A non-training elevation of 2,100–2,500 metres (6,900–8,200 ft) and training at 1,250 metres (4,100 ft) or less has shown to be the optimal approach for altitude training. Good venues for live-high train-low include Mammoth Lakes, California; Flagstaff, Arizona; and the Sierra Nevada, near Granada in Spain.Altitude training can produce increases in speed, strength, endurance, and recovery by maintaining altitude exposure for a significant period of time. A study using simulated altitude exposure for 18 days, yet training closer to sea-level, showed performance gains were still evident 15 days later.Opponents of altitude training argue that an athlete's red blood cell concentration returns to normal levels within days of returning to sea level and that it is impossible to train at the same intensity that one could at sea level, reducing the training effect and wasting training time due to altitude sickness. Altitude training can produce slow recovery due to the stress of hypoxia. Exposure to extreme hypoxia at altitudes above 16,000 feet (5,000 m) can lead to considerable deterioration of skeletal muscle tissue. Five weeks at this altitude leads to a loss of muscle volume of the order of 10–15%.