14 Sep 2007
The relationship between the duration of exercise and the amount of glucose used as a fuel depends upon the availability of oxygen. Oxygen plays a key role in the workings of the muscle's metabolic engines. With ample oxygen, muscles can extract all available energy from glucose in three to 20 minutes of moderate exercise. During this period of aerobic glycolysis the muscles and liver pour out their stored carbohydrate for use by the muscles.
However, the muscles and liver can only store and use a specific amount of glycogen before it will run out. Therefore, a person who continues to exercise moderately for longer than 20 minutes will need to find another source of fuel. At this point (after 20 minutes) the body will begin to use less glycogen and more and more fat for fuel.