Heart rate zones: what they can do and where they mislead
Zone models are practical because they put intensity into numbers. But they rest on assumptions that can be considerably off in an individual case — starting with the formula from which most models derive their upper limit.
Why zones at all?
Heart rate is easy to measure and rises with effort. That makes it an obvious measure of intensity — and the basis for models dividing the range between rest and maximum into sections.
These sections are meant to correspond to different training purposes: easy base work, longer sessions at the edge of sustainability, short intense efforts. The idea behind it is sound, because different intensities do favour different adaptations.
How many zones a model has is convention. Three, five and seven zones are common; they describe the same range at different resolution. Numbers from different models therefore cannot be compared directly — “zone 3” means something different depending on the system.
The formula and its problem
Most zone models work with percentages of maximum heart rate. This is usually estimated via a formula, best known as “220 minus age”.
The formula comes from a synthesis of older data and was never intended as a precise method. Its spread is considerable: actual maxima of people the same age regularly lie ten to twenty beats above or below the estimate, in individual cases further still.
That is no small problem, because the error propagates. Deriving a zone ceiling from an underestimated maximum means training harder than planned throughout; from an overestimate, correspondingly easier. Newer formulas represent the average somewhat better but change little about individual spread.
More reliable are values from measurement — either from a performance test or from the highest value actually observed in intense sessions. The latter approaches from below and will usually underestimate the true maximum.
What else influences heart rate
Even with a known maximum, heart rate remains a sluggish and easily disturbed measure. It responds with a delay to changes in effort — in short intervals the value is often only reaching the intensity at the end of the interval.
Then there are influences unrelated to intensity: heat drives the rate up at the same power output, as does fluid loss. Lack of sleep, stress, caffeine, altitude and time of day all act as well. In longer sessions the rate rises gradually at constant output — a known phenomenon that further complicates assigning a zone.
And measurement itself has limits. Optical sensors at the wrist work by a different method than chest straps and more frequently show deviations during rapid changes and certain movements.
Usable alternatives
For everyday planning, several measures are often more useful than a percentage.
Ability to speak is the simplest: holding a conversation in full sentences means the easy range; managing only short sentences, the middle; unable to speak, the high range. This criterion needs no device and is barely affected by the disturbances listed.
Perceived exertion on a scale works similarly and can be logged. It is subjective but remarkably consistent within one person over time.
In sports where actual output can be measured — power on a bike, speed on a known course — that measure is superior to heart rate, because it responds immediately and without delay.
Resting heart rate
Alongside exercise heart rate, resting heart rate is frequently used. In most people it falls over weeks of endurance training, because the heart ejects more volume per beat.
As a steering tool it is of limited use, as an observational measure it works well — provided it is measured under comparable conditions. Usual practice is measuring in the morning after waking, before getting up and before the first caffeine.
A resting heart rate markedly elevated across several days is interpreted in training practice as an indication of insufficient recovery or a developing infection. That is a rule from experience, not a diagnostic procedure: the value also varies with sleep, room temperature, fluid balance and the time of measurement. With persistent anomalies or complaints, interpretation belongs in medical hands, not in training planning.
What zones are still good for
Despite all limitations, zones are not worthless. They counter the most common recreational error: doing easy sessions too fast. A ceiling visible during the session disciplines more effectively than the intention to run slowly.
They are also useful for comparison with yourself over longer periods. If the same route is covered faster at the same heart rate, that is information — regardless of whether the underlying percentage calculation is right.
What they do not do is compare between people. Two people in “zone 2” are not necessarily working in the same physiological range, and a zone figure from an app says less about actual load than the number suggests.
The key points
- Zone models differ in resolution; “zone 3” means something different depending on the system.
- “220 minus age” was never intended as a precise method; individual spread is considerable.
- An error in the estimated maximum propagates into every zone derived from it.
- Heat, fluid loss, sleep, stress and caffeine influence heart rate independently of intensity.
- Ability to speak and perceived exertion need no device and are less easily disturbed.
- Zones work for comparison with yourself over months; not for comparison between people.
Last reviewed: 14.08.2026
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