13/10/2016
Apple Watch performs very well compared to other leading wristband heart rate monitors, 91% percent accuracy.
http://uk.businessinsider.com/the-apple-watch-has-the-most-accurate-heart-rate-monitor-according-to-new-research-2016-10?
Cleveland Clinic researchers find Apple's wearable gives the most accurate readings, but it’s no substitute for a chest strap