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To clarify a point of confusion, it appears as though the milanese band is not suitable for accurate heart rate monitoring during a workout. As I suspected, it seems the sports bands are pretty much required to get a decently accurate reading from the HRM.

From Joanna Sterns review:
At the end of three workouts, both the Polar and the watch reported similar average beats per minute. That’s far more accurate than the Fitbit Charge HR and Microsoft’s Band. Apple’s sport band isn’t the most stylish, but it is the one you want for working out. With the looser steel Milanese Loop band, the watch struggled to lock in on my heart rate.
 
To clarify a point of confusion, it appears as though the milanese band is not suitable for accurate heart rate monitoring during a workout. As I suspected, it seems the sports bands are pretty much required to get a decently accurate reading from the HRM.

From Joanna Sterns review:

That's concerning for someone who works out almost daily. Not sure if I'll want to be swapping bands every day. Making me lean more towards a sport model... Or just using a sport band most of the time...
 
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