If you’ve ever worried about your heart rate during exercise being too high or too low, you’ll want to read this. Your “heart rate zones” might be completely wrong. Not only are zones defined differently in different apps, they are also usually calculated based on your maximum heart rate. And that maximum heart rate calculation? It’s incorrect for huge swaths of the population.
What does it mean to know your “maximum” heart rate?
Your maximum heart rate is, by definition, the fastest your heart can possibly beat. If a watch tells you that your max is 180 beats per minute, and then you go for a run and your heart is beating at 190 beats per minute, you haven’t gone “over” your max. You have simply found out that 180 isn’t your max at all. Your actual max must be at least 190.
The only way to truly know your max heart rate is to test it with intense exercise. I’ll give you some ways to do that below. Fitness gadgets and apps (and, in the olden days, books and other sources of fitness advice), meanwhile, try to skip that step by using a formula that estimates your maximum heart rate based on your age. The most popular formula simply subtracts your age from 220.
But there are problems with that formula, and even with the alternative equations that have been proposed to replace it. There is no formula that can tell you what your own personal max heart rate actually is.
Why you shouldn’t trust any max heart rate formula
These formulas—no matter which one you choose—are one-size-fits-all calculations that will be roughly accurate for finding the average max heart rate of people of a given age. But it doesn’t matter what the average is, if you’re trying to figure out the max heart rate for you as an individual.
Think about how you shop for shoes. You don’t tell an app that you’re 5 feet 6 inches tall, and then trust it if it says that the average 5-foot-6 person wears a size 8 shoe. You need to try on different shoes, or at least measure your feet. Maybe you’re a size 6. Maybe you’re a size 9. It doesn’t matter at all what size the average person wears, because plenty of people will have larger or smaller feet than the average.
It’s the same for max heart rates. The idea of “calculating” a max heart rate has become so widespread that people assume the calculation is correct, or at least very close. But check out this graph from a 2012 study where researchers measured the actual maximum heart rates of over 3,000 people. (The lines represent two of the supposedly more accurate max heart rate formulas.)
If you’ve ever been confused about your own max heart rate, this will make you feel validated. For example, I’m 43, and the “220 minus your age” formula would have me believe my max is 177. But my harder workouts tend to see my heart rate somewhere in the 190’s, and I’ve clocked over 200 several times with a chest strap. Looking at this graph, heart rates of 200+ are certainly high for 43-year-olds, but they’re not unheard of and not even rare.
The same study also found that the formulas get even further from accurate as you get older. Check out the averages they found for different age groups:
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Age 19 to 29: 195 plus or minus 9.9
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Age 30 to 30: 189 plus or minus 10.1
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Age 40 to 45: 183 plus or minus 10.9
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Age 50 to 58: 176 plus or minus 11.6
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Age 60 to 69: 171 plus or minus 12.3
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Age 70+: 164 plus or minus 12.4
The “plus or minus” in this sense refers to a standard error, meaning that most people will fall within that range, but by no means all. So even the formula that is supposed to find the average for a given age is not keeping up with how people’s heart rates actually change with age.
And while I have a higher max heart rate than most people my age, there are also plenty of folks who have a lower heart rate than the formulas would predict. Bottom line, not only is there a wide range in what’s normal, but the common formulas get less and less accurate the older people get. I would not consider any heart rate calculation to be accurate enough for setting your own personal zones or exercise targets.
How can the formula be so wrong?
The “220 minus age” formula was based on observations from sparse data, as detailed in this paper on the history of the formula. That same paper notes that all of the formulas developed since then have error bars that are just as bad (plus or minus 10 beats per minute, or more in many cases—which matches the from above). “Currently, there is no acceptable method to estimate HRmax,” they wrote in 2002, and that’s still true today.
I feel like I should say something here about the mysteries of the heart being unknowable. (Surely some poet has beaten me to it.) The truth is that age alone doesn’t determine a person’s max heart rate, so no matter what numbers you use to create an age-based formula, you’re just not going to get a useful result. If there were other obvious relationships, like if your weight or exercise habits affected your max, surely a more accurate formula would be possible. But people are different in their heart rates for reasons we don’t entirely understand, just like they are different in their shoe sizes, so this doesn’t seem to be a problem that math can solve.
Unfortunately, the makers of smartwatches and other devices want to have some default zones to present when you begin using their product, even if you’ve never done a max-effort sprint in your life. That’s why we keep seeing these flawed numbers over and over again. I can understand the smartwatch makers’ dilemma: on the one hand, only an intense test of a workout will find your actual max. But on the other, most beginners won’t want to do an intense test and won’t have any sense of how to pace themselves appropriately to reach their max, anyway. People with cardiovascular, metabolic, or renal diseases shouldn’t do all-out exercise unless they check with a doctor first, anyway.
What to do instead of relying on a max heart rate calculation
Because of the problems above, reputable organizations have mostly backed away from the idea of “calculating” your max heart rate. Runner’s World took down their target heart rate calculator. The American Council on Exercise, one of the major organizations that issues personal training certifications, instructs trainers not to use max heart rate calculations, but to do real-world tests to help clients match their own heart rate to appropriate intensities of exercise. (I’m certified through ACE, and can confirm that this is what’s in the textbook.)
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If you’re a beginner, you don’t need a heart rate goal at all; if you can exercise without getting out of breath, you’re in a good “zone.” No need to overthink it.
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If you want to put a number on that, you can do a submaximal talk test. The informal way is to just notice what number you see on your watch when you can no longer speak comfortably; the more formal way is this treadmill test that accomplishes the same thing.
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If you really truly want to know your max HR, you can test it. I’d only recommend this for relatively experienced folks, and obviously skip this if a medical professional has ever told you to stick to lower-intensity exercise.
How to test your max heart rate
Although we tend to match “heart rate” to “effort,” a maximal exercise effort doesn’t always bring you up to your max heart rate. You need that hard effort to be sustained for at least a few minutes, as you push harder and harder.
That’s why it helps to be experienced at running races to do one of these tests. If you intend to run a fast mile, but you go too hard at the start and slow down by the end, your heart rate may never hit a true max. On the other hand, if you start at a challenging pace and then push the pace slightly as you go, finishing in an extended near-sprint as you approach the finish line, you’ll hit your max or something pretty close to it. An FTP test on a bike or a 5K race will often look exactly like that, so if you’re an experienced runner or cyclist, you can probably just look at the heart rate from your last hard race, and consider that number to be more or less your max.
All that said: you’re reading this far because you want a field test, yes? Here’s the usual protocol, most famously shared by Pete Pfitzinger in Advanced Marathoning.
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Jog for 10 to 15 minutes to warm up.
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Run hard up a moderately steep hill that is at least a quarter-mile long (Pfitzinger recommends a 600-meter hill).
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As soon as you get to the top, jog back down and repeat.
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After three repeats, the test is over. The highest number your watch recorded is your new max.
If you don’t have a smartwatch, you can measure your pulse with two fingers on your neck at the top of each hill repeat. The highest number you see is your max.
Note that your max heart rate for running may be different from your max heart rate for other sports, like cycling and, most notoriously, swimming. Your heart has to work harder to pump blood around when you’re upright versus horizontal. If you determine your max heart rate with a running test, then use that to guide pool workouts, you’ll be chasing numbers you can’t actually achieve in the pool.
All that said, heart rate numbers are only as good as the training they guide you to do, so whether you should use heart rate percentages to run your workouts depends on whether those mathematically guided workouts are helping you get faster, stronger and healthier. If you work best without numbers, that’s fine; if you do use numbers, make sure they’re accurate.
