Validity of the Apple Watch to monitor heart rate at different intensities

How valid is the Apple watch to measure heart rate during exercise?

The Apple Watch is one of the most sold Smartwatches and might be used durign different exercise to monitor heart rate at the wrist.  But how accurate is the Apple Watch for this purpose? Results from a study with n=20 particpants (10 male, 10 female) using the Apple Watch Series 6 & 7 exercising from standing still to high intensity running:

„The Apple Watch Series 6 and 7 are highly valid smartwatches at rest and different exercise intensities, with validity decreasing with increased running speed.“

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