Endurance athletes spend weeks living at elevation before major competitions. The intended effect is a specific physiological change, and the practical difficulty is a well-known trade-off.
Low pressure, not low oxygen concentration
The proportion of oxygen in air is essentially the same at altitude as at sea level. What falls is atmospheric pressure, and with it the pressure driving oxygen into the blood.
Blood leaving the lungs therefore carries less oxygen per unit of haemoglobin, and the body registers this shortfall through sensors in the kidney.
The response is increased release of a hormone that stimulates red blood cell production in bone marrow, raising the blood's oxygen-carrying capacity over subsequent weeks.
The adaptation takes time and is not permanent
Meaningful increases in red cell mass require sustained exposure, which is why camps run for several weeks rather than days.
The gain also fades after returning to sea level, over a timescale of weeks, which forces athletes to time descent relative to competition carefully.
There is individual variation in how strongly athletes respond, and some show little change despite identical exposure, which programmes discover through testing rather than assumption.
Training quality falls at elevation
The same reduced oxygen delivery that triggers adaptation also limits how hard an athlete can work, so high-intensity sessions are slower than at sea level.
An athlete who spends weeks training below their usual speeds risks losing the neuromuscular sharpness the intensity was maintaining.
This is the central problem altitude training has to solve, and it is why the naive approach of simply moving a squad to a mountain has fallen out of favour.
Live high, train low resolves the conflict
The common arrangement now is to live and sleep at elevation while descending for the hardest sessions, capturing the adaptation without sacrificing training speed.
Where geography does not permit daily descent, altitude tents and chambers are used to reproduce the exposure at home, though the hours achieved are usually fewer.
Programmes also monitor iron status closely, because producing additional red cells requires iron, and an athlete who is deficient will not respond however long the camp lasts.
Not every sport benefits equally
The advantage is largest for events limited by oxygen delivery, which means middle-distance and endurance disciplines rather than sprints or strength sports.
For very short events the reduced air density at altitude has a separate and unrelated effect, since less drag can make sprinting and throwing marginally faster.
Squads therefore make different decisions by discipline, and a national programme may run altitude camps for its distance runners while its sprinters remain at sea level throughout.
