Elevation training is the practice by some endurance athletes of training for several weeks at high elevation, preferably over 2,400 metres (8,000 ft) above sea level, though more commonly at intermediate altitudes due to the shortage of suitable high-elevation 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.
 
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