Mechanisms
Athletes engaged in exercise over a long period of time produce energy via two mechanisms, both facilitated by oxygen:
- via fat metabolism and
- via breakdown of glycogen into glucose, followed by glycolysis.
How much energy comes from either source depends on the intensity of the exercise. During intense exercise that approaches one's VO2 max, most of the energy comes from glycogen.
A typical untrained individual on an average diet is able to store about 380 grams of glycogen, or 1500 kcal, in the body, though much of that amount is spread throughout the muscular system and may not be available for any specific type of exercise. Intense cycling or running can easily consume 600-800 or more kcal per hour. Unless glycogen stores are replenished during exercise, glycogen stores in such an individual will be depleted after less than 2 hours of continuous cycling or 15 miles (24 km) of running. Training and carbohydrate loading can raise these reserves as high as 880 g (3600 kcal), correspondingly raising the potential for uninterrupted exercise.
Neurochemical changes in the brain are also very likely to be involved, according to J. Mark Davis, PhD
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