Cardio fitness is the most physiologically meaningful number your Apple Watch produces, and the one most people either ignore or over-read. It is worth ten minutes to understand what it is and is not.
What VO₂max actually is
The maximum rate at which your body can take in, transport and use oxygen during exercise. It is the best single predictor of endurance performance available, and it correlates strongly with long-term health outcomes, which is why it has become the fashionable metric it now is.
Measuring it properly means a lab, a mask, and running to exhaustion. Everything a consumer device gives you is an estimate.
How the watch estimates it
By watching the relationship between how fast you are moving and how hard your heart is working. Fitter people move faster at a given heart rate, and the gap between those two numbers is what the estimate is built on.
That requires knowing your pace, which requires GPS, which is why only outdoor walking, running and hiking produce readings. It also requires your personal details, because body mass sits directly in the arithmetic. If your weight in Health is five kilos out of date, so is your cardio fitness.
How far to trust it
The honest position is the same as for wrist HRV: good for your own trend, poor for comparison. The error on an individual estimate is large enough that comparing your 44 with a friend’s 47 is meaningless, while your own move from 44 to 48 over six months is real information.
Things that produce a bad reading without any change in fitness: heat, hills, illness, dehydration, a loose watch, and stopping at road junctions. Medication that blunts heart rate, beta blockers in particular, breaks the relationship the estimate depends on entirely.
Reading the classification
Apple sorts your figure into Low, Below Average, Above Average and High against population norms for your age and sex. That banding is more useful than the number, because it handles the fact that VO₂max declines with age and differs by sex, and it saves you looking up a table.
Worth knowing that Low carries genuine health meaning rather than being a fitness insult. Low cardiovascular fitness is among the stronger modifiable risk factors there is, and a Low classification that persists is worth acting on.
What actually moves it
Aerobic volume at easy intensity. The bulk of the adaptation, and the part most people skip because it feels too easy. Getting that zone right is zone 2 heart rate, and why yours is probably wrong.
A smaller amount of genuinely hard work. Intervals near maximum are the most potent single stimulus, in small doses.
Time. Months. And it will move in steps rather than smoothly, because it only updates when you give it a qualifying effort.
The full protocol is in how to increase your VO₂max, and you can sanity-check the number against a field test with the VO₂max calculator.
If the number is not updating at all, that is a separate and very common problem: Apple Watch cardio fitness not updating.