Metabolic fatigue requires working until a muscle is incapable of performing another rep. When this occurs, the muscle is depleting its stored glycogen leading to acidosis in the blood stream. Acidosis creates damage to individual muscle fibers, which initiates a repair response that results in muscle growth. Furthermore, depleting muscle glycogen increases the muscle’s ability to store more glycogen. One molecule of glycogen can hold 3 to 4 molecules of water, which leads to an increase in muscle volume.