Last night I got no VO2max reading at all, although that could be because I had to undo and retighten the strap (Trail Loop on U2) after the downpour. I am really getting tired of having to stuff my shoes full of paper towels every night; can't remember the last time I had a dry run. Other than that it's not so bad as, besides than the shoes, I don't usually get that much wetter than just from sweat.

 

I’m waiting for 26.1 to be released. I’ve had the beta on my iPad for a couple months (and now 26.0) because it doesn’t really matter if things are janky, but I’m holding off on my main gadgets.

 

Still at the elevated level last night (in the dry!), so it wasn't just a one-off thing.

Looking back at older data, there was a significant (but less in absolute terms) drop a couple of months ago, and if the trend prior to that had continued without that drop the current numbers would be more or less on that curve, so they aren't necessarily completely impossible. But, while I'm not "unhappy" with the levels it's reporting now, it's not possible to say if the levels reported before or after the jump are more correct.

 

You haven’t started hanging out with Lance Armstrong lately, have you?

 

VO2max still at the elevated levels, although it has dropped ~ -0.5 mL/(kg·min) since the jump. (Still raining, too.)

But, apparently, this is pretty common and lots of people are reporting significant increases And, someone over at MacRumors pointed to this bit from Apple,

If the Health app detects that your previous cardio fitness estimates could be more accurate, you'll have the option to update your data. In Cardio Fitness, tap Update under Update Your Data. After you update, the Cardio Fitness Data Updated notification shows you the results of the update and how your estimates changed.

— Track your cardio fitness levels - Apple Support

And I did have the option to update that I either hadn't noticed before or just recently showed up, so I "updated" my VO2max history and, at least according to the new algorithm it's been at this "new" level for quite a while.

 

Actually, looking at the "new" values in more detail, while last night's values stayed the same after the update, the previous "elevated" values went down a bit, so that what I originally reported as a change of -0.5 now looks like an increase of +1.0. And, interestingly, the missing value from 9/18, after the recalculation, now has a value, as do several other past workouts that had no VO2max value calculated at the time.

And it's also nice to see that Tempo picks up these new values when I view the workouts in that app. (Indicating that it doesn't store them but reads them out of Health data each time you view one.)

So, apparently, you can increase your fitness just by installing watchOS 26. (Or iOS 26, or both.)

 

So — it updated the old data?

I haven’t paid much attention to my VO2Max because I’ve never had a lab test done to get a good baseline.

 

Yes, it updated all the historical "Cardio Fitness"/VO2max values

The latest studies I've seen indicate that Apple's VO2max calculation tends to be slightly above laboratory tests. However, other research shows that repeated laboratory tests tend to be slightly higher than a one-off test, indicating that there is likely a "training effect" involved that improves performance in repeats of those tests. Bottom line is that Apple's calculation is pretty accurate. But, even if it is off by some amount, steady changes in it should still reflect real changes in oxygen uptake and utilization.