Heart rate reserveHRR · Karvonen method
The gap between your resting and maximum heart rate, used to set training zones.
Heart rate reserve is the difference between your maximum heart rate and your resting heart rate. It represents the range your heart rate can actually move through, rather than the range from zero.
It exists because percentage of maximum heart rate ignores where you start. A person with a resting rate of 45 and one with a resting rate of 75 have quite different working ranges, and prescribing both of them 70% of maximum asks for different relative efforts.
The Karvonen method uses it. Target heart rate is calculated as resting rate plus a percentage of the reserve, so 70% of reserve for someone with a resting rate of 50 and a maximum of 190 gives 50 plus 0.7 times 140, which is 148 beats per minute.
The result is generally a better match to physiological intensity than the simple percentage of maximum method, particularly for fitter people whose low resting rates would otherwise make the standard zones too easy.
It also gives an incidental fitness signal. Reserve widens as training lowers resting heart rate, since maximum barely moves, so a widening reserve over months reflects genuine cardiovascular adaptation.
Its accuracy still depends on the inputs. An estimated maximum from an age equation carries a standard deviation of ten beats or more, so the zones inherit that uncertainty however carefully the reserve arithmetic is done.
In practice, measure resting heart rate properly on waking, use our heart rate zone calculator which can apply the Karvonen method, and cross check the resulting zones against the talk test.