Large stores of intramuscular substrates are found in the different fiber types of human skeletal muscle, and with prolonged exercise both glycogen and triglyceride stores are utilized. The contribution from intramuscular glycogen stores is greatest at higher work intensities while triglyceride stores are utilized at moderate intensities. In man all fiber types have a similar glycogen content whereas the highest lipid content is found in the more oxidative fibers. The muscle metabolism is well adapted to the supply of substrate as well as to the demand for energy. Among several regulatory mechanisms, changes in citrate concentration seems to be an important factor in the interplay between carbohydrate and lipid metabolism.