During my recent uptick in running volume, I began following the changes in V02 Max as measured by my Apple Watch. While I did not set any formal running volume or pace goal, I aimed to generally improve my capacity — and ultimately, correlate how training associated with changes in VO2Max.

In a 4-month trading window, from April to September, my VO2MAX roughly improved by 4 units.

Apple iOS 26 Update Changed its Calculation

In research terms, we have a threat to internal validity on our hands!

What changed?

Instrumentation, or how Apple calculated VO2Max for Apple Watch Users. Some users saw a prompt to recalculate cardio fitness scores, some did not. I was in the latter group. Maybe it flashed across the screen, I cannot remember. Either way, the calculation changed, and there is no documentation on Apple on what it changed. We only see the output.

Like any other tech company, Apple’s calculation has its own secret sauce and is rather vague:

“Apple Watch Series 3 or later can record an estimate of your VO2 max using the heart and motion sensors during an Outdoor Walk, Outdoor Run, or Hiking workout in the Workout app” — Source: Apple

I readily downloaded the iOS 26 and related watchOS update on September 15th, 2025. My devices updated on a Monday, which left me with my regular running days on Tuesday and Thursday.

Now, before this update, I did not know the VO2Max was even affected until I looked at the data.

How I Noticed

After each run, I looked at the Cardio Fitness graph provided in the iPhone Health app. At least once per week, I then graphed and analyzed my training volume and VO2Max in my own Power BI file like so:

My data show a nice, gradual increase in VO2Max that correlates with my increased running volume (miles).

After the update, my first 4 running workouts shifted dramatically (about 6 units) and stayed steady around 49 mL/kg/min.

The finger-drawn purple line indicates when the update occurred. Runs before the update are left of the line. Runs after the update are right of the line.

Given my consistent running volume and pacing, I doubt that my VO2Max changed overnight, but instead, resulted from the iOS 26 update.

Other Users Noticed Too…and Research!

When researching the topic, I came across a thread on a MacRumors.com forum. Other users noticed changes in their VO2Max calculations after the update, and one even said she saw a ‘recalculate’ screen prompting a change in the math. Like I mentioned above, I did not catch this screen or remember seeing it.

One user on a reddit thread cited a validation study in May 2025 indicating that the old VO2Max algorithm underestimated the calculation by 6.07 units, which mapped onto the changes that I observed. The timing of this published article and the Apple iOS update aligns with the update. If new validation research found that the Apple Watch data were off compared to lab testing, then it makes sense that Apple would have tweaked its algorithm with the new update.

Take a look at the threads for different examples of Cardio Fitness data changing after the iOS26 update.

A few Notes about my devices:

• iPhone 11 — yes, it still works!
• Apple Watch Series 9 (GPS)