How come the maximum heart rate is estimated using age only, and not also resting heart rate? The maximum safe heart rate during rigorous exercise is a function of 80% of the MHR so it implies to me that it would be 160BPM for a 20 year old to exercise safely, regardless of the resting heart rate. That is 3x the heart rate of a 20 year old who rests at 50BPM while only 1.5x the heart rate of a 20 year old who rests at 100BPM and it shouldnt be considered equally safe? Shouldnt someone with a tripled heart rate be much more concerned?