Heart rate while training: why your zones are probably wrong

My watch logged 38 minutes in zone 4 last week. I was running an easy ten kilometres at conversation pace, talking the whole way. Search for heart rate training zones and you see where that comes from: every page sets your zones with 220 minus your age.

That sum was held up against the measured data in 2001 and came off badly (PMID 11153730). It still sits in almost every watch app.

So you are training on an estimate of an estimate.

Heart rate while training: why 220 minus your age does not hold up

The formula did not come out of a large study. It was pulled from a handful of small datasets in the seventies and stuck around because you can do it in your head.

Tanaka and colleagues ran a meta-analysis against it and landed on 208 minus 0.7 x your age (PMID 11153730). That version fits the measured data better. It does not solve the real problem.

Around any age-based estimate sits a spread of roughly ten beats per minute. Two people of the same age can therefore be twenty beats apart. No formula knows where you sit in that spread.

Imagine two runners, both 40, whom the formula hands the same 180 as a maximum. The first never gets past 172 in the field, the second touches 195. On the same zone boundary they end up more than fifteen beats apart.

They train off the same chart anyway.

How many beats does that cost you?

In younger athletes about four to five beats, and in older athletes the same amount the other way. The two formulas cross each other around age forty. Below that, 220 minus age reads too high; above it, too low. On your zone 2 boundary that lands as a few beats of difference, every session again.

The table breaks it down by age. The last column carries it through to a zone 2 boundary of 70 percent of your maximum.

Four or five beats sounds small. On an hour-long steady run it decides whether you hold pace or keep pulling back.

And this is still the tidy version of the error. The spread between people of the same age is a multiple of it.

What counts as too high a heart rate while training?

There is no number that is too high for everyone. A high heart rate during effort mostly says how hard you are working right then, not whether something is wrong. What does count are symptoms: chest pain, dizziness, breathlessness out of proportion to the effort, or a beat that feels irregular.

Those belong with your GP or a sports physician, not with a sum you found online.

The Hartstichting holds the same line: how you feel weighs more than the number on your screen. Going above your calculated maximum is rarely news either. Usually it means that calculation was set too low.

When a bigger heart is normal in athletes and when a cardiologist takes a look, sits in the athlete's heart explained.

What does a low heart rate while training mean?

Usually that your fitness is improving. If you need fewer beats at the same pace, your heart is moving more blood per beat. That is exactly the adaptation you are training for. A low heart rate can also have other causes, medication that slows your heart rate among them, so read it next to how you feel.

Under load, a low heart rate only reads properly alongside your pace or your power. Without that second axis you are measuring nothing.

At rest it works differently, and that has a story of its own. What your resting value can say about recovery sits in resting heart rate and what it says about recovery.

How long does your heart rate stay up after training?

That depends on the effort, but the first minute is the interesting one. In most trained athletes the heart rate drops sharply within that first minute, then falls more slowly. After a hard session it can take hours before you are back at your usual resting value.

That first minute has a name: heart-rate recovery.

Cole and colleagues followed 2,428 adults after an exercise test. A fall of 12 beats or fewer in the first minute was associated with a higher risk of death over the following six years (PMID 10536127). That is an association in a group, not a statement about you.

Those 12 beats also come from a tight protocol with a cool-down walk. Your watch measures something else. Use your own figure as a trend line, not as a cut-off.

What keeps surprising me: almost no page about heart rate zones mentions this number, while your watch has been storing it for you free of charge for months.

Why does your heart rate creep up at the same pace?

Within one session that is usually cardiac drift: you lose fluid, your blood volume drops slightly and your heart compensates with more beats per minute. Across weeks it is a different story. A structurally higher heart rate at the same pace can then relate to sleep, heat, stress or an infection still running its course.

Sometimes something in your blood sits behind it. Low iron stores or a low haemoglobin can limit your oxygen transport, and your heart then turns faster to deliver the same thing.

That is a possible explanation, not a diagnosis.

Which values move most often in endurance sport sits in blood values for endurance athletes, and on oxygen transport itself hematocrit is the value to know. If your heart rate creeps up while your performance drops, spotting overtraining through your blood values is the logical next step.

What your zones are actually worth

More than this piece has suggested so far. The point is not that zones are nonsense. The point is that their boundaries come from a sum that does not know you.

The fitness you build with those zones is the interesting part. Mandsager and colleagues looked at 122,007 people who underwent an exercise test, and higher fitness was associated there with lower mortality, with no clear ceiling (PMID 30646252).

That too is an association, not a promise.

Why the standard risk calculation serves fit people poorly in the first place sits in cardiovascular risk in athletes.

What do you do next?

Replace your estimated maximum with a measured one. A well built-up hard effort with a few all-out minutes at the end gives you a more realistic peak than any formula does. Build towards it calmly, and talk to your GP or a sports physician first if you have symptoms.

My other suggestion: note your heart-rate recovery over the first minute after your hardest block each week. One number, always taken at the same moment, tells you more after two months than your whole zone chart.

If your heart rate keeps creeping up at the same pace while your recovery lags, blood can be a logical next place to look. Iron, haemoglobin and inflammation markers sit together in 360 Health.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 2001;37(1):153-6. PMID 11153730.
• Cole CR, Blackstone EH, Pashkow FJ, Snader CE, Lauer MS. Heart-rate recovery immediately after exercise as a predictor of mortality. New England Journal of Medicine, 1999;341(18):1351-7. PMID 10536127.
• Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open, 2018;1(6):e183605. PMID 30646252.
• Hartstichting. Exercise and your heart rate.