VO2max: what it tells you, and what it doesn't

0:00 · The number everyone quotes

Your watch has an opinion about your VO2max. In a laboratory that number is measured with a mask over your face, and it is a real physiological measurement. What it tells you about Saturday's race is a different question, and that is the one this video is about.

0:20 · What it actually measures

VO2max is the highest rate at which your body can take in oxygen, deliver it to the muscles, and use it. It is the top of the aerobic engine, and everything else you do in a race has to be paid from under it. The number is written in millilitres of oxygen, per kilogram of body weight, per minute. That division by body weight is the useful part: it makes a seventy-kilogram runner and a ninety-kilogram one comparable, because both bodies absorb oxygen but they also have different amounts of tissue to move. The clearest place it earns its keep is on a climb, where carrying an extra ten kilos up an alpine col costs real power.

1:08 · Same number, different race

Here is the sentence worth repeating: it is an association with performance, and not performance itself. Two runners can share a VO2max and finish minutes apart, because a second number is quietly doing work the first one cannot see. That second number is running economy, which is how much oxygen a given pace actually costs you. The pace you can hold depends on your ceiling, on how efficiently you turn oxygen into motion, on how long you can hold a fraction of that ceiling, and on the day itself. Your watch answers one of those four questions.

1:51 · It is not your threshold

There is one more thing the number is not, and this one gets confused constantly. VO2max is the ceiling, while your lactate threshold is the fraction of that ceiling you can hold before fatigue starts to build sharply. The threshold is what actually decides your race pace, and two runners with the same ceiling can hold very different fractions of it. That is a whole piece in itself, and we have one in the queue.

2:20 · Three routes to a value

There are three routes to a VO2max value, and they arrive at the same units with very different meanings. The reference version is a laboratory test, on a treadmill or a bike, taken to voluntary exhaustion while a mask analyses every exhalation. That has been the standard for roughly a century. The very first values were measured in 1923, on an eighty-four metre grass track, into a canvas Douglas bag that collected the exhaled air for later analysis. Modern equipment reads breath by breath instead of emptying a bag, but it answers the same question. The second route is a submaximal field test, which skips the exhaustion by measuring your heart rate at a known power or pace and working backwards from that pair, and sport physiology has used it since the 1950s. The third route is the one on your wrist.

3:20 · What your watch really knows

Your watch does not measure oxygen. After a run where your heart rate sits above about seventy percent of its estimated maximum for at least ten minutes, it combines heart rate, pace and the profile you typed in. It fills in the gaps from who you told it you are. Fourteen studies pooled by an international consortium checked those estimates against a real laboratory test. On average the exercise-based algorithms barely miss, while the resting-based ones sit about two millilitres per kilo per minute too high. The catch is not the average, it is the range around it: even the good algorithm is nearly twenty millilitres per kilo per minute wide on one person. Those reviewers concluded that such estimates suit population-level assessment, and not individual clinical assessment. For a study of a thousand people the errors average out, and for your own reading they do not.

4:26 · Can you raise it?

So, can you actually improve your VO2max? Yes, and the honest version has a caveat. A scoping review published in 2026 maps six hundred and seventeen studies of aerobic training. That is the good news, because a lot of people have looked. The less good news is that the map is very lopsided. Two in every three studies that reported sex enrolled only men or a male-biased mix, and only about one in seven was female only. Most interventions ran three months or less, and only nine studies in the whole sample followed anyone for a year or more. Training does raise it, and higher intensities raise it faster for the time invested. How much you can raise it depends on how much headroom you started with, because beginners see the biggest jumps.

5:24 · Whose number is this?

So whose number is this really? Both the elite runner and the recreational one, but for different reasons, and it is worth knowing which one applies to you. At the sharp end of the sport it is a physiological ceiling that matters because everything else has to be paid from under it, and small differences at that level track with real differences in race outcomes. For a recreational runner or cyclist it is a fitness indicator: where you sit against the population, whether the last block moved you, and what the trend looks like as you age. That is a legitimate use of the number, and a watch estimate is a reasonable tool for it. What the same number is not built for is telling you your goal race pace.

6:14 · What age does to it

VO2max drops as you get older, in a pattern that is remarkably regular across large populations and much more variable in any one person. The most-cited reference set in the world was built from nearly eight thousand maximal treadmill tests across eight American laboratories. Median values for men fall from forty-eight in the twenties to twenty-four in the seventies, and for women from thirty-eight to eighteen across the same span. That works out at roughly ten percent per decade in both sexes.

6:52 · Bending the line

People who keep training appear to bend that line rather than escape it. A study published in 2026 looked at three hundred and forty lifelong marathoners from twenty-four countries, with an average of a hundred and twenty-one marathons behind them and an average age of fifty-two. They sat well above the reference median in every age band, and the drop from decade to decade was more gradual. Read that as a shape rather than as tabled values, because those numbers are wearable-estimated in a very self-selected group. A single reading, yours or your neighbour's, sits somewhere on a wide distribution around the average, so a number above or below the median at your age is information rather than a judgement.

7:43 · Which method for which question

Most people asking about their VO2max are really asking one of three different questions. If you want to know where you sit against the population, the number your watch already produces is fine. If you want to know whether the last training block moved you, repeat the same field test on the same course under the same conditions. If you want your ceiling measured to the decimal, you want the laboratory. That middle option is under-used, because it is free, it is repeatable, and a consistent baseline tells you more than the absolute number does.

8:23 · The mortality tail

You may have read that VO2max predicts how long you live. A meta-analysis brings together thirty-three cohort studies, covering about a hundred and three thousand healthy adults. It found that each extra MET of aerobic capacity travelled with a thirteen percent lower rate of death from any cause. A MET is a metabolic equivalent: one MET is three and a half millilitres per kilo per minute. Two footnotes keep that number honest. The first is that these are observational cohorts and not trials, and fitter people are also on average thinner and less likely to smoke. The second is that what holds across a whole population does not tell you what happens when one person changes their own number. None of that makes the association a mirage, because higher aerobic fitness is one of the best corroborated markers of long-term health we have.

9:25 · What to remember

So the number in your watch is a lens on your aerobic engine. It is not a photograph of your race day, and it is not a promise about your ninetieth birthday. Both of those depend on what you do with it. The whole piece, with every source, is at enduranceproof.com.