VO2 max is an estimate of how much oxygen your body can use at maximum effort, in millilitres per kilogram per minute. It is the single best summary of your cardiorespiratory fitness, and one of the strongest predictors of long-term health in the research literature.
Where the number comes from
NOYU does not calculate your VO2 max. Every value you see is your device's own estimate, passed through.
A true VO2 max needs a lab, a mask and a treadmill test to exhaustion. Watches estimate it from the relationship between your pace and your heart rate: if you can run faster at the same heart rate than you could last month, your fitness has probably improved.
That method works reasonably well in aggregate and poorly in specific cases. It needs sustained outdoor efforts with good GPS and good heart rate data. Which is why many devices report it sparsely, often only after a hard outdoor run, and why NOYU looks over a 30-day window and averages what it finds rather than reacting to single readings.
Why this one is not baseline-relative
Most day-to-day metrics in NOYU are compared with your own normal. VO2 max is a deliberate exception.
When NOYU weighs up your VO2 max, it reads it as a percentile against published norms for your age and sex, drawn from the Cooper and ACSM reference data.
The reasoning: unlike HRV, VO2 max is comparable between people once you account for age and sex, and the population data is good. Knowing you are around the 70th percentile for your age is real information. Knowing your VO2 max is 47 in isolation is not.
Two caveats about the reference tables. If NOYU does not know your age, it uses the 30 to 39 band. If it does not know your sex, it uses the male tables. Both are on your profile, and filling them in makes this number meaningful.
It should move slowly
VO2 max responds to months of training, not days. Meaningful change in a trained adult is on the order of a few points over a season.
If yours jumps several points in a week, your device has revised its estimate; your physiology doesn't move that fast. Common triggers are a change in route or terrain, a new device, a heart rate strap replacing wrist sensing, or the first hard outdoor run in a while.
Because it moves so slowly, we keep VO2 max out of the correlation tool. Two weeks is nowhere near long enough for it to respond to anything, so any pattern found would be noise.
What moves it
The evidence is consistent and unglamorous:
- A base of easy aerobic work. The largest share of the benefit comes from sustained, conversational-pace training.
- Some hard intervals. Work near your maximum is what shifts the ceiling, in much smaller doses than most people assume.
- Consistency over months. This is the one that decides it.
Age lowers it gradually and training offsets a good deal of that decline. Comparing yourself with your own trajectory over years is more useful than comparing yourself with anyone else.
If you want to work on it, the Increase Zone 2 cardio experiment is built around exactly this, and sets a target from your own recent volume rather than a generic prescription.
