At least in the case of Apple I have met a couple people who work on the fitness products and their accuracy. One of the people does real world testing so they

At least in the case of Apple I have met a couple people who work on the fitness products and their accuracy. One of the people does real world testing so they are diving one day and coding the next. Great work if you can get it and your hobbies align. The lab setup you describe is basically what they have to setup to correlate their proxies to energy. I’d expect the variability is going to come as a factor of body efficacy. I used to be in much better shape (resting heart rate in the 50s) but I’m not much heavier and haven’t lost much of the muscle I gained (70ish percent of my former one rep max achievable). My day to day non-workout energy burn is measured higher now than it was when my heart rate was lower for all activity levels. I don’t think that it’s true I’m suddenly +15-20% energy consumption simply because I put on 10lbs but rather having a 10bpm increase in heart rate is probably what is being measured. Short of that though it does a pretty good job. If I’m doing things that are strenuous but my arms are stationary it picks up the work. Very old wrist trackers needed to have an arm pumping or otherwise moving to do anything aside from a pre-calculated “you are alive” burn. Given the level of effort for what we get, I’ll take it as a basic way to answer “did I actually get some exertion from watching my 2 year old today?”