Apple says;
Your Apple Watch gives you a cardio fitness ESTIMATE by measuring how hard your heart is working during an outdoor walk, run or hike.
To estimate your cardio fitness level, your Apple Watch also takes into account your:
Age
Sex
Weight
Height
Medications that may affect your heart rate
It can take at least 24 hours wearing your Apple Watch, followed by several workouts and passive measurements with your Apple Watch before you receive an initial ESTIMATE.
Certain conditions or medications that limit your heart rate may cause an overestimation of your VO2 max.
Are VO2 max predictions accurate?
The relationship between heart rate and VO2 max is not precise, and it varies from one individual to another. So Apple’s predicted VO2 max may not be very accurate. The estimate is also dependent upon your Apple Watch’s heart rate sensor. And while that’s pretty good, it is not infallible.
Why is there no VO2 max data in the Health app?
If you can’t find any VO2 max readings in the Health app, that is probably because you have not yet logged the right kind of workouts with your watch. Only three workout types can produce an estimated VO2 max on Apple Watch: Outdoor Walk,Outdoor Run and hiking.
VO2 max relates to maximal exertion. So you’ll need to log a pretty intensive workout to generate a reading. And you’ll need to sustain it for at least 20 minutes.
