Disclaimed: I used to work in this field, but not on the Apple Watch. The wearing a bunch of them at the same time is going to be wrong, yeah. Wrist mounted fi

Disclaimed: I used to work in this field, but not on the Apple Watch. The wearing a bunch of them at the same time is going to be wrong, yeah. Wrist mounted fitness devices are calibrated to read on the top side of the wrist. Even reading on the bottom side of the wrist would be a bit different, way up the arm? Very different. It is possible to make accurate devices that read there, they just need to be constructed to do such! Wearables are also meant to give an accurate rating over the long term. If you go running for 30 minutes, are the overall calorie burn, average HR, max HR, and min HR, all correct? Tracking in perfect sync with the heart is nice, and of course everyone tries to do that, but if the number is off for a second here or there, well so long as the overall result is the same the usefulness hasn't really changed. Finally, due to the way optical HR works, random unexpected, movements are really hard to deal with. It can take some time for an HR sensor to lock on to the heart rate. This isn't too long (measured in seconds, and each generation of devices is getting better and better), but it relies on regular patterned movement, such as arms pumping while running or walking, the regular motion of a bench press, etc. Stopping and checking the HR value every few seconds to see if it is correct is literally the worst case scenario for a lot of wrist mounted sensors! Ground truth is strapping a heart rate strap to your chest, putting on the wearable, going for a run as you normally would, and comparing the two at the end. Multiple sources have done this, and found the Apple Watch to be best in class when compared to directly measuring HR. tl;dr: Stopping and checking wrist mounted devices can confuse them, Apple Watch, and everyone else in this field who is serious, are tested against HR straps and they come within spitting distance of accuracy.