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. Oxygen debt - lactic acid buildup . Muscle pain - cramps. As muscle glycogen is used up during exercise, the muscle cells become ravenous for more.
Blood glucose is used by muscles to replace the glycogen.
It takes about 90 minutes of vigorous exercise for the glycogen stores in a trained athlete to be depleted. Train the cells to store more glucose. 389 Body fat is an unlimited source of energy. Early in exercise, blood fatty acids drop. After about 20 minutes fatty acid levels rise higher, as cells begin to empty out.