I'd also take the results of that study with a pinch of salt - even if it is "the best" among the very small number of watches tested, most athlete focused reviews of the Apple Watch's HR performance show it being noticeably worse than a dedicated Bluetooth or ANT+ chest strap monitor, like the kind many cyclists and runners use. Speaking from my own experiences, the collection of ANT+ HR straps I've amassed over the years all work better than my Apple Watch's optical system does - way less odd outlier values etc. This seems to be true of most wrist worn optical heart rate measurement systems. If you are really keen to use heart rate as a training tool, the Apple Watch arguably shouldn't be your first choice.

"What’s notable to start with is that it took almost 2 minutes after pressing the start button for the Apple Watch to decide on a heart rate. At which point that heart rate was incorrect by 40 beats. It wasn’t until 7 minutes into the run that it agreed with everyone else."

https://www.dcrainmaker.com/2017/02/apple-watch-series2-nike...