FortiorCardio
What is VO2 max?
VO2 max is one way to describe aerobic fitness: the most oxygen your body can use at full effort. Fortior shows an estimate together with the method and date behind it, because a watch or a running pace can only approximate a lab test.
What VO2 max is
VO2 max is the highest rate at which your body can take in and use oxygen during hard exercise. It is given in millilitres of oxygen per kilogram of bodyweight per minute (mL/kg/min). A lab measures it with a mask during a graded test to exhaustion. Most people instead see a VO2 max estimate from a watch or an app.
A higher number is associated with better endurance performance, but it is only one piece of the picture. How efficiently you run and how long you can hold a pace matter too, which is why it is best read next to your pace and endurance over similar stretches.
Where Fortior's VO2 max estimate comes from
Fortior uses the most trusted of three methods that has data, and always labels which one you are looking at:
| Method | What it uses | When Fortior uses it |
|---|---|---|
| Apple Watch estimate | The VO2 max value in Apple Health, which an Apple Watch estimates | Whenever Apple Health has one. A reading from the last year takes priority over Fortior's own estimates. |
| Running-performance estimate | Your fastest run of at least 3 km in the last six weeks, converted with a running-performance formula based on Jack Daniels' VDOT work | When there is no recent watch value |
| Submaximal estimate | Pace and heart rate from your recent steady sessions | A coarse fallback with the widest margin, used only when neither method above applies |
Each estimate keeps its method and date. The method matters: a watch estimate and a run-based estimate can disagree because they use different inputs, so the number can move when Fortior switches methods even if your fitness hasn't changed.
A worked example
For the running-performance method, Fortior treats your fastest recent run as an all-out effort. Take a 5 km run in 25 minutes. The pace is 5,000 m ÷ 25 min = 200 m/min. The formula first estimates the oxygen cost of that pace: −4.60 + 0.182258 × 200 + 0.000104 × 200² = 36.0 mL/kg/min. It then divides by the share of VO2 max a runner can hold for 25 minutes, about 94%, which gives 38.3 mL/kg/min.
| 5 km time | Estimated VO2 max |
|---|---|
| 22:00 | 44.5 mL/kg/min |
| 25:00 | 38.3 mL/kg/min |
| 28:00 | 33.5 mL/kg/min |
Behind the scenes Fortior treats this method as good to about 4% either way, so the 25-minute run is roughly 36.8 to 39.8. The bigger caveat is that Fortior can't know whether your fastest run was a full effort. If your last six weeks were all easy runs, the estimate will read low.
Why it's an estimate
- A lab test measures oxygen directly. Watches and apps infer it from heart rate, pace or power.
- Heat, hills, wind, stops and an off day change a run's pace without changing your VO2 max.
- Different methods use different inputs, so they can give different answers on the same day.
- A higher or lower number alone doesn't explain why your performance changed.
Using the estimate
- Compare like with like. The chart legend names each method, and a trend within one method is the cleanest to read.
- Judge by weeks, not days. Small moves are noise. Look at the direction over several weeks.
- Read it alongside other signals. Pace, heart rate drift on long runs and how sessions feel give the number context.
- Give the running method something real to work with. A recent race or hard time trial of 3 km or more gives it a true effort to convert.
The Aerobic Fitness attribute in Progress uses the same estimate on one common scale, with its measurement method and date shown.
In the app: open Progress, choose Cardio, and find the VO2 max card to see each estimate, its method and the trend. To see it as an attribute, open Progress, choose Attributes, then Aerobic Fitness. The estimate needs heart rate or run data, and some views are part of Fortior Premium. Plans and prices appear in the app.
VO2 max values are estimates, not lab results or medical measurements, and Fortior is not medical advice.