At the beginning of exercise, Glucose alone is mobilized for the extra energy burst. Then as activity continues, fats and other stored carbohydrates begin to be available to the muscles from the blood. Glucose as fuel for activity 382 Liver and muscle store glucose as glycogen. Liver also makes glucose. Liver glycogen is circulated.
Distance runners have more endurance if they eat high carbohydrate meals before a competition.
Exercise intensity Glycogen stores are finite, so length of exercise possible depends on intensity. The more intense the exercise, the shorter the duration. Energy production for exercise also depends on O 2 availability. In moderate exercise the body does aerobic metabolism, and uses both glucose and fat for energy. If exercise causes the body to exceed its O 2 capacity, muscles must turn to anaerobic metabolism of glucose for energy.