Muscle contractions are fueled by ATP (adenosine triphosphate) . ATP is an energy storing molecule, available in several pools or forms to the muscle filaments. The immediate source of ATP can power only a few twitches. However, this pool is easily and quickly replenished for full muscle contraction and sustained exercise. Three sources supply the muscle's ATP pool: (1) creatine phosphate, (2) glycogen, and (3) cellular respiration in the mitochondria of the fibers.
Creatine phosphate, in a reaction with ADP (adenosine diphosphate), produces ATP through loss of the phosphate group, which is added to the formed ATP molecule creating a high-energy phosphate bond. This is a reversible reaction, with creatine able to accept the high energy phosphate group from ATP to fuel further reactions with ADP. Phosphate groups available for the conversion of ADP to ATP exist as creatine phosphate in pools that are 10 times larger than the pools of ATP within muscle fibers.
Glycogen, through conversion to glucose-1-phosphate via the process of glycolysis, produces a minimal amount of ATP. The end product of the pathway, particularly in the absence of oxygen, is lactic acid. This pool of ATP is used in the absence of oxygen, and thus the ability to produce ATP by way of cellular respiration.
Cellular respiration provides ATP to muscles during prolonged activity, provided sufficient oxygen is available. Furthermore, following anaerobic respiration, cellular respiration converts lactic acid to glycogen, thus restoring the pool for short-term muscle contraction. Therefore, cellular respiration not only is required to meet the ATP needs of a muscle engaged in prolonged activity (thus causing more rapid and deeper breathing), but is also required afterwards to enable the body to resynthesize glycogen from the lactic acid produced earlier (deep breathing continues for a time after exercise is stopped). The body must repay its oxygen debt.
Chronic loading, or frequent vigorous physical exercise, at levels typical of endurance training (or aerobic conditions), causes several alterations to the metabolic pathways that serve to provide ATP to muscles. Breakdown of glycogen is decreases, thus decreasing lactic acid production. Further, the type of molecules used for cellular respiration shifts from simple sugars to fatty acids, which is offset by the decrease of the contribution of the glycogen pools to muscle metabolism. In addition, chronic loading increases the expression of the enzymes involved in fatty acid activation, translocation, and oxidation leading to an increased rate of fatty acid oxidation. As a consequence, chronically-loaded muscles rely more on fats as the main source of fuel for ATP synthesis, which is offset by a reduction in the relative contribution of intramuscular glycogen.
Chronic unloading, or lack of vigorous physical acitivty, often associated with bed rest, sees opposite patterns. Carbohydrates and glycogen contribute significantly more to reactions producing ATP.
Such differences due to chronic differences in vigorous activity are evident in acute exercise responses. For example, muscles conditioned by chronic loading preferentially using creatine phosphate stores first, and at a faster rate, than muscles subjected to chronic unloading.