Athletes in endurance events practice a training technique called "living high, training low". Many years ago, scientists noticed that people who live in the mountains, where the air contains lower levels of oxygen, have higher than normal blood oxygen levels. A limiting factor in events that require endurance is the time it takes to move oxygen from the lungs into the muscles. Since more than 98 percent of the oxygen in the blood is bound to red blood cells, people with high numbers of red blood cells should have higher levels of oxygen and therefore have more oxygen available for their muscles, giving them greater endurance. It appears that living and training at high altitude would improve performance even more, so theoretically, all long distance runners, cross country skiers, bicycle racers and other athletes in endurance sports would benefit from living and training at high altitudes.

However, you can't train as intensely in the mountains where oxygen is sparse. Lack of oxygen during hard exercise slows you down. One group of researchers decided to see if living at high altitudes would increase red blood cell concentration, and training at low altitude would allow the athletes to take harder workouts. Eleven trained middle-distance runners were tested before an 18-day training session in which they slept in special low-oxygen pressure chambers and trained at sea level with oxygen-rich air (Journal of Applied Physiology, January 2006). The tests were repeated 15 days after the training. The athletes who lived high and trained low had higher maximal oxygen uptakes, higher maximal aerobic power and lower resting heart rates than the control group. The blood of these athletes could carry more oxygen, and the oxygen concentration in their bloodstream would return to normal earlier after intense competitions so their performance would improve.