Cardio Age explained: how the Ultrahuman Ring turns fitness into an age

What is Cardio Age?

Fitness declines with age, so the same VO2 max means very different things for a 25-year-old and a 55-year-old.

Cardio Age handles that comparison for you. It places your estimated fitness against population norms for your sex, then reports the age at which it would be average.

The number worth watching is the Cardio Age gap: your Cardio Age minus your real age. A negative gap means your fitness is ahead of your years. A positive gap means there's room to improve.

How Ultrahuman Ring estimates Cardio Age

Cardio Age starts from the Ring's VO2 max estimate, which you'd otherwise need a lab test to measure. The ingredients are things a ring worn every night already records.

An optical sensor reads your pulse across the night, giving your overnight resting heart rate and your heart rate variability. The motion sensor counts your daily steps, which tell the VO2 max estimate how active you are. With your age, sex, height and weight, these produce a fitness estimate. Cardio Age then reports that estimate as the age at which it would be typical for your sex, smoothed so it doesn't jump from one night to the next.

This is Ultrahuman's own model. VO2 max explained covers how close the underlying estimate comes to a lab test.

How to read Cardio Age in the app

Your Cardio Age appears alongside the VO2 max estimate it comes from, on the same card.

Because it's re-estimated from your latest readings each night, it shifts as your resting heart rate, HRV and step patterns change. A sustained drop in resting heart rate over several weeks, from more consistent training or better sleep, moves your Cardio Age younger.

The main lever is aerobic conditioning. The blog covers how to train your aerobic capacity across the different intensity zones.

A gap that concerns you, or one that won't move despite consistent training and better sleep, is worth raising with a physician.

What the research says about fitness and aging

Aerobic capacity falls by roughly 3 to 6% per decade in your twenties and thirties, then more than 20% per decade after 70, according to the Baltimore Longitudinal Study of Aging.

Cardio Age turns your fitness into an age using published norms for how fitness changes with age in men and women, from a study that followed more than 20,000 adults through repeated treadmill tests. That study found the decline is not steady: it speeds up after about 45.

Regular activity slows that decline, so keeping your Cardio Age steady as the years pass counts as progress. A 2016 American Heart Association scientific statement said cardiorespiratory fitness may be a stronger predictor of long-term mortality than smoking, high blood pressure, high cholesterol or type 2 diabetes, and made the case for treating it as a clinical vital sign.

What Ultrahuman's data shows

In a peer-reviewed study of 442 Ring members by Ultrahuman Science, 82.6% had a Cardio Age younger than their real age, with a mean gap of 1.84 years. Ring members aren't a random sample of adults, so this says more about members than about people in general.

Members who improved consistently across 12 months lowered their Cardio Age by an average of 3.24 years, alongside a fall in resting heart rate.

Members whose Cardio Age sat furthest below their age slept more efficiently than those furthest above it, with a median sleep efficiency of 90.4% against 88.6%.

The gap is small, and the two groups also differed in sex and BMI. When the study accounted for BMI, sex and age, sleep efficiency kept its link with a younger Cardio Age, while total sleep time did not. The study was observational, so it shows what goes together, not what causes what.