VO2

This information comes from:

https://www.cultofmac.com/610268/vo2-max-apple-watch-aerobic-fitness/

Apple Watch may log your VO2 max, but you can’t currently check it on the watch itself. To see your stats, you must use the Health app on your iPhone. Go to Health Data > Activity and scroll down to find it. If you have readings for the current month, they will be highlighted in a big orange box. If you have no VO2 max data, it will be languishing at the bottom of the list, under No Recorded Data.

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 two workout types can produce an estimated VO2 max on Apple Watch: Outdoor Walk and Outdoor Run.

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.

The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Make sure the your personal information is up to date in the health App.

Height, weight, age, sex

Then Reset your fitness calibration data 

From your iPhone>Watch>Privacy>Reset Fitness Calibration Data>Tap this

You will just get a confirmation box that tells you that your metrics will not be as good if you do this.

Tap confirm and do it. You will get nothing else, no confirmation but it is done.

Then go for a 20 minute outdoor run or walk using the workout app at your normal speed on a flat road/path etc

This will re-calibrate the watch.

The next time after this workout (not this time) it should record correctly.

Start Workout on Apple Watch.

Start a Workout using the native Apple Workout app (3rd party apps like the Nike Run club app or Strava won’t calculate V02 max for you)

Choose the Right Workout Type. You will have to choose from the following Workouts: Outdoor Run, Outdoor Walk, and Hiking (you’ll notice these are the workouts which use outdoor GPS… this means Indoor Run won’t work)

Workout for 20 Minutes or More. Do the above workouts for at least 20 minutes straight without pause

Check the Health App. After you end the Workout open the Health app and you should see VO2 max as a new data point

Use Apple Watch Heart Rate Monitor. 3rd party heart rate monitors might affect the reading (some people have reported not getting the measurement with their chest strap heart rate readers so only use the Apple Watch)

You won’t see Cardio Fitness level data points for every workout recorded with Apple Watch, here’s what Apple says:

Apple Watch can record an estimated VO2 between 14 and 60 mL/kg/min when you do a brisk hike, walk, or run outdoors with persistent heart rate measurement. It can estimate your VO2 max if you start one of those workouts in the Workouts app

VO2 max is validated for users 20 and older. Most people can improve their VO2 max with more intense and more frequent cardiovascular exercise. Certain conditions or medications that limit your heart rate may cause an overestimation of your VO2 max. You can indicate you’re taking certain medications in the Health Details.

You can read more specifics about Cardio Fitness in the Cardio Fitness section in the Health app.

Something important to keep in mind: this means you won’t really see Cardio Fitness data measured with Apple Fitness+ workouts (unless Apple expands which workout types record VO2 max).