Energy Production and Release
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. Anaer-obic glycolysis produces ATP quickly to sustain muscle
actions for a couple of minutes but the end products , 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. It is important to note that the utilization of these meta-bolic 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. Carbohydrates (plasma glucose and muscle glycogen) and fats (plasma free fatty acids and muscle triglycerides) are the two main fuels utilized by the muscle cell to pro-duce ATP. Metabolism of amino acids may contribute a small percentage of the total energy production. Again, the selection of the specific fuel depends on the intensity and duration of the exercise. In general, at high intensities,
muscle actions are mainly fuelled by muscle glycogen
stores. On the other hand, lower intensity and long duration
exercise utilizes the metabolism of free fatty acids for most
of the energy needs. In real life, most activities activate
different pathways at different points in time and use a
combination of fuels to produce the ATP needed for muscle actions.