Although aerobic respiration produces more ATP molecules than anaerobic respiration for each glucose molecule metabolized, anaerobic respiration can occur faster and is important when O2 availability limits aerobic respiration. By using many glucose molecules, anaerobic respiration can rapidly produce much ATP, but anaerobic respiration can proceed for only a short time. Lactic acid is an end product of anaerobic respiration. Some of the lactic acid can diffuse out of the muscle fiber into the blood, but some remains in the muscle fibers. Lactic acid can irritate muscle fibers, causing short-term pain. Muscle pain that lasts for several days, however, indicates mechanical injury to the muscle.