Altitude training is the practice by some endurance athletes of training for several weeks at high altitude, preferably over 2,400 metres (8,000 ft) 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 very much on the protocols used, the body may adapt 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.
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 2,240 metres (7,349 ft). It was during these Olympic Games that endurance events saw significant below-record finishes and 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.
Live high, train low 
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 1,500 metres (5,000 ft).
One suggestion for optimizing adaptations and maintaining performance is the live-high, 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.
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.
Artificial altitude 
Altitude simulation systems have enabled protocols that do not suffer from the tension between better altitude physiology and more intense workouts. Such simulated altitude systems can be utilized closer to competition if necessary.
For example, 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 have a low concentration of oxygen, about 15.3% (below the 20.9% at sea level), which is roughly equivalent to the amount of oxygen available at the high altitudes often used for altitude training due to the reduced partial pressure of oxygen at altitude. 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.
Principles and mechanisms 
Altitude training works because of the difference in atmospheric pressure between sea level and high altitude. At sea level, air is denser and there are more molecules of gas per liter of air. Because atmospheric pressure is lower at high altitudes, air is less dense and there are fewer molecules of gas per liter of air; this causes a decrease in partial pressures of gases in the body, which elicits a variety of physiological changes in the body that occur at high altitude.
The physiological adaptation that is mainly responsible for the performance gains achieved from altitude training, is a subject of discussion among researchers. Some, including American researchers Ben Levine and Jim Stray-Gundersen, claim it is primarily the increased red blood cell volume. Others, including Australian researcher Chris Gore, and New Zealand researcher Will Hopkins, dispute this and instead claim the gains are primarily a result of other adaptions such as a switch to a more economic mode of oxygen utilization.
Increased red blood cell volume 
At high altitudes, there is a decrease in oxygen hemoglobin saturation. In order to compensate for this, erythropoietin (EPO), a hormone secreted by the kidneys, stimulates red blood cell production from bone marrow in order to increase hemoglobin saturation and oxygen delivery. It is uncertain how long this adaptation takes because various studies have found different conclusions based on the amount of time spent at high altitudes.
While EPO occurs naturally in the body, it is also made synthetically to help treat patients suffering from kidney failure and to treat patients during chemotherapy. Over the past thirty years, EPO has become frequently abused by competitive athletes through blood doping and injections in order to gain advantages in endurance events. Abuse of EPO, however, increases RBC counts beyond normal levels (polycythemia) and increases the viscosity of blood, possibly leading to hypertension and increasing the likelihood of a blood clot, heart attack or stroke. 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.
Other mechanisms 
Other mechanisms have been proposed to explain the utility of altitude training. Not all studies show a statistically significant increase in red blood cells from altitude training. One study explained the success by increasing the intensity of the training (due to increased heart and respiration rate). This improved training resulted in effects that lasted more than 15 days after return to sea level.
Another set of researchers claim that altitude training stimulates a more efficient use of oxygen by the muscles. This efficiency can arise from numerous other responses to altitude training, including angiogenesis, glucose transport, glycolysis, and pH regulation, each of which may partially explain improved endurance performance independent of a greater number of red blood cells. Furthermore, exercising at high altitude has been shown to cause muscular adjustments of selected gene transcripts, and improvement of mitochondrial properties in skeletal muscle.
In a study comparing rats active at high altitude versus rats active at sea level, with two sedentary control groups, it was observed that muscle fiber types changed according to homeostatic challenges which led to an increased metabolic efficiency during the beta oxidative cycle and citric acid cycle, showing an increased utilization of ATP for aerobic performance.
See also 
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