Altitude training is the practice by some endurance athletes of training for several weeks at high altitude, preferably over above sea level, though more commonly at intermediate altitudes due to the shortage of suitable high-altitude locations. At intermediate altitudes, the air still contains approximately 20.9% oxygen, but the barometric pressure and thus the partial pressure of oxygen is reduced. Depending on the protocols used, the body may acclimate to the relative lack of oxygen in one or more ways such as increasing the mass of red blood cells and hemoglobin, or altering muscle metabolism. Proponents claim that when such athletes travel to competitions at lower altitudes they will still have a higher concentration of red blood cells for 10–14 days, and this gives them a competitive advantage. Some athletes live permanently at high altitude, only returning to sea level to compete, but their training may suffer due to less available oxygen for workouts. Altitude training can be simulated through use of an altitude simulation tent, altitude simulation room, or mask-based hypoxicator system where the barometric pressure is kept the same, but the oxygen content is reduced which also reduces the partial pressure of oxygen. Hypoventilation training, which consists of reducing the breathing frequency while exercising, can also mimic altitude training by significantly decreasing blood and muscle oxygenation. Background history The study of altitude training was heavily delved into during and after the 1968 Olympics, which took place in Mexico City, Mexico: elevation . It was during these Olympic Games that endurance events saw significant below-record finishes while anaerobic, sprint events broke all types of records. It was speculated prior to these events how the altitude might affect performances of these elite, world-class athletes and most of the conclusions drawn were equivalent to those hypothesized: that endurance events would suffer and that short events would not see significant negative changes. This was attributed not only to less resistance during movement—due to the less dense air—but also to the anaerobic nature of the sprint events. Ultimately, these games inspired investigations into altitude training from which unique training principles were developed with the aim of avoiding underperformance. Training regimens Athletes or individuals who wish to gain a competitive edge for endurance events can take advantage of exercising at high altitude. High altitude is typically defined as any elevation above . Live-high, train-low One suggestion for optimizing adaptations and maintaining performance is the live-high,