There are a number of formuale used to determine maximum / optimum heart rates during exercise. If you use 220 - age - resting heart rate will give you your first number. So for a 40 year old with a resting heart rate of 60 bpm you get 220 - 40 - 60 = 120.
You then assign a number between 65% and 85 % of 120, which is your upper and lower limit, so you get between 68 and 102 bpm, to which you add your resting heart rate.
This gives a heart rate of 128 (low) to 172 (high). so below about 128 bpm you are in the fat burning zone wherby a greater number of calories of fat are burned off, and between 128 and 172, you are in the cardiovascular zone, where your heart and lungs get good exercise.
Imperator