I've also been wondering this. Mine reports a number that's way below what I get from other trackers.
I suspect it might use the age and gender predicted max heart rate in the calculations instead of the values set by the user in the workout settings. My MHR has been roughly 25 beats above the average throughout my adult life. As far as I know, above average MHR is not an indicator of poor cardio fitness in it self (even though being fit affects heart rate response).
> In addition to chronotropic incompetence, other medical conditions can also decrease the accuracy of VO2
max estimates on Apple Watch ... medical
conditions that severely limit exercise tolerance, preventing patients from reaching heart rates close to
their predicted maximum heart rate
I suspect it might use the age and gender predicted max heart rate in the calculations instead of the values set by the user in the workout settings. My MHR has been roughly 25 beats above the average throughout my adult life. As far as I know, above average MHR is not an indicator of poor cardio fitness in it self (even though being fit affects heart rate response).
The discussion part of this paper lists some causes for incorrect VO2MAX prediction, and has a line that may support my thesis https://www.apple.com/healthcare/docs/site/Using_Apple_Watch...
> In addition to chronotropic incompetence, other medical conditions can also decrease the accuracy of VO2 max estimates on Apple Watch ... medical conditions that severely limit exercise tolerance, preventing patients from reaching heart rates close to their predicted maximum heart rate