After intense exercise, the respiration rate and volume remain elevated for a time, even though the muscles are no longer actively contracting. This increased respiration provides the O2 to pay back the oxygen debt. The oxygen debt, or excess postexercise O2 consumption, is the amount of O2 needed in chemical reactions that occur to (1) convert lactic acid to glucose, (2) replenish the depleted ATP and creatine phosphate stores in muscle fibers, and (3) replenish O2 stores in the lungs, blood, and muscles. After the lactic acid produced by anaerobic respiration is converted to glucose and creatine phosphate levels are restored, respiration rate returns to normal. The magnitude of the oxygen debt depends on the intensity of the exercise, the length of time it was sustained, and the physical condition of the individual. The metabolic capacity of an individual in poor physical condition is much lower than that of a well-trained athlete. With exercise and training, a person’s ability to carry out both aerobic and anaerobic activities is enhanced.