What do heart rate training zones actually mean?

The problem with 220 minus age

Almost everyone has met the formula: maximum heart rate equals 220 minus your age. It is memorable, it is everywhere, and it was never derived from a proper study. It came from an informal fit to scattered data in the 1970s and was published without a stated error margin.

The error margin turns out to be large. Individual maximum heart rates vary by roughly ten to twelve beats per minute either side of any age-based estimate, and 220 minus age additionally has a systematic bias: it tends to overestimate for younger people and underestimate for older ones.

The Tanaka formula — 208 minus 0.7 times age — comes from a proper meta-analysis and fits observed data considerably better across the age range. At thirty it gives 187 rather than 190. At sixty it gives 166 rather than 160, a six-beat difference that shifts every zone boundary upward.

Neither formula is a measurement. Both are population averages, and your actual maximum could sit a fair way from either. That matters less than it sounds, though, because zones are broad bands rather than precise thresholds — but it does mean treating a calculated number as gospel is a mistake, and it means paying attention to how the effort actually feels.

What heart rate reserve changes

The simplest way to build zones is to take percentages of maximum heart rate directly: seventy percent of 187 is 131. The Karvonen method does something different, and the difference is worth understanding.

Karvonen works from heart rate reserve — your maximum minus your resting rate — which is the range your heart actually operates across. It takes a percentage of that range and adds it back to your resting rate. With a maximum of 187 and a resting of 60, the reserve is 127. Seventy percent of 127 is 89, added to 60 gives 149.

That is eighteen beats higher than the straight percentage-of-maximum answer, and the gap is not a rounding difference. It reflects a real fact: your heart is never doing zero work, so the bottom of your operating range is your resting rate, not zero.

The practical consequence is that a fit person with a low resting heart rate gets a wider reserve and therefore differently shaped zones from someone the same age with a higher resting rate. That is correct, and it is the reason the resting figure is worth measuring properly — on waking, before getting up, ideally averaged across several mornings.

What the five zones are for

Zone 1, roughly 50 to 60 percent of reserve, is recovery. Conversational, easy, and its purpose is to move blood without adding fatigue. It is genuinely useful between hard sessions and almost universally skipped.

Zone 2, 60 to 70 percent, is endurance, and it is where the largest share of most sensible training plans lives. It builds aerobic capacity, mitochondrial density and fat oxidation, and it does so with a fatigue cost low enough to be repeated often. You should be able to hold a conversation, albeit not comfortably.

Zone 3, 70 to 80 percent, is tempo — the moderately hard middle. It is productive in small doses and it is where undisciplined training accumulates by accident, because it feels like real work without being hard enough to drive adaptation the way zone 4 does.

Zone 4, 80 to 90 percent, sits around the lactate threshold and drives the ability to sustain hard efforts. Zone 5, above 90 percent, is maximal and can only be held in intervals of a few minutes at most. Both are effective and both are expensive in recovery, which is why they occupy a small fraction of a well-structured week.

Why most people train in zone 3

There is a well-documented pattern in amateur endurance training: sessions intended as easy drift upward into zone 3, and sessions intended as hard drift downward into zone 3. Everything converges on the moderately-hard middle.

The reason is that zone 3 feels correct. Zone 2 feels almost embarrassingly easy, especially when running, where the required pace is often slow enough to be socially uncomfortable. Zone 4 and 5 feel genuinely unpleasant. Zone 3 feels like productive effort, which is precisely why it accumulates.

The cost is that it is hard enough to require meaningful recovery while not being hard enough to drive the adaptations that zone 4 and 5 produce. A week of consistent zone 3 generates fatigue without the corresponding stimulus, and progress flattens while effort stays high.

The standard corrective is polarised training — the large majority of volume genuinely easy, a small minority genuinely hard, and very little in between. Whether the ratio should be eighty-twenty or something else is debated, but the underlying observation is consistent: the easy sessions need to be easier than feels natural, and that is the harder discipline of the two.

When to stop trusting the number

Heart rate is a useful signal, not a precise instrument, and several things move it independently of effort.

Cardiac drift means heart rate climbs over a long steady session at constant pace, driven by dehydration and rising core temperature. An hour into a run, the same effort can read five to ten beats higher, which will push you out of your intended zone without your having done anything differently.

Heat, altitude, caffeine, poor sleep and illness all raise heart rate for a given workload. Being under-recovered can raise it noticeably, and — confusingly — deep fatigue can sometimes suppress the maximum you can reach at all, so the same session reads differently for opposite reasons.

There is also a lag. Heart rate takes a minute or two to respond to a change in effort, which makes it close to useless for short intervals; by the time the reading is right the interval is over. For short, hard work, pace or power is the better guide. Use heart rate for the long, easy sessions where its slowness does not matter and its main job — keeping you honest about how easy easy is — is exactly what you need.

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