How it works VO2 max
VO2 max
The most oxygen your body can use during exercise, and the single best marker of your endurance. Stamio recalculates it after every run.
What Stamio measures
- Your heart rate, second by second, from the wrist or a chest strap.
- Your speed and elevation, from your watch's GPS.
- Your resting HR (from your nights) and your max HR, which tracks your peak efforts.
How it's calculated
- 1
What your pace costs
Running at a given speed, on the flat or uphill, costs a known amount of oxygen. That's the basis of the metabolic equations used in exercise physiology.
- 2
How hard your heart is working
Your heart rate, as a share of your heart rate reserve (between resting HR and max HR), shows how much of your capacity you're using at that moment.
- 3
Putting the two together
The faster you run at the same share of your heart rate reserve, the higher your VO2 max. Stamio only keeps clean stretches: steady pace, a fairly sustained effort, minus the first minute and any stops.
- 4
A value that moves gradually
Each run gives an estimate that nudges your VO2 max, never all at once: a steady, sustained workout weighs more than a very slow easy run.
When it changes
- After every outdoor GPS run, recorded by Stamio or by the Workout app.
- A run that's fast for your heart rate raises it; one that's slow for your heart rate (fatigue, heat) lowers it slightly.
- Treadmill runs, runs without heart rate and runs that are too short count toward your load and your plan, but not your VO2 max.
- If you run slowly with walk breaks, Stamio still learns from it, using your steadiest running stretches.
What can throw it off
- A wrist sensor that's poorly placed or too loose (heart rate too high or dropping out): you can exclude the run, and your VO2 max is recalculated without it.
- A wrong max HR: by default it starts at 220 − age, then follows your peaks. If you know yours (from a stress test), enter it.
- Poor GPS (tall buildings, dense forest, tunnels): sketchy stretches are filtered out, though not always all of them.
- Medication that slows your heart: the link between heart rate and effort changes, and so do the values.
What you can set
- Your max HR and resting HR (Settings › Heart rate and zones).
- A known VO2 max, from another watch or a test: it becomes your starting point from that day on, and your past trend is kept. By default there's nothing to enter: it's learned from your runs.
- Exclude a run with bad heart rate data (run details).
How it was checked
Calibrated, then checked on 109 runs, day by day, against a high-end GPS watch worn at the same time: an average difference of 0.28.
Just one runner so far: me. The beta will tell the rest.
Questions
Why doesn't it match the Health app?
Stamio has its own engine, built for running, and never reuses the Health app's VO2 max. That one is estimated differently, from walking and running, and it often underestimates runners: 6 points below a lab test on average in an independent study (PLoS ONE, 2025). Stamio calculates its own from your runs, using your heart rate reserve and max HR, and never mixes the two.
My VO2 max dropped after an easy run. Am I losing fitness?
No. An easy run in serious heat, on tired legs, or with a sketchy sensor can pull it down a little. What matters is the trend over several weeks, and your training status keeps an eye on that for you.
How long until my first value?
One or two outdoor runs, at a steady pace for at least a few minutes. If you were already wearing your Apple Watch, Stamio reads your past runs and your value is there from the first calculation.
What about wheelchair, cycling or swimming?
Stamio only calculates VO2 max for running, where the method is reliable. In wheelchair mode it's hidden, so you never get a wrong value.
Further reading
- American College of Sports Medicine, ACSM's Guidelines for Exercise Testing and Prescription: metabolic equations for running and walking.
- Lambe R., O'Grady B., Baldwin M., Doherty C. (2025), "Investigating the accuracy of Apple Watch VO2 max measurements: A validation study", PLoS ONE, 20(5), e0323741.
- Swain D. P., Leutholtz B. C. (1997), "Heart rate reserve is equivalent to %VO2 reserve, not to %VO2max", Medicine & Science in Sports & Exercise, 29(3), 410-414.
An estimate for wellness and training, not a lab measurement. It can vary with your watch and how you wear it.