The higher the exercise intensity, the greater the relative proportion of energy output derived from carbohydrate metabolism. Muscle glycogen stores are the most important source of carbohydrate during heavy exercise and in normal dietary conditions. At the same relative energy output, a muscle rich in type II fibers obtains a relatively larger energy contribution from carbohydrates, in terms of local muscle glycogen utilization, than one rich in type I fibers. This pattern can be modified by different dietary means.