VO2 max and fitness age, from your Apple Watch

What the Cardio Fitness card shows

The card on the Heart tab is titled VO2 max - your aerobic engine. It shows your most recent VO2 max in millilitres of oxygen per kilogram of body weight per minute, a sparkline of recent readings, and your level for your age and sex: Low, Below Average, Above Average or High.

Below it, the Estimated Fitness Age strip places you on a track from younger to older. It reads as a number of years younger or older than your actual age, or the same as your age, with a short verdict such as your heart performing like someone several years younger.

The detail screen shows your current VO2 max with a Fitness Level for Your Age bar, your fitness age, a trend you can view over one month, six months, one year or two years, and insight cards: Your Next Target, How to Improve It and The Longevity Metric.

Why VO2 max matters

VO2 max is the maximum amount of oxygen your body can take in and use during hard exercise. It reflects how well your heart, lungs, blood and muscles work together, which is why it is often called the best single measure of aerobic fitness.

It also matters well beyond sport. Large long-term studies have repeatedly found that people with higher cardiorespiratory fitness tend to live longer and have lower rates of heart disease, even after accounting for other factors. VO2 max naturally declines with age, but regular aerobic exercise slows that decline and can raise it at almost any age. That makes it one of the most useful numbers to watch for long-term health, and one you can actually change.

Fitness age expresses the same idea in a more intuitive way: how your aerobic fitness compares with what is typical at different ages. It is not your biological age, and FitRest describes it as an estimate, not a clinical measurement.

FitRest is a wellness app, not a medical device, and it does not diagnose or treat any condition. If a heart reading worries you, talk to a doctor.

Where the readings come from

FitRest does not run its own fitness test. It reads the VO2 max estimates Apple Watch saves to Apple Health. Apple Watch records these during brisk outdoor walks, runs and hikes, so new readings appear after that kind of activity. If you mostly train indoors, readings may be sparse.

To rate your level and estimate fitness age, FitRest uses the date of birth and sex stored in the Health app. If those are missing, add them in the Health app on iPhone.

How to improve your number

VO2 max responds to aerobic training over weeks and months. A mix of plenty of easy, steady exercise you can sustain and some harder intervals that push your breathing tends to work best for most people. Consistency matters more than any single session, and changes show up slowly, so judge progress on the six-month or one-year trend rather than week to week.

Body weight also plays a part because VO2 max is measured per kilogram, so changes in weight can move the number even when your heart and lungs have not changed.