Wearable devices estimate VO2 max using algorithms that infer gas exchange from heart rate, pace, power, age, sex, and weight, rather than directly measuring it.

“None of the V2 max numbers that you see on wearables are actually measuring anything. They're estimating V2 max. And the first problem is that they're not actually measuring gas exchange. So if you don't if you can't measure gas exchange, you can't you can't estimate V2 max accurately. So they rely on algorithms that try to infer gas exchange from heart rate, from pace, power, and then they combine them with demographic variables and you know things like age, sex, and weight.”