Then there is the myth that low intensity burns more fat than high intensity Then there’s the “myth” that it doesn’t. Don’t get me wrong. I happen to believe the latter “myth”. But my point is: no one has offered any numbers to support (or dispel) either “myth”. Perhaps someone can tell us: (1) the percentage fat burned as a function of percent MHR; and (2) the calories burned as a function of MHR for specified classes of exercise. During a hard ride I can burn close to 1000 kcal.
One hour at rest I burn approximately 2500/24 = 104 kcal – nearly ten times less. During a high intensity ride it is certain that I burn more fat than during a low intensity ride. I have even seen an exercise physiologist put the numbers out (Burke maybe ?) The amount of fat burned per hour does not change much as exercise intensity rises from 60% MHR to 85%, but it does rise. There are good reasons to work out at 60% MHR though.It’s more complicated than that. % fat used is a ramp function, but probably not a straight line.
And it’s probably better to consider oxygen consumption since the total % of the body’s energy that is consumed by the muscles changes in a slightly different manner that the changes in heart rate. A general approximation is that if your maximal sustainable intensity is 10X your resting metabolic rate, then 30% of the energy will come from fat.