The concentration of muscle and liver glycogen prior to exercise plays an important role in endurance exercise capacity. In exhaustive exercise many studies have observed significant depletion of both liver and muscle glycogen. It is interesting to recognize that the point of exhaustion seems to occur upon the depletion of liver glycogen. Conversely, muscle glycogen reserves, though significantly lower are only 65-85% depleted, versus the 85-95% depletion exhibited for liver glycogen. This should make it readily apparent that liver glycogen is an integral determining factor in an athlete's time to exhaustion. It follows that endurance athletes who maintain a daily regimen of endurance training without glycogen repletion may severely deplete their glycogen reserves.