Hi
There is no stated heart rate threshold. To credit progress towards your Exercise goal, the Activity app aims to identify activity that equals or exceeds the intensity of a brisk walk. This requirement cannot be changed and it applies both during general daily wear and when recording workouts via the Workout app.
Exercise credit is awarded for each minute during which this requirement is met. It is possible that, during your workout, you are only meeting that requirement for part of the time and/or your arm movements are not resulting in your activity being tracked accurately.
The definition of a brisk walk varies from person to person and depends on your personal information. To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
When recording fitness activities, be sure to use the Workout app (or a third-party app) and to choose the activity type that most closely matches your workout, including choosing Other for workouts that are neither included within nor sufficiently similar to those in the main list. This enables your watch to use the most appropriate sensors and data sources when tracking results.
For example, with Apple Watch (first generation) and Series 1 models, the watch can access Location Services data from the paired iPhone if it is taken along during outdoor walking, running and cycling workouts. Series 2 has built-in GPS.
During general daily wear and when using the Workout app to record workouts when GPS data is either not relevant or not available (eg Indoor Walk), the Activity app and Workout app track your arm motion (measured by the accelerometer) when estimating results including pace and progress towards the Exercise goal.
For the best results, allow the arm on which you are wearing your watch to swing naturally as you move around during daily wear and to move as expected during workouts. Calibrating your watch can improve the accuracy of estimations made by Activity app and Workout app. More information and instructions for calibrating your watch are available via the link below. The article also includes instructions for resetting existing calibration data and starting afresh. (Resetting your calibration data will not erase your Activity history).
If you would prefer to receive Exercise credit for the full duration of your workouts, regardless of their estimated intensity levels, track them via the Workout app using "Other" as the activity type. This will credit one minute of Exercise for each full minute of the workout. Active calories will be estimated at a rate equivalent to a brisk walk or based on data recorded by the heart rate sensor, whichever is higher. (Note that neither distance nor a route map are recorded under this workout activity type).
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