, a substance similar to ATP. Creatine phosphate and ATP are on opposite ends of a chemical spectrum. ATP releases energy and creatine phosphate gains it. When a creatine phosphate molecule separates into creatine and phosphate, it releases energy to make a molecule of ATP.

This action takes place whether the cell has oxygen or not, because the reaction does not occur in the mitochondria. It happens within the muscle's protein filaments. During the short bursts of intensity demanded by power-strength creatine phosphate is both the first source of energy summoned and the first depleted. The reserves of this compound in the muscle can only provide energy for five to eight seconds. The benefit of the process is to allow the muscles to react instantly. This avoids waiting for the cardiovascular system to provide the extra oxygen that the aerobic pathway needs to work. Like ATP, creatine phosphate must be rebuilt when depleted. Rebuilding creatine phosphate uses some of the energy liberated from ATP and the aerobic pathway.

Anaerobic glycolysis is the second process that supplies energy to the muscle fiber without oxygen. Gycolysis is the breakdown of sugar. The muscles borrow from their stores of glucose to make ATP. Anaerobic glycolysis is an intermediary energy source used after the creatine phosphate is depleted but before the aerobic pathway starts. Enzymes transform glucose into pyruvic acid and make ATP in the process. If the muscle fiber has enough oxygen, the pyruvic acid enters the mitochondria for further energy use. But if the muscle fiber lacks sufficient oxygen, the pyruvic acid cannot enter the mitochondria and changes into lactic acid. The lactic acid builds up in the cell and seeps into the blood. The accumulation of lactic acid in the muscle fiber makes the cell's environment more acidic. High acidity interferes with the breakdown of glycogen, so anaerobic glycolysis cannot take place. Creatine phosphate and anaerobic glycolysis simultaneously can only supply the muscle with enough energy for two to three minutes of intense exercise.

Almost any exercise uses all three energy pathways. Creatine phosphate and anaerobic glycolysis drive the muscles, while the body makes the adjustments to supply more oxygen. If the activity is not too strenuous, the anaerobic pathways soon shut down and the body reaches an "aerobic steady state." At this point not enough lactic acid has built up to cause exhaustion. The aerobic pathway can use part of the energy it creates to turn the lactic acid back into pyruvic acid that the mitochondria can use to generate more energy. The liver and muscles also can manufacture glycogen from lactic acid. This is still another energy source. Together, these processes limit the build-up of lactic acid and forestall exhaustion. The BioFitness Institute can personalize a prescriptive workout plan for you that will effect each of your energy pathways.