The Apple Watch can estimate your VO₂ max, that is a good indicator of your overall fitness.
Apple Watch records your predicted VO₂ max when you do a rigorous Outdoor Walk or Outdoor Run in the Workout app for at least 20 minutes with a persistent heart rate measurement.
It does need to be a walk or a run, you need to use the Workout app with activated GPS, and you have to walk/run at a reasonable pace to provide enough elevated heart rate data for the prediction algorithm to work.
The Apple Watch writes the data in the Health Store. With this app you always see your latest value directly on your
wrist. This App supports all complications. So make your Apple watch a real Fitness Gadget!!
This app needs to connect with Apple’s Health app to displays the recorded data on a Watch App, a complication and your phone.
Attention: This app only works good if you have an Apple Watch Series 3 and 4.