Since, as a person increases their cardiovascular fitness, their resting heart rate will drop over time, the the heart rate reserve will increase.
HRR = HRmax − HRrest
This formula is often used to gauge exercise intensity (first used in 1957 by Karvonen). Hmmm....but as we've already learned, an individual's maximum heart rate is an estimate only, as the only way to truly gauge one's rate is to stress the heart out too much (and while I love science, I'm not willing to die for it!).
Another failing...Karvonen's study findings have been questioned since the study did not use VO2 data to develop the equation AND only six subjects were used AND the correlation between the percentages of HRR and VO2 max was not statistically significant.
So, HRR is a theory only. It can't even be described as a hypothesis since there is truly no ethical way to test it. And thus, its tossed around like a scientific term related to exercise that has no significance.