Almost always because your recent workouts did not qualify. Apple Watch only estimates VO2 max, which the Health app calls Cardio Fitness, during Outdoor Walk, Outdoor Run and Hiking workouts, and only when the conditions are right. Indoor runs, cycling and every other workout type never produce one.
What has to be true for an estimate?
These conditions come from Apple’s own documentation:
| Condition | What Apple says |
|---|---|
| Workout type | Outdoor Walk, Outdoor Run or Hiking. Indoor workouts do not count. |
| Terrain | Relatively flat ground: less than 5% incline or decline. |
| Signal | Adequate GPS and heart-rate signal quality. |
| Effort | Your heart rate rises by roughly 30% of the range between your resting and maximum heart rate. |
| History | You have worn the watch for a day, and it is not your first qualifying workout. The first one never produces an estimate. |
| Watch | Apple Watch Series 3 or later. |
You do not need to go all out. The estimate is based on how your heart rate responds to steady effort, not on reaching your maximum.
Why did it stop updating?
Go through these in order. The first three explain most cases.
- You moved indoors or changed sport. Treadmill runs, indoor walks, cycling and strength workouts do not count. A winter on the treadmill means a winter without new values.
- Your routes are hilly. Trail runs and steep hikes can fall outside the 5% limit for much of the workout.
- The effort is too low. A slow stroll may never raise your heart rate enough.
- The watch is loose. A poor heart-rate signal means no estimate. Wear it snug, above the wrist bone.
- Your details are missing or out of date. Apple uses your age, sex, height and weight. Check them under Health Details in the Health app.
- You record runs in another app. Apple describes the estimate as coming from workouts in its own Workout app. If another app gives you no values, record a few runs in the Workout app and compare.
How often should it update?
Not after every workout. A value appears when conditions were good enough, so several runs in a row without one is normal. Apple does not state a minimum workout length; people commonly report that workouts shorter than about 20 minutes rarely produce a value.
Why did my VO2 max suddenly drop?
Single values move around. Heat, hills, fatigue, illness, a bad night or a loose watch can all push one reading down without your fitness changing. Look at the direction over months, not at one number.
One thing can push it the other way: medication that limits your heart rate, such as beta blockers, can make the estimate too high. You can record this under Health Details so Apple can take it into account.
Where do I see it?
On iPhone, open the Health app, then Browse, Heart, Cardio Fitness. Apple Watch supports values from 14 to 65, and shows how your value compares with others of your age and sex if you are 20 or older.
Kiiro shows the same Apple Health values as a trend, next to your resting heart rate and HRV. If Kiiro tells you that VO₂Max has not been recorded for a while, the cause is one of the points above: Kiiro reads the value from Apple Health and does not calculate its own. See VO2 max trends in Kiiro and the glossary entry.
Related questions
- What does Training Load mean on Apple Watch?: Apple Watch Training Load compares your last 7 days of workouts with the previous 28 and labels the result from Well Below to Well Above. Here is how to read it and what to do about it.
- How can I tell if I'm overtraining?: No single number tells you. Look for several signs together: training load well above what you are used to, a raised resting heart rate, a lower HRV, and runs that feel harder than they should.
