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Go to Settings and toggle "Manual Override" under Heart Rate Max. If you do not override it, the app estimates your HRmax from your age using the Tanaka formula (208 - 0.7 x age).

Heart rate needs a connected source: a paired Apple Watch running the companion app, or any Bluetooth heart rate monitor such as a chest strap. Make sure you have granted Health and Bluetooth permissions, and that your monitor is selected in Settings.

The 4x4 protocol uses fixed 4-minute intervals and 3-minute recoveries. You can adjust the warm-up and cool-down durations in Settings.

Your workout data is saved on your device and to Apple Health. The app keeps your workout history locally on your device so you can review past sessions, and you can delete any workout from the History screen. The app has no server and no analytics, you do not need an account, and it does not transmit your data off your device. Anything you share with another service is your choice.

If you track with a Bluetooth heart rate monitor whose own app also logs to Apple Health, such as Whoop, Garmin Connect, or Polar Flow, both that app and Norwegian 4x4 can save the same session, so it appears twice.

To fix it, open Norwegian 4x4, go to Settings > Data, and turn off "Save workouts to Apple Health." After that only your monitor's app logs the workout, so there is a single entry. Your history inside Norwegian 4x4 is unaffected.

One exception: if you use a plain chest strap with no companion app writing to Apple Health, like a Garmin HRM-Dual or a Polar H10 in broadcast mode, leave this setting on, otherwise your workouts will not reach Apple Health at all. Only turn it off if you actually see duplicates.

This setting was added in version 1.6.1.

Email us with a description of what happened. Include your iPhone model and iOS version if you can.