Fitbit has added a bunch of features to its older devices, ahead of the launch of its newly released smartwatches.

The company has updated older Charge and Versa devices with Active Zone Minutes, its new metric based on heart rate activity.

Active Zone Minutes are a new take on the idea of measuring active time, and you only clock a minute if your heart rate rises into the fat burn zone. Get it into cardio or peak zones and you get double credit.

The feature was debuted on the Charge 4 and rolled out to the Versa 3 – but now the Charge 3 and older Versas are also getting the metric.