What a wrist can and cannot tell you about VO₂max
It is not a measurement. It is a regression, and knowing that changes how you should read it.
Your watch tells you your VO₂max is 52.3. The decimal place is doing a lot of work there, because nothing on your wrist has ever measured a millilitre of oxygen.
A real VO₂max test involves a mask, a metabolic cart, and a treadmill protocol that ends when you cannot continue. What your watch does instead is fit a model: at this pace, on this gradient, your heart rate settled here — and in the population the model was built on, people with that pace-to-heart-rate relationship had roughly this VO₂max. It is a regression from two things it can measure to one thing it cannot.
That is not a criticism. It is a useful regression. But it explains every strange behaviour you have ever seen from the number.
Why it moves when nothing has changed
The estimate depends on the pace–heart-rate relationship, so anything that disturbs either side moves it.
Heat is the big one. Cardiac drift in warm weather raises heart rate at the same pace, and the model reads that as a fitness loss. A run in 30 °C can knock a point or two off an estimate that will come straight back in October. The same happens with dehydration, poor sleep, a hard session two days earlier, or a chest strap swapped for a wrist sensor.
Terrain matters too. Most estimates need flat, steady running to work — enough minutes at a stable pace for heart rate to plateau. Feed the model an interval session, a trail run, or anything with stop-start traffic lights, and it either declines to update or updates on bad data.
Why the absolute number is not the point
Two watches on the same wrist will disagree by several points, because they use different models fitted on different populations. Comparing your 52 to a friend’s 55 on another brand tells you nothing at all.
What is worth something is your own series. The regression’s biases are mostly constant for a given person on a given device, so while the absolute level may be off by five points, the direction over eight weeks is real. If your estimate climbs three points across a block of consistent training, in similar conditions, that is a signal. If it moves two points between Tuesday and Thursday, that is weather.
The rule I use, and the one the app applies: never read a single value, always read a trend of at least four to six weeks, and discount anything that coincides with a heat wave or a cold.
The one thing it is genuinely good for
Detecting the loss you did not notice. Fitness declines slowly enough that perceived effort adapts with it — you feel roughly the same, just slower, and you attribute it to a bad day. A downward trend over six weeks, in comparable conditions, is the kind of thing a person will rationalise away and a series will not.
That is the honest scope of a wrist VO₂max: a slow-moving trend detector for your own aerobic fitness, on your own device. Not a number to compare, not a number to screenshot, and definitely not a number that deserves a decimal place.
Run Analytics computes it from what your watch already recorded and shows it as a series with the conditions attached, because the conditions are usually the explanation.