A study published in the Medicine Science of Sports and Exercise found that cyclist that trained when they had low levels of muscle glycogen increased fat use during endurance exercise. Normally, the body uses mainly carbohydrates for fuel during exercise intensities above 60% of maximum effort. However, fuel use depends on availability. The muscles will use whatever fuel is at hand. Scientists from the University of Birmingham, compared fat use during exercise in cyclists who had high or low levels of muscle glycogen. They manipulated glycogen levels by altering the training schedule. Fat use increased during periods of glycogen depletion, but power output also decreased by approximately 10%. While this was a sophisticated study, its practical application is questionable. Training while glycogen depleted is stressful and miserable, and increases the risk of injury. It is not clear whether this technique is valuable for improving exercise capacity or promoting weight loss.