VO₂max: the number your watch guesses, and what it means

What the lab measures

VO₂max is the most oxygen your body can use in a minute, per kilogram of body weight — millilitres per kg per minute. The real test happens in a lab: you run or ride against a climbing load, wearing a mask, until you cannot continue, and a gas analyser reads where your oxygen uptake stops rising. One conversion is worth memorising: 3.5 mL/kg/min equals one MET, the oxygen cost of sitting still. A VO₂max of 35 means you can briefly work at ten times your resting rate.

What your watch actually does

Your watch has no gas analyser. It watches heart rate against pace (or power) during steady outdoor work and fits a line: the faster you go at a given heart rate, the higher the estimate. Validation studies typically find errors of 5–15% against a lab test. For a displayed reading of 45, that is a plausible band from about 38 to 52.

So be clear about what the number cannot tell you: your true lab value, and anything on a daily timescale. A one-point wobble is heat, hills, sleep or caffeine — and illness or medication that lowers heart rate, like a beta-blocker, makes the same pace read as more fitness than it is. A three-month drift in one direction is signal. Read the absolute value as a postcode and the trend as the address.

Where the bands sit

VO₂max falls with age — roughly 1% per year after 30, faster if you stop training. Large exercise-testing registries put the middle half of men in their 20s between about 38 and 53 mL/kg/min; by the 60s the same band is 23 to 34. Women's bands run roughly 15–20% lower — mostly body composition and oxygen-carrying capacity. Note what the bands do not say: they describe the population, not your ceiling. A trained 55-year-old and an average 25-year-old can carry the same number.

Why the number earns attention

Cardiorespiratory fitness is one of the strongest single predictors of how long you live. A 2009 meta-analysis of 33 cohorts put each extra MET — 3.5 mL/kg/min — at about 13% lower all-cause mortality. A 2018 Cleveland Clinic cohort of over 120,000 exercise-tested patients found no upper limit: the fittest group kept gaining. Those studies used lab tests, not watches. The guess on your wrist inherits the direction of that evidence, not its precision.

What actually moves it

Mostly easy volume, some hard work. Analyses of trained endurance athletes keep landing on roughly 80% low intensity, 20% hard — the pattern Seiler documented. Untrained people commonly gain 10–15% in 8–12 weeks of consistent aerobic work; the well-trained fight for 1–3% a season. And because the unit is per kilogram, losing fat raises the number without the engine changing at all. The watch will congratulate you either way.