How is an HRM supposed to work?

jonnythan

I want to preface this by saying I am a very scientifically literate person, so I'm not looking for some basic explanation.

How well does heart rate actually correlate to oxygen consumed in untrained individuals? I can understand that if you're already well-trained, your cardiovascular system has reached peak or near-peak ability to transport oxygen. However, a lot of the people buying these things are completely untrained so their CV is very inefficient at delivering oxygen. These people will have high heart rates when burning relatively few calories, and you get results like the 350-lb guy whose HRM said he burned 1200 calories walking 3 miles.

Untrained people will experience fairly rapid adaptation. Walking on a treadmill may make a newbie's heart go up to 160, but within a few months it might be down to 120 when walking at the same pace and burning almost exactly the same number of calories.

So what's the theory? Why is the HRM held as the gold standard for calorie measurement among consumer devices? Are there any studies actually correlating calories burned vs heart rate for untrained people with any semblance of accuracy, especially over time?