Resting muscle or muscles undergoing long-term exercise, such as during long-distance running, depend primarily on aerobic respiration for ATP synthesis. Although some glucose is used as an energy source, fatty acids are a more important energy source during both sustained exercise and resting conditions. During short periods of intense exercise such as in sprinting, ATP is used up more quickly than it can be produced by aerobic respiration. Once the ATP reserves begin to decrease, the energy stores in creatine phosphate is used to maintain ATP levels in the contracting muscle fiber. Once the creatine phosphate stores are depleted, anaerobic respiration becomes important. Anaerobic respiration rapidly metabolizes available glucose to provide enough ATP to support intense muscle contraction for up to 2-3 minutes. During intense exercise, glycogen stored in muscle fibers can be broken down to glucose, which can then be used to produce more ATP. Anaerobic metabolism is ultimately limited by depletion of glucose and a buildup of lactic acid within the muscle fiber.