Open the Fitness app on your iPhone. Tap Cardio Fitness. Scroll back six months.
For a lot of Apple Watch users, the chart tells a strange story. A number that barely moves. 41, 42, 41, 42, 43, 42. Week after week, month after month, despite what feels like meaningful improvement in training.
This is one of the most quietly frustrating signals in the entire wearable ecosystem. You’re doing the work. Your runs feel better. Your recovery between intervals is faster. You can hold a pace today that would have destroyed you in January. And the number on the watch sits there like it’s painted on.
Here’s why.
How Apple Calculates VO2 Max
Apple’s VO2 Max estimate is based on a specific formula that relies on two inputs. Heart rate response during outdoor walking or running. And GPS-derived pace.
That’s it. Two variables. Apple takes your heart rate at a given pace, compares it to age and sex adjusted expected values, and produces a single estimate. Rolling average over the past several workouts. Very conservative in how it updates.
The data source matters. Apple only uses workouts where you are outside, using GPS, moving at a walking or running pace that is sustained for long enough to produce a stable heart rate signal. Short runs don’t count. Treadmill runs don’t count. Interval sessions where your pace is constantly changing don’t count. Any workout tagged as cycling, swimming, weight training, or HIIT contributes nothing.
If your training is mostly indoor, mostly short, or mostly not running, you have effectively zero inputs to the algorithm. The number you see is months out of date.
The Ceiling Effect
Even when you do provide clean inputs, there’s a second problem. Apple’s algorithm has a documented conservative bias at higher fitness levels.
For trained athletes with VO2 Max estimates above 45, the algorithm becomes increasingly resistant to upward revision. A 10 percent improvement in actual fitness might show up as a 1 or 2 percent change in the estimate. Sometimes no change at all for months.
This is not a bug. It’s a deliberate choice by Apple’s algorithm designers to avoid false positives. The cost is that for anyone serious about tracking real fitness progression, the VO2 Max on an Apple Watch becomes directionally useless.
I’ve seen athletes add 20 watts to their FTP, drop 90 seconds off their 5km time, and double their weekly mileage, only to watch their Apple Watch VO2 Max number move from 46 to 47.
The fitness improved dramatically. The number on the watch did not.
What Actually Moves the Number
If you want to see your Apple Watch VO2 Max actually update, here’s what works and what doesn’t.
What works. Outdoor runs longer than 20 minutes at a steady sustained effort. Tempo runs at the high end of your aerobic range. Hilly runs where sustained heart rate is elevated. Walking workouts over 30 minutes at brisk pace for people starting from lower fitness.
What does not work. Indoor treadmill runs, even if you manually tag them. Interval training of any kind. Cycling, even outdoor cycling. Strength training. HIIT classes. Short runs under 15 minutes. Any run where you stop and start frequently.
If you’re training for Hyrox, marathon, or any hybrid endurance discipline, a huge portion of your weekly training either doesn’t count or actively suppresses the number. A 90 minute long run with a lunch break in the middle produces less signal than a flat 30 minute tempo.
The algorithm wasn’t designed for you. It was designed for the average Apple Watch owner who walks their dog and occasionally goes for a jog.
Why This Matters Beyond the Vanity
VO2 Max matters for two reasons that have nothing to do with bragging rights.
First, trend direction. A rising VO2 Max suggests improving cardiovascular fitness. A falling one suggests the opposite. Individual data points are noisy. The trend is the signal. When your watch shows no trend in either direction for six months, you lose access to one of the clearer indicators of training effect you have available.
Second, training zone derivation. Many wearable platforms use VO2 Max as an input to calculate heart rate training zones, estimate lactate threshold, and model training load. If your VO2 Max is stuck at a number from last year, every downstream zone calculation is built on a wrong foundation.
You end up training in what your watch thinks is Zone 3 when you’re actually in Zone 4. Accumulating more anaerobic load than you think you are. Wondering why your recovery feels worse than the training looks.
How to Actually Track Fitness Progression
If your Apple Watch VO2 Max is stuck, here are three signals that work better for tracking real fitness change.
Resting heart rate trend. Your RHR responds to aerobic fitness changes within weeks, not months. A dropping RHR over 4 to 8 weeks is one of the clearest fitness improvement signals your watch can give you. Apple tracks this. Most people never look.
Submaximal heart rate at a reference pace. Pick a pace you can hold for 20 minutes. Run it every two weeks on the same route in similar conditions. Track your average heart rate. If you’re getting fitter, your heart rate at that pace will drop by 3 to 8 beats per minute over a training block.
Heart rate recovery after hard efforts. How fast does your heart rate drop in the 60 seconds after you stop a hard effort? This is trainable. It improves with aerobic fitness. Most watches record it. Nobody looks at it systematically.
None of these require Apple to tell you a new VO2 Max number. They’re direct measurements. They update on meaningful timescales. They respond to actual training.
The Bigger Point
VO2 Max on a consumer wearable is a compromised metric. It’s derived from limited inputs, conservatively updated, and increasingly resistant to change at exactly the fitness levels where real athletes want to track progression.
The number has branding power. Apple puts it in a big font. Garmin makes it a centrepiece of their fitness dashboard. People pay attention to it because the interface tells them to.
But the number is downstream of a lot of assumptions that don’t apply to serious athletes. For you, the more useful signals are the ones your wearable quietly records and rarely features. Resting heart rate trends. Submaximal heart rate at known effort. Heart rate recovery curves. Simple, direct, physiological.
Your VO2 Max number might be stuck. Your fitness almost certainly is not. The gap between those two statements is what the wearable industry has built a market on and what properly interpreted data is finally starting to close.
Green score. Destroyed legs. There are 6 blind spots in your wearable data. We wrote a free guide covering every one of them.
Green score. Destroyed legs. There are 6 blind spots in your wearable data. We wrote a free guide covering every one of them.
Download the Free Guide