But it is not straight forward.
The HR to kcal correlation is not linear. At least not always.
There exists an HR that defines when your body goes into "I'm doing physical activity"-mode.
This HR is called "Flex HR" or "Flex Point".
An HR lower than that seems to always burn an almost fixed amount of kcal, independently of your HR. Your maintenance energy.
An HR higher than "Flex HR" seems to be linearly correlated to the amount of kcal you burn.
The higher your HR, the more kcal you burn.
But how do you get this "Flex HR"?
Unfortunately I didn't find a formula to estimate it without actively following a protocol.
The "Flex HR" was originally calculated by this formula:
The mean of the highest HR for a standing activity and the lowest HR of exercise activities. [1]
Since the purpose of the "Flex HR" is to decide if you are at rest or doing physical exercise, it is nowadays defined more broadly:
The average of the highest resting HR and the lowest exercising HR. [2]
[1] LIVINGSTONE, M. B., et al. Simultaneous measurement of free-living energy expenditure by the doubly labeled water method and heart-rate monitoring. The American journal of clinical nutrition, 1990, 52. Jg., Nr. 1, S. 59-65.
[2] LEONARD, William R. Measuring human energy expenditure: what have we learned from the flex‐heart rate method?. American journal of human biology, 2003, 15. Jg., Nr. 4, S. 479-489.
