On the watch's bottom, there's a small optical heart rate sensor sitting in the middle. (techradar.com)
A study of more than 9,000 people shows that fitness trackers and smartwatches with optical heart rate sensors can predict atrial fibrillation with 97... (tomsguide.com)
If you count yourself among those who hate trying to jog or exercise with a chest strap in place, Polar has some good news: a new optical heart rate sensor designed to be worn on the arm. (slashgear.com)