Context: I was in Japan and we went to a hot bath (onsen). The GF reasoned that she could skip the gym next morning because her heart had been pounding by the end of 30 minutes in the 140 degree water. I suspected that this was wrong, but couldn’t quite say why – it’s obviously not “aerobic,” but it is putting your cardiovascular system into overdrive and raising your heart rate. Does heart rate elevation alone, for a substantial time, have a sufficient metabolic effect to be analogous to actual exercise in terms of fat-burning?
No. Heart rate is a rough gauge that is used to measure oxygen consumption during physical exercise. It’s rather inconvenient to measure oxygen consumption directly, unless you have a decent research lab.
But if a person is exercising, and isn’t on medications which either artificially slow or accelerate the heart, heart rate will work to determine when one has burned significant oxygen, and when someone enters aerobic metabolism, where fat is burned.
So no, you can’t just raise your heart rate by itself to create that effect. That’s like revving the engine of your car in the driveway to break in the tires, transmission, differential, etc.