Apple is expanding the range of cardiorespiratory fitness that can be measured on the Apple Watch. Starting today, the Health app will classify users as having “high,” “above average,” “below average,” or “low” cardio fitness. Low levels will trigger a notification.

The Watch tracks fitness through a metric called VO2 max, which is the amount of oxygen your body consumes during movement or exercise. The Apple Watch was already tracking average and high VO2 max for users during exercise. This update expands the range it measures and allows users to see their levels when they’re walking around, not just when they’re working out.

Image: Apple

Apple says it tracks lower ranges of VO2 max by using Watch sensors like the heart rate monitor and accelerometer. Typically, VO2 max is calculated using specialized equipment in a lab while someone exercises on a treadmill or other piece of equipment.

“Apple Watch now brings estimation of low cardio fitness levels from clinics directly to a user’s wrist, so people have more insight into how they can improve their long-term health through daily activity,” said Jeff Williams, Apple’s chief operating officer, in a press release.

Wearing a Watch is certainly less time-intensive than going into a lab, but it may not be as accurate. Studies comparing Garmin and Polar smartwatches to typical lab testing found that the watches tended to be off by about 5 percent. Apple does not report how well the Watch compares to traditional, clinical VO2 max measurement methods.