Updated: Mar 1, 2019
The mathematical formula to determine maximal heart rate (MHR) is based upon one’s age, with the assumption that all people of the same age have the same MHR. Obviously this isnt and will never be the case. Your maximal HR is determined primarily by genetics and individualized to each person rather than generalized by a formula. Here are a few factors that does affect your Heart Rate, age and fitness level.
All machines and most programs are designed with the generalized above formula and because we are not all clones you should always thrive for fitness performance as a basis of your goal and not focus on staying within the "fat burning zone". Why? well maybe you don't fit in that zone, so quit it and push yourself.