VO₂max on Apple Watch: What Your Cardio Fitness Estimate Means
Learn what Apple Watch Cardio Fitness estimates, how VO₂max is measured, and how My Bio Age compares each week with age- and sex-based reference data.
By My Bio Age
VO₂max is the most oxygen your body can take in and use during hard exercise, and it is widely used as a measure of cardiorespiratory fitness. Apple Watch estimates it during outdoor walks, runs and hikes, and saves the result to Apple Health as Cardio Fitness.
My Bio Age reads that estimate; it does not measure VO₂max itself. Here is what the number describes, how the app uses it, and how to read slow changes calmly.
What VO₂max describes
Apple's HealthKit documentation summarises the clinical test: you exercise on a treadmill or bike while the intensity rises every few minutes until exhaustion. Results are given in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min), so the figure reflects your weight as well as your aerobic capacity.
Values tend to fall with age and differ between men and women in US laboratory reference data, so there is no single “good” VO₂max for everyone. A watch doesn't analyse your breathing. It predicts VO₂max from how your heart rate responds to the effort of a walk or run, so you never need to push to your maximum.
How Apple Watch estimates cardio fitness
Apple's Cardio Fitness guide lists Outdoor Walk, Outdoor Run and Hiking workouts as the activities it uses; indoor workouts don't count. It also draws on your Health profile, including age, sex, height, weight and medication that might affect heart rate. Apple's developer notes add that a workout needs fairly flat ground, good GPS and heart-rate signals, and enough effort, so not every session produces a value.
Apple's validation paper describes comparing the watch estimate with VO₂max projected from submaximal laboratory exercise tests in several hundred adults, and notes that more qualifying outdoor workouts improve the chance and accuracy of an estimate. That is manufacturer evidence rather than independent research, so treat the watch value as a reasonable estimate, not a lab result.
Why researchers pay attention to fitness
A 2009 meta-analysis in JAMA pooled 33 observational cohort studies of healthy adults. Better fitness, assessed with exercise testing, was associated with lower all-cause mortality and fewer cardiovascular events. A 2016 American Heart Association scientific statement went on to argue that fitness should be assessed in clinical practice as a vital sign, alongside traditional risk factors such as blood pressure and cholesterol.
Keep these findings in proportion. They are associations from groups of people whose fitness was measured in exercise tests, not by a wrist sensor. A watch estimate that rises or falls a little does not show that your own risk has changed; it is a prompt to look at your routine.
How My Bio Age uses your VO₂max
My Bio Age reads the Cardio Fitness estimates saved in Apple Health. VO₂max appears in the dashboard's Overview section, and its card opens a chart of your history. The app does not generate estimates of its own, so a gap in Health is a gap in My Bio Age too.
For the weekly MyBioAge Index™, the app scores complete Monday-to-Sunday weeks, not the week in progress. It compares each week's estimates with age- and sex-specific percentile tables that the app attributes to the FRIEND registry of laboratory exercise tests (for background, see Myers et al. (2017) and Kaminsky et al. (2022), listed on our Methods & Sources page). The result is one of several Index inputs. Since a watch estimate is being compared with lab-measured values, treat the comparison as approximate.
If a week has no new estimate, the app can reuse the most recent one from within 14 days of that week's end. VO₂max is also one of three inputs, with overnight HRV and sleep regularity, that the Index's data checklist treats as critical. It looks for readings on at least three separate days in the past 21 days; with none in that window, the Index is shown as Limited, VO₂ is listed among the missing inputs (for example, “VO₂ (3 runs)”), and the weekly digest suggests logging an outdoor run. See how Apple Health metrics become the MyBioAge Index™ for the wider picture.
Reading a change
Before deciding what a shift in your estimate means:
- Compare months, not days. One estimate can sit above or below your usual level; look for a direction that holds over several weeks.
- Check dates and source. In Apple Health, confirm when each estimate was recorded and by which device. A flat line may mean no new estimate.
- Keep your workout mix similar. Swapping flat routes for hills, or outdoor sessions for indoor ones, changes which workouts can produce an estimate.
- Note profile edits. A change to your Health details, such as weight or medication, can coincide with a step in the chart.
- Consider medication and conditions. Apple notes that conditions or medicines that limit heart rate may cause an overestimate. Take questions to a clinician, and never change medication or treatment to move a score.
- Treat the in-app range as a rough guide. The range note on the VO₂max detail screen is general orientation, not a clinical target.
If readings stop appearing, see our VO₂max troubleshooting checklist. For practical routines, read how to improve VO₂max, and for a signal that updates more often, see our guide to your walking heart rate average.
Using it alongside your other data
VO₂max is most useful next to signals that update more often, such as your walking heart rate average. Our guide to easy aerobic training covers practical routines, and VO₂max, HRV and Recovery together explains how cardio fitness relates to those scores. In My Bio Age, you can also choose VO₂max in AI Coach and ask a Status, Why or Improve question for a summary of your own data, prepared on your iPhone.
Ready to look at your own cardio fitness trend? Download My Bio Age free on the App Store to see the VO₂max readings already in Apple Health alongside your other signals. The iPhone app is completely free, with no in-app purchases, subscription or account required. Health data stays on your iPhone; the optional AI Coach is fully on-device too.
My Bio Age offers wellness information, not medical advice, and its VO₂max view is not a fitness test or exercise clearance. If you have symptoms such as chest pain or unusual breathlessness, or you're unsure what exercise suits you, a clinician is the right person to advise. Our Methods & Sources page explains the Index in more detail.