What actually goes on in the muscles during exercise has been speculated upon in muscle and fitness magazines endlessly. Like many things in product/industry financed publications, not all of it is factual or scientifically based. Here, David Cohen explains energy metabolism during exercise from a detailed kinesiological perspective.
Energy for muscular contraction is produced when the ATP (adenosine triphosphate) molecule loses phosphorus atom and the energy that bound the phosphorus to the rest of the molecule is released. After ten seconds of supreme effort, the supply of ATP in the muscle is completely converted to ADP (adenosine diphosphate). Now a molecule called creatine phosphate (CP) is broken down to creatine and one phosphorus, which is supplied to the ADP molecules so they become ATP and be broken down to provide energy again. After thirty seconds, all the CP is used up. Any continuing effort now requires burning of glucose in a reaction called anaerobic glycolysis. This produces great amounts of lactic acid, pyruvic acid and carbon dioxide. A burning sensation is perceived in the muscles being used. That shows that anaerobic glycolysis is occurring and the intensity of effort cannot be continuously performed much longer. The muscles need to rest before resuming exercise at this level of intensity, to replenish their supplies of ATP and CP and the lungs must dispose of all the excess carbon dioxide produced.
A lower intensity effort can take place for a much greater period of time if aerobic metabolism is used. It produces energy less quickly than anaerobic metabolism, but can continue for a much greater period of time. Sugar is broken down for energy by aerobic glycolysis and fat is burned by the Krebs’ cycle; both of these methods of energy production require the presence of oxygen to proceed. The fat is burned systemically, so there is no truth to the idea that swimming will burn fat from the upper body and cycling or other aerobic exercises involving primarily leg motion which will selectively burn fat from the legs.
Research has shown that, after 90 minutes of aerobic exercise, amino acids are burned as fuel in a process known as Gluconeogenesis. The amino acids come from protein from muscle tissue and are converted into glucose to be burned. It is an inefficient way to produce energy and increases ammonia and pH in the blood. This process occurs when the glycogen stores in the muscle tissue and liver are exhausted. The greater the amount of glycogen in the muscles and liver, the longer one can exercise without resorting to using one’s own muscle tissue as fuel. That is why Swedish researchers had found that carbohydrate loading improves the performance of Marathon runners and other endurance athletes. The original method involves high intensity training combined with a carbohydrate poor diet to deplete the glycogen stores. On the last few days before a race, training is curtailed and carbohydrate intake is greatly increased, causing super-compensation and a greater concentration of glycogen in the muscles and liver. The modified method avoids such drastic depletion and doesn’t require as great an intake of carbohydrates during the replenishment phase. While the effect of the modified method is not as pronounced, it is much easier on the digestive system and energy level of the athlete during the loading and depletion phases of the process.
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