Energy metabolism consists of a series of chemical reactions that break down foodstuffs and thereby produce energy. The body traps about 20 percent of the energy that is produced and releases the remaining 80 percent as heat. This is why the body heats up during exercise.

Energy production in the human body revolves around the rebuilding of ATP molecules after they have been broken down for energy. ATP is the molecule that stores energy in a form that the body can use. This rebuilding of ATP molecules is accomplished in a number of ways, all of which correlate to the four main purposes for which energy is utilized during athletic performance - power, speed, strength, and endurance - and to the four basic types of physical activity - strength - power, sustained power, anaerobic power - endurance, and aerobic endurance.

When resting, the body derives most of its energy from the oxidative energy systems. But when physical activity begins, the nonoxidative energy systems kick in. When the exercise level becomes very intense, the nonoxidative immediate energy systems predominate. First, the ATP reserves in the muscle cells are drafted. However, these ATP reserves are depleted instantly - within a second. Therefore, if the exercise level remains very intense, resting CP stores are called upon, regenerating the ATP molecules, which can then continue to serve as the intense, the next nonoxidative energy system, glycolysis, jumps in. Nonoxidative glycolysis functions during near-maximum efforts lasting up to about one and half minutes. If helps maintain the intensity of the muscle contractions, even though the muscles' force output becomes diminished. When an ATP molecule releases its energy to power a muscle contraction, it is reduced to 1 adenosine- diposphate (ADP) molecule and 1 phosphate atom. In nonoxidative glycolysis, a glucose molecule is split in half to regenerate ADP back to ATP. Every glucose molecule releases enough energy to regenerate 2 ATP molecules. Nonoxidative glycolysis also results in the formation of lactic acid. Glycolysis takes place in the cytoplasm of a cell. Since the amount of free glucose in a cell is generally low, the glucose that is used is usually derived from the breakdown of glycogen. During intensive exercise, the oxidative energy systems also supply energy, but they contribute a much smaller share.

As the intensity of the exercise is reduced and the duration is increased - generally to more than three to four minutes - the oxidative systems include oxidative glycolysis and beta oxidation. In oxidative glycolysis, when the glucose molecule is split in half, it forms 2 molecules of pyruvate. These pyruvate molecules enter the Kerbs cycle, a process in which short chains of carbon atoms from glucose, fatty-acid, or protein molecules are broken down and the energy that is released is used to regenerate ATP molecules. The breakdown products of the Kerbs cycle are then shuffled into the electron transport system, a process in which electrons are passed between certain protein molecules, releasing energy that is used to regenerate additional ATP molecules. The complete catabolism of 1 molecule of glucose yields about 36 ATP molecules. In beta oxidation, fatty acids are metabolized. Each fatty-acid molecule is broken down into 2 carbon acetyl fragments. The acetyl molecules then combine with coenzyme A to form acetyl-coenzyme A (actryl-CoA). The acetyl-CoA molecule is shuffled to the krebs cycle and broken down, yielding energy to regenerate ATP. Oxidative energy metabolism is a slower process that nonoxidative energy metabolism, but it more completely breaks down the energy molecules - for example, glucose and fatty acids - and generates a lot more energy. The complete breakdown of  3 molecules of fatty acids containing 18 carbon atoms each yields about 441 ATP molecules.

Glycogen is essential for good performance in both anaerobic and aerobic activities. Muscles that are being strenuously exercised rely on glycogen to power their strength - generating muscle contractions. In aerobic - endurance activities, the primary fuel is fatty acids, but glycogen is also utilized. In fact, fat catabolism works better when carbohydrates are metabolized at the same time. Research on aerobic - endurance exercise and work performance has shown that when glycogen is depleted in the body, fatigue sets in. Therefore, adequate carbohydrate intake and glycogen replenishment are vital factors for peak energy output. 

Glycogen depletion is just one factor that contributes to the onset of fatigue, however. Some other factors are ATP and CP depletion, accumulation of lactic acid, buildup of calcium ions in the muscles, oxygen depletion, dehydration, and decreased blood pH. The proper conditioning of the body to enable it is  to appropriately utilize the preferred energy systems is also important. If the body is poorly conditioned, it cannot properly use the energy systems appropriate for the activity, which can lead to early onset of Fatigue. To avoid this, focus your physical - conditioning efforts on developing the primary energy systems needed for your diet supplies the ideal fuel mix to enhance your physical conditioning.

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