Cardio fitness (VO2 max)
The most oxygen your body can use during hard exercise. Apple Watch estimates it during outdoor walks, runs and hikes.
VO2 max is the most oxygen your body can take in and use when you're working flat out. It's a good overall measure of cardio fitness, and Apple Health calls it Cardio Fitness.
Apple Watch estimates it without making you go flat out. During an Outdoor Walk, Outdoor Run or Hiking workout on fairly flat ground, with good GPS and enough effort, it reads how your heart responds and predicts your maximum. Indoor workouts don't count. It also uses your age, sex, height and weight from Health, so keep those up to date. Medicines or conditions that limit heart rate can make it read high; Apple lets you note such medicines in Health.
It's an estimate, not a lab test, and single readings wobble. From age 20, Apple shows how your number compares with cardio fitness levels for your age and sex.
Regular endurance training and interval training raise it. It drifts down with age and falls surprisingly fast after a few weeks of much less activity. Pajdo looks for a steady trend over about three months rather than reacting to one reading.
What tends to push it up
- Regular endurance or interval training
What tends to pull it down
- Several weeks of much less exercise
- Getting older
Worth a try
Mix steady cardio with intervals
Apple says most people can improve VO2 max by increasing the intensity and frequency of cardio exercise. A meta-analysis in healthy adults aged 18 to 45 found both steady endurance training and interval training brought large gains, with intervals a little ahead.
Apple Watch estimates cardio fitness during Outdoor Walk, Outdoor Run and Hiking workouts, so those keep your readings coming.
Try it: keep your regular walks or runs, and once a week swap one for intervals, like a few minutes brisk then a few minutes easy. Give it weeks, not days.
Keep reading
Sources
- Track your cardio fitness levels, Apple
- Using Apple Watch to Estimate Cardio Fitness with VO2 max (May 2021), Apple
- Effectiveness of high-intensity interval training and continuous endurance training for VO2max improvements (Milanović, Sporiš and Weston 2015), Sports Medicine
- Detraining: loss of training-induced physiological and performance adaptations, part II (Mujika and Padilla, 2000), Sports Medicine
- Age-associated changes in cardiovascular structure and function (Fleg and Strait, 2012), Heart Failure Reviews
Checked against these sources on September 15, 2026.
This is general wellness information, not medical advice. If a number worries you, or stays away from your usual for a while, talk to a doctor.