Apparently the Apple Watch is the most accurate of the wrist HR sensors.
Apple Watch is most precise wrist-worn heart rate tracker at 90% accuracy, study says
When the subject is exercising, Apple Watch is 90% accurate, other wrist sensors like Fitbit are closer to 80%, and a chest strap is 99%.
So 170 bpm could be 168-172 with chest strap, 153-187 with Apple Watch, and 136-204 with other wrist sensors . . .
For serious exercise use, seems like there's no substitute for a chest strap.
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