train-low principle. This training idea involves living at higher altitudes in order to experience the physiological adaptations that occur, such as increased erythropoietin (EPO) levels, increased red blood cell levels, and higher VO2 max, while maintaining the same exercise intensity during training at sea level. Due to the environmental differences at high altitude, it may be necessary to decrease the intensity of workouts. Studies examining the live-high, train-low theory have produced varied results, which may be dependent on a variety of factors such as individual variability, time spent at high altitude, and the type of training program. For example, it has been shown that athletes performing primarily anaerobic activity do not necessarily benefit from altitude training as they do not rely on oxygen to fuel their performances. A non-training elevation of and training at 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 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%. Live-high, train-high In the live-high, train-high regime, an athlete lives and trains at a desired altitude. The stimulus on the body is constant because the athlete is continuously in a hypoxic environment. Initially VO2 max drops considerably: by around 7% for every 1000 m above sea level. Athletes will no longer be able to metabolize as much oxygen as they would at sea level. Any given velocity must be performed at a higher relative intensity at altitude. Repeated sprints in hypoxia In repeated sprints in hypoxia (RSH), athletes run short sprints under 30 seconds