UPDATE December 14, 2020

Apple Watch can monitor and classify cardio fitness, a strong predictor of overall health

Now, with watchOS 7, Apple Watch uses multiple sensors, including the optical heart sensor, GPS, and the accelerometer, to estimate lower levels, too. This is significant because direct measurement of VO₂ max typically requires a rigorous clinical test with specialized equipment that is not readily accessible to most people. watchOS 7 also allows Apple Watch to take cardio fitness measurements as users walk throughout the day, whether or not they are tracking a workout. With this innovation, Apple Watch is better able to measure VO₂ max for users with low cardio fitness, who may not complete high-intensity workouts.