Heart rate reserve: definition, formula, example

Heart rate reserve is the range your heart actually has to work with: your maximum heart rate minus your resting heart rate. A runner who rests at 50 and tops out at 190 has 140 beats of reserve. Training zones built on that range, rather than on a plain percentage of maximum, are what most people mean by the Karvonen method, after the Finnish physiologist Martti Karvonen, who used it in a 1957 study of how training lowered heart rate.

The appeal is that it accounts for where you start. Two runners with the same maximum but different resting heart rates do not have the same easy pace in beats per minute, and the reserve sees that. It also lines up better with physiology than percentages of maximum do: Swain and Leutholtz (1997) showed that a given percentage of heart rate reserve matches the same percentage of oxygen uptake reserve, while a percentage of maximum heart rate does not match the same percentage of VO2max.

How it is calculated

The usual five zones are bands of 10% of the reserve: 50–60%, 60–70%, 70–80%, 80–90% and 90–100%. The percentages are conventions, not thresholds measured on you, which matters later.

A worked example

Resting heart rate 50 bpm, maximum 190 bpm, so a reserve of 140 beats.

• Zone 1 (50–60%): 50 + 0.5 × 140 = 120 to 134 bpm
• Zone 2 (60–70%): 134 to 50 + 0.7 × 140 = 148 bpm
• Zone 3 (70–80%): 148 to 162 bpm
• Zone 4 (80–90%): 162 to 176 bpm
• Zone 5 (90–100%): 176 to 190 bpm

With percentages of maximum, the same zone 2 would be 114 to 133 bpm, a good 15 beats lower. For a fit runner, 114 bpm is barely a jog, which is why people who switch to Karvonen often find their easy runs stop feeling like walking.

Now keep the maximum at 190 but raise the resting heart rate to 65. The reserve drops to 125 beats and zone 2 becomes 65 + 0.6 × 125 = 140 to 65 + 0.7 × 125 = 152.5, so roughly 140 to 153 bpm. Same maximum, a different zone, because the starting point moved.

Getting the two numbers right

Resting heart rate is best taken lying down on waking, before coffee, over several mornings; take a typical value, not the lowest one ever seen. Watches often report the minimum during sleep, which can sit a few beats lower and inflates the reserve slightly.

Maximum heart rate is the harder one. The highest value you have seen at the end of a hard race, or after the last of a set of hard hill repeats, is worth more than any formula. If you have nothing, Tanaka, Monahan and Seals (2001) give 208 − 0.7 × age, so 180 bpm at 40. It is a population average: many people sit 10 beats or more either side, and a maximum that is 10 beats too high shifts every zone up by about 6 to 9 beats.

How to use it in training

Heart rate zones work best for long, steady efforts, where the heart has time to settle. Easy runs and long runs in zone 2 are the obvious case: for the runner above, 134 to 148 bpm, whatever the pace on a hilly or windy day. Tempo runs of 30 to 40 minutes in zone 3, or 2 × 15 minutes near the top of zone 4 (around 169 to 173 bpm, 85–88% of reserve), are also easy to steer by heart rate.

For short intervals such as 5 × 4 minutes at threshold, heart rate lags behind the effort: it takes a minute or two to climb, and the first rep will look easy on the watch when it is not. Steer those by pace or power and use heart rate afterwards to check that the session did what you wanted. The same reserve fraction also feeds load scores such as TRIMP, so getting your resting and maximum values right pays twice.

The heart rate zones calculator does the arithmetic for both methods. In OpenAthlete, heart rate zones can be set from a percentage of maximum or of reserve, the reserve is calculated from your resting and maximum heart rate metrics, and workout steps can target a percentage of heart rate reserve.

Limits

Everything rests on two inputs, and both are noisy. Resting heart rate moves by several beats with sleep, stress, heat or an oncoming cold; heart rate during exercise rises with heat, dehydration, caffeine and altitude, and drifts upward over long efforts. Some medications, beta blockers in particular, change both ends of the range.

The bigger limit is that 60%, 70% or 80% of reserve are the same for everyone, while physiology is not. One runner’s lactate threshold sits at 82% of reserve, another’s at 90%. If you know your threshold heart rate from a lab test or a 30-minute race effort, anchoring zones on it is more accurate. Karvonen is a sound default when you don’t.