Why do endurance athletes train high in the mountains?

Endurance athletes train at altitude so their bodies build more oxygen-carrying capacity. Mountain air still contains about 21% oxygen, but at lower pressure, so each breath delivers less of it. The body responds: the kidneys release a hormone called erythropoietin, or EPO, which tells the bone marrow to make more red blood cells. Back at sea level, proponents say, athletes keep that extra red blood cell boost for 10 to 14 days, just in time to race.

The idea took off after the 1968 Olympics in Mexico City, 2,240 metres above sea level. There, endurance events came in well below record pace, while sprinters broke records in the thin air, which offers less resistance. Coaches and scientists started asking how to turn altitude from a handicap into an advantage.

The catch is that hard training is harder up high. When athletes live and train at altitude, their maximum oxygen uptake first drops by around 7% for every 1,000 metres. Hence the popular recipe live high, train low: sleep at 2,100 to 2,500 metres to trigger the adaptations, and train at 1,250 metres or lower to keep workouts intense. A Finnish scientist, Heikki Rusko, even built a high-altitude house at sea level, with air containing only about 15% oxygen.

It doesn't work equally for everyone. Some athletes respond strongly, others hardly at all, and researchers still argue about whether the gains come mainly from extra red blood cells or from muscles using oxygen more economically.