In order to meet the energy demands of the muscles during exercise, ATP is produced by both anaerobic and aerobic metabolism. The amount of ATP produced by each depends on how intense the activity is. During moderate-intensity exercise, the heart and lungs can deliver enough oxygen to the muscle to support aerobic metabolism. Aerobic metabolism can use both glucose and fat to produce ATP.
As exercise intensity increases, oxygen cannot be delivered to the muscle quickly enough to meet energy demands through aerobic metabolism alone, so anaerobic metabolism of glucose increases to provide additional ATP. The more intense the exercise is, the greater the proportion of ATP that is provided by anaerobic metabolism of glucose.
As was previously discussed, exercise training increases the amount of oxygen delivered to the muscles. It increases the amount of blood pumped with each heartbeat, as well as the volume of blood, the number of red blood cells, and the number of capillaries in the muscles.
At the muscle cell level, exercise training increases the number and size of mitochondria, the ability to metabolize fatty acids, and the amount of stored glycogen. These physiological and biochemical effects increase a person's ability to deliver oxygen to muscle cells, and the ability of cells to produce ATP by aerobic metabolism.
Comparing the performance of a trained and an untrained person in identical tasks reveals the impact of exercise training. If two men of the same age, height, and weight were to try to ride a bicycle uphill for an hour at a set speed, the level of training would affect the type of metabolism, the fuels burned, and how long each person could continue.
The trained person would be able to ride at the required speed while using predominantly aerobic metabolism. He would use mostly fatty acids as an energy source, so his glycogen stores would not be depleted. He would be able to ride for the entire hour.
The untrained man would not be able to ride at the required speed using aerobic metabolism alone. He would use anaerobic metabolism of glucose to supply some of the needed energy. Before the hour was up, his glycogen stores would be empty and he would have to slow down or stop.
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