Heart rate reserve (HRR) is defined as: HRR = HRmax − HRrest. It represents the functional range of your heart rate — from its lowest to its maximum. A runner with a maximum heart rate of 190 bpm and a resting heart rate of 50 bpm has a heart rate reserve of 140 bpm.
The concept is central to the Karvonen method for calculating heart rate training zones (published by Martti Karvonen in 1957). Instead of calculating zones as simple percentages of maximum heart rate, the Karvonen method calculates zones relative to the reserve:
Target HR = ((HRmax − HRrest) × intensity%) + HRrest
For example, 70% Karvonen intensity for the runner above: ((190 − 50) × 0.70) + 50 = 148 bpm. The equivalent %HRmax calculation (70% of 190) gives 133 bpm — a significant difference.
Why reserve matters: resting heart rate reflects fitness level. A well-trained runner with a low resting heart rate will train at proportionally higher absolute heart rates at any given intensity percentage compared to a less-fit runner with the same max HR. The Karvonen method captures this; %HRmax does not.
Resting heart rate typically decreases with aerobic fitness training. Elite distance runners often have resting heart rates of 35–45 bpm; untrained adults typically 60–80 bpm. As resting HR decreases with training, the heart rate reserve increases, and the Karvonen zones adjust accordingly.