Smartwatches makeit easy to see a number that could reflect how old you are more accurately than your age: the VO2max, the maximum amount of oxygen your body can use during intense exercise.

The higher your VO2max, experts say, the better your cardiovascular fitness and, potentially, the longer your life. In the past, only serious athletes sought a traditional VO2max test but now anyone can get an estimate with a smartwatch.

Is it good to have access to this kind of information? In the last five months, when I fell down a VO2max rabbit hole, I learned some uncomfortable truths about my health and the limits of smartwatches.

In November, my Apple Watch gave a notification of a high heart rate. That led me to look at my VO2max, which the Apple Watch said was 32, well below average for a man in his late 30s.

I bought a membership at a high-intensity interval gym. Five months later, I felt progress. The Apple Watch gave me a VO2max of 40. Garmin rated me at 45.

All that was left to do was to take a real VO2max test. A few hours after pedalling with an oxygen mask strapped to my face, I got my lab results: 25, a rating of poor, far below the flattering results from the Apple Watch and Garmin.

Dr Ethan Weiss, a cardiologist in San Francisco, US, who has studied wearable technology, said my experiment underscored the pros and cons of using smartwatch data to explore health.

“On the one hand, you can give it credit for kicking you in the butt to tell you to go work out,” he said. “But on the other hand, now you’re sort of burned with this real test and like, ‘What do I do with this number?’”

After learning how the wearable algorithms work and talking with health experts, I arrived at a positive conclusion: even if the smartwatch numbers were wrong, they were correct in broad strokes, and I was probably better off wearing one than not.

A deeper look at data

The purpose of a clinical VO2max test is to measure your maximum oxygen intake at the point you reach exhaustion. This metric — breathe in oxygen and produce carbon dioxide during exercise — is a strong indicator of cardiovascular fitness.

My gym’s owner, Cassie Hecker, strapped an oxygen mask over my face and a heart monitor to my chest. After I reached my maximum heart rate of 182 beats per minute and began to struggle from exhaustion, the test was done. It was very different from the way the wearables estimated my VO2max. They study your heart rate and movement while you’re walking or running for at least 10 minutes and tally a score.

Spokespeople for Apple and Garmin referred to documents describing their methods. The key word is “estimate”. The watches aren’t actually measuring your oxygen intake and, therefore, aren’t actually measuring your VO2max.

“It’s at best an imputed VO2max,” Weiss said. “Not only do you not have an oxygen mask, you’re not actually exerting yourself to exhaustion.”

A reason my wearable estimates were so far off from my real VO2max result is that the way my body works doesn’t match the heart rate and oxygen-intake patterns of participants in the Apple and Garmin studies.

That left me with no choice but to embrace the hard truth: my VO2max result in the lab test was very low. But that statistic was only one data point. The report also showed many positives, including a very high metabolic rate and fat-burning efficiency and healthy breathing patterns.

Hecker rated my fitness level “average”, higher than the Apple Watch’s rating of “below average”.

Ignore the numbers

All the experts I interviewed agreed that even though the wearable data — much of it flawed — had given me anxiety, I had reached a net positive. The Apple Watch nudged me to pay closer attention to my health, and as a result, I’m healthier now.

The broad trends shown were accurate: after the pandemic had taken a toll on my body and mind, I was in my worst shape in years. Now, even though the watch numbers are too high, I do look and feel better, and that’s all that really matters.

That might be the best way to approach wearables — view them as a directional arrow rather than a precise measuring tool, said Steven Adams, a sports medicine doctor and personal trainer in Danville, California, US.

“It’s the trend that’s important, not the absolute number, because this stuff’s not accurate,” Adams said.

NYTNS