I worked in wearable electronics back in 2013. At the time, everyone was trying to crack wristworn heart rate monitors. It’s a challenging problem to solve: having to detect a faint color change in human skin while ignoring the massive shifts in ambient light from sunlight to shade all while bouncing around on a wrist.
Different vendors had different solutions and couldn’t even agree on what color LED was best for illuminating the skin.
Anyway, when the next generation of Nike Fuelband came out and didn’t include heart rate monitoring, I’ll never forget one of the comments I saw on a review.
“Come on Nike, it’s not hard to add a heart rate monitor. Just use pulse tracking.”