The consumption of energy by all cells, including those of muscle tissue, in order to perform work. The source of chemical energy, adenosine triphosphate (ATP), is metabolized to adenosine diphosphate (ADP). If the energy requirement is short term, the ADP is converted back to ATP. This process goes at a rate too slow to keep up with energy demands during long-term exercise; thus, consumption of other fuels is required. The main sources of fuel for muscles are carbohydrates and lipids. Prior to being available for intracellular metabolism, glycogen is obtained when glucose is converted to glycogen. The main lipid fuel is free fatty acids present in plasma. Carbohydrates can be metabolized in either the presence (aerobically) or absence (anerobically) of oxygen, but lipids can be metabolized only aerobically. During light exercise or when the body is at rest, muscle metabolism is usually entirely aerobic and the source of fuel is the free fatty acids in plasma. During intense exercise, metabolism of the fatty acids cannot keep up with the demand; thus, glycogen is used. However, as intense exercise continues, glycogen stores are exhausted and free fatty acids become the principal source of energy. Trained athletes have an increased ability to metabolize fatty acids as compared with sedentary individuals. This permits athletes to exercise longer and at higher work rates than would be the case if they were not trained. Athletic trainers found that muscle glycogen stores could be increased by what is known as carbohydrate loading. This regimen will permit the athlete to exercise for a much longer period than would be possible if carbohydrate loading had not been done prior to exercising.