Altitude training

Why do athletes go and train where there is less oxygen to breathe?

An endurance athlete's whole problem is delivering oxygen to working muscle. So the decision to spend a month somewhere with less oxygen available looks like deliberately making the problem worse. Either the athletes are wrong, or the thing that improves is not the thing that gets harder. Which is it — and if it is the second, what exactly is being trained?

Start with what "less oxygen" means, because the usual phrasing is misleading. The air at 2,500 metres is still 21 percent oxygen. What has dropped is the total pressure, and with it the partial pressure of oxygen — the push driving oxygen across the lung membrane into blood. So arterial blood arrives at the muscle carrying less oxygen than it would at sea level, and the body notices.

Where would you put the sensor for that? The kidney, as it happens, which is unusually well placed to detect how much oxygen its blood is delivering. Sensing a shortfall, it secretes erythropoietin, which instructs the bone marrow to produce more red blood cells. Over weeks, total haemoglobin mass rises. And here is the point: haemoglobin mass is not altitude-specific. Bring that athlete back down and each litre of their blood now carries more oxygen than it did before, at sea level, where the partial pressure is high again.

So the deficit is the stimulus and the surplus is the payoff. Does that mean more altitude is straightforwardly better?

No — and the reason is the second half of the problem. Training is not just a stimulus you receive, it is work you perform. At altitude the athlete cannot sustain the same absolute pace. A session that would be run at race speed at sea level is run measurably slower for the same internal effort. Do that for four weeks and the haematology improves while the ability to hold race pace, and the neuromuscular qualities that go with it, quietly erode.

Now the two halves are in tension, and stating a tension usually suggests its resolution. The stimulus comes from being at altitude — from hours of exposure, most of them not spent training. The cost comes from training at altitude. Do those have to happen in the same place?

They do not, and that is the protocol: live high, train low. Sleep and spend the day at altitude for the hypoxic dose, descend for the hard sessions so they can be run at full speed. The controlled comparisons that separated living-high-training-high, living-high-training-low and living-low-training-low found the mixed group came out best, which is what the reasoning predicts.

Two practical constraints follow from the mechanism rather than from tradition. Red cells are built from iron, so an athlete with depleted iron stores will produce the hormone and not the cells. And marrow is slow: the exposure needs roughly three to four weeks at something like 2,000 to 2,500 metres to move haemoglobin mass appreciably. A long weekend on a mountain does nothing.

The mechanism above is well established. The size of the benefit is genuinely argued about, and it would be dishonest to present the practice as settled.

The central complication is that people respond very differently. Studies tracking individuals through identical camps find some athletes with a substantial rise in red cell mass and others with essentially none, and the split does not appear to be explained by fitness or effort. If a squad contains responders and non-responders, a group average understates the first and overstates the second, which is one reason published effects vary so much.

The second complication is that altitude camps are impossible to blind. An athlete who has flown somewhere striking, trained with focus, slept well and eaten carefully for four weeks will usually improve, and disentangling the hypoxia from the camp is very hard. Some analyses that attempt it find the specific altitude effect is modest — smaller than the practice's reputation.

There are also proposed non-haematological benefits: improved muscle buffering, better economy of movement, adaptations at the level of the muscle cell. These are plausible and much weaker in the evidence than the red-cell pathway. Treat them as open, not as an additional reason.

One clarification worth making explicitly, because it is often blurred. Acclimatising in order to compete at altitude is a different problem with a different answer: there you are managing an unavoidable deficit, not banking a surplus for later.

• How normobaric tents and simulated altitude compare with real elevation for the same daily exposure.
• Why the anti-doping rules treat raising red cell mass by hypoxia and by injection so differently.

Every term the collection defines is gathered in the glossary.