What is the formula used by the Activity app to calculate VO2 max?

Thanks for the information but there still must be a formula that is used to arrive at the estimate. If it is based on "changes in your heart rate" then how is this data converted into a percentage based on mL/kg/min (where the percentage is expressed in mL)? I understand the limitations of estimating VO2 max without measuring air flow. However I am trying to better understand what the watch is telling me via this estimate. So I'm looking for the specific formula used to arrive at the estimate. Does anyone know what it is?

If I calculate VO2 max using the first formula I get values which are higher than what my Apple watch is giving me. E.g., for today's exercise your formula gives me a VO2 max of 37.88 (15 x (149/59)). The activity app gave the same workout a value of 34.5. On the other hand the HRmax formula gives me a HR max of 157.85 which is higher than that measured by the watch (i.e., 149). So whatever the best formula is for either of these values, it doesn't appear that these are what Apple watch is using. Moreover your VO2 formula gives me a value which elevates my VO2 outside of the range for my age group! I guess that's not a bad thing but who do I believe? I know the only way to settle this is to get on a machine and measure it properly and then compare the values obtained. Maybe one day I'll do that. Thanks for your help.