While it readies replacements for these watches with its forthcoming Versa 3 and Sense devices, Fitbit has rolled out the feature for its original smartwatches, and also its discontinued Charge 3 activity tracker.

Fitbit OS 4.2 is an update to June's 4.1.3. Your Fitbit app will alert you to the tracker update.

AZM was first introduced on the non-smartwatch activity tracker Charge 4: read our Fitbit Charge 4 review, Fitbit Versa 2 review.

Active Zone Minutes replace the less sophisticated Active Minutes. The measurement uses your resting heart rate as a baseline to target motivational heart-rate zones during exercise.

Fitbit Active Zone Minutes

They are earned for time spent in the different Heart Rate Zones: Fat Burn, Cardio, or Peak. These are personalised based on your fitness level and age.

You earn two Active Zone Minutes for each minute you’re in the Cardio or Peak zones.

The default goal is to earn 150 Active Zone Minutes each week, which is based on recommendations from the American Heart Association and World Health Organization, which both recommend at least 150 minutes of moderate-intensity activity or 75 minutes of vigorous activity each week.

Fitbit Active Zone Minutes points score

Users can change that goal in the Fitbit app.

More intense workouts - such as spinning, interval training, or running - increase your opportunities to earn Active Zone Minutes. If you run for 20 minutes and spend 10 minutes in the Fat Burn zone and 10 minutes in the Cardio or Peak zones, you earn 30 Active Zone Minutes.

Health and Wellness Reminders

Fitbit OS 4.2 also adds Health and Wellness Reminders, such as Get your heart pumping, Moment of mindfulness, Eat with the sun, Stay hydrated, and Wash your hands.

Health Metrics Dashboard

To build support for Fitbit Premium's Health Metrics Dashboard, users of the Inspire HR, Charge 3 and 4, Versa, Versa 2 and Ionic will now see heart-rate variability and breathing rate.

The Health Metrics dashboard - available "soon" to Premium members - will display five health metrics to show trends in your overall wellbeing: Skin temperature variation, breathing rate, resting heart rate, oxygen saturation (Sp02), and heart-rate variability (HRV).