Altitude training traditionally called training at an altitude camp, or now commonly using altitude simulation tents or mask based hypoxicator systems is the practice by some endurance athletes of training at high altitude, usually over 2,500 m (8,000 ft) above sea level, for several weeks. At this altitude although the air still contains approximately 20.9% oxygen, the barometric pressure and thus the partial pressure of oxygen is reduced. The body adapts to the relative lack of oxygen by increasing the concentration of red blood cells and hemoglobin. Proponents claim that when such athletes return to sea level (where they are competing) they will still have a higher concentration of red blood cells for 10-14 days. 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.
A larger concentration of red blood cells allows more oxygen to be supplied to the muscles allowing higher performance. Increases in red blood cell mass are stimulated by an increase in EPO. The body naturally produces EPO to regulate red blood cell mass and should not be confused with synthetic EPO. Synthetic EPO injections and blood doping are illegal in athletic competition because they cause an increase in red blood cells beyond the individual athlete's natural limits. This increase, unlike the increase caused by altitude training, can be dangerous to an athlete's health as the blood may become too thick and cause heart failure. The natural secretion of EPO by the human kidneys can be increased by altitude training, but the body has limits on the amount of natural EPO that it will secrete, thus avoiding the harmful side effects of the illegal doping procedures.
Scientific studies have shown that altitude training can produce increases in speed, strength, endurance, and recovery. 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. With the advent of altitude simulation systems that are portable, however, none of these detrimental side effects should be a problem.
In Finland, a scientist named Heikki Rusko has designed a "high-altitude house". The air inside the house, which is situated at sea level, is at normal pressure but modified to a low concentration of oxygen, about 15.3% (below the 21% at sea level), the same concentration as that at the altitudes often used for altitude training. Athletes live and sleep inside the house but perform their training outside (at normal oxygen concentrations at 20.9%). Rusko's results show improvements of EPO and red-cell levels. His technology has been commercialized and is being used by thousands of competitive athletes in cycling, triathlon, olympic endurance sports, professional football, basketball, hockey, soccer, and many other sports that can take advantage of the improvements in strength, speed, endurance, and recovery.