Why altitude training works
Athletes from many sports have used altitude training to prepare for a big match or event, and not just when the event will be at a high altitude. They do this because the air is "thinner" at high altitudes meaning there are fewer oxygen molecules per volume of air. Every breath taken at a high altitude delivers less of what working muscles require.
While the effect is most dramatic at altitudes greater than 8,000 feet (2,438 meters) above sea level, it is noticeable even at 5,000 feet (1,524 meters) above sea level.
To compensate for the decrease in oxygen, one of the body's hormones, erythropoietin (EPO), triggers the production of more red blood cells to aid in oxygen delivery to the muscles.
You might have heard of EPO in news stories about performance-enhancing drugs. A synthetic version of EPO has been used by endurance athletes to mimic the body's natural process of red blood cell creation. So far, most sports organizations are more concerned with this artificial version rather than triggering it naturally up in the mountains.
By training at high altitudes, athletes aim to allow their bodies to produce extra red blood cells. Then, they head to a competition at lower elevations to take advantage of their changed physiology, which should last for 10 to 20 days.
While the benefits of altitude training have been demonstrated, specifics on how to best undertake it have remained elusive.