All muscle actions require energy in the form of ATP. The metabolic energy pathway used to produce and sustain muscle actions depends on the duration and intensity of the activity. The three basic energy pathways in the muscle fiber are stores of ATP and CP, anaerobic glycolysis, and oxidative phosphorylation. The first supports high intensity short duration (few seconds) activities because the amount of ATP and CP reserves in muscle fibers is small. Anaerobic glycolysis produces ATP quickly to sustain muscle actions for a couple of minutes but the end products (H?, lactate) impair muscle function and are associated with muscle fatigue. Finally, the energy for exercise performed at intensities that can be sustained for longer duration (minutes to hours) is supplied by oxidative phosphorylation within the mitochondrial network. A network of capillaries transports oxygen to the active muscle fibers. The extent of this network correlates with the metabolic demand on the muscle fiber. The architectural relationship between these two structures was described very early in the twentieth century by August Krogh and recently discussed [31]. It is important to note that the utilization of these metabolic pathways is not an ‘‘all or none’’ phenomenon. Pathways overlap and can be activated at different points in time during a single session of exercise depending on the intensity of the effort.