Heart rate reserve (HRR) is the difference between resting heart rate (HRrest) and maximum heart rate (HRmax).
HRR = HRmax - HRrest
Heart rate reserve is used when determining exercise heart rates.
The Karvonen formula is used to calculate exercise heart rate at a given percentage training intensity. Add the given percentage of heart rate reserve to the resting heart rate.
Exercise HR = % of target intensity (HRmax – HRrest) + HRrest
For example: Target intensity 70 % HRR for a person with HRmax 201 bpm and
HRrest 50 bpm
Exercise HR= 70% (201-50=151) + 50
Exercise HR=155 bpm
This method gives an exercise intensity that is equivalent to the desired percentage of VO2R (maximal oxygen uptake reserve).
To use Karvonen formula you need to know your true HRmax and HRrest. When using estimated HRmax or/and HRrest the values are always predictions.