I’ve always kept track of my mileage and time spent on feet, but I hadn’t paid much attention to any other stats until 2025, when I was returning to running after a three-month hiatus due to an injury. Wanting to proceed safely, I tried to make my comeback slow, committing to only easy runs and rebuilding my base.
I figured the best way to make sure I was where I needed to be was to run by my heart rate. I used the generally accepted formula MHR=220–age, which, being 32 at that time, put my max heart rate at 188, and my supposed zone 2 anywhere between 113 and 132 beats per minute (BPM).
But no matter how slow I felt like I was running, I was unable to keep my easy runs in my predetermined zone 2. My watch would categorize every one of my runs as zone 3 or even 4—usually a combination of the two (and no zone 2). First I thought that perhaps that’s what rebuilding fitness from scratch is like, but when it hadn’t improved over time, I became confused—I wasn’t exhausted, my breathing was even, my pace was mostly conversational.
If my runs felt easy, why was my watch insisting they weren’t?
Maybe it’s the tech, I thought, so for even more precise data, I upgraded to a clip-on heart rate monitor and swapped my Garmin Forerunner 265S for the 570. Then I wondered, maybe there is something else going on with me that could explain the elevated heart rate. I started obsessing over various data points my watch was serving me—heart rate, max heart rate, heart rate variation—to understand what was going on in my body.
I would not rest until I figured it out!
The Problem Starts With the MHR Formula
I first questioned the very formula that threw me for a loop: MHR=220–age. It’s attributed to Samuel S. Fox and William L. Haskell, who published it in 1971, and which the Journal of Exercise Physiology later described as “a superficial estimate, based on observation, of a linear best fit to a series of raw and mean data.” Fox and Haskell’s study wasn’t original but rather a compilation of 11 existing ones, which included data from smokers and cardiac patients—but also very few women.
The American College of Cardiology, after reviewing the archaic formula and analyzing over 25,000 stress tests, concluded the formula doesn’t account for sex differences. A woman’s heart, on average, weighs about 2 ounces less than a man’s, and therefore beats about eight times more per minute to allow it to maintain cardiac output since it pumps less blood per beat. Women experience a more gradual decline in MHR, which leads to the formula’s overestimation of MHR in younger women and underestimation in older women. A 2014 study by the American College of Cardiology added that “younger men have a lower resting heart rate and higher peak heart rate than women and men’s heart rates rise more dramatically during exercise and return to normal more quickly after stopping.”
While many other formulas exist, Fox and Haskell’s is still the most widely accepted as accurate—although because our individual MHRs differ drastically, any formula will only provide a rough estimate.
Smartwatch manufacturers like Garmin, for example, employ it as a starting point. “Formula initializes according to 220–age if we have no other information, such as in the case of a brand new user,” wrote Garmin’s Lead Product Manager, Joe Heyes, in an email, adding that default heart rate zones follow 10 percent increases starting at 50 percent for zone 1. “As long as the Auto Detection setting is on (which is the default), the max heart rate can start adjusting right away, but to set expectations, it can take a while (weeks) for max heart rate to drift to its true value.” Garmin also allows you to input the number manually if you know your precise MHR.
TIP: If you’re a Garmin user, you can find your heart rate stats by opening the Garmin Connect app, tapping More → Garmin Devices → Your Device → User Profile → Heart Rate & Power Zones.
How do you actually find out your MHR? The most precise way to measure it is in a lab with a VO2 Max test, says Tamanna Singh, MD, director of the Sports Cardiology Center at Cleveland Clinic. If you’d like to figure out your MHR outside of a lab, you could do a 30-minute run at threshold (a little bit faster than your race speed) and increase progression, then average your heart rate over the last 10 to 15 minutes, she suggests.
The Problem Continues with the Heart Rate Zones
Let’s say we figured out the MHR by going for a run. Next up would be determining the heart rate zones. But if everything about our hearts is so different, shouldn’t they also vary and not precisely follow the commonly used five heart rate zones that increase by 10 percent from 50 to 100 percent of MHR (zone 1: 50-60 percent of MHR, zone 2: 60-70 percent of MHR, etc)?
“Heart rate zones are individual, and our current algorithms do not account for that,” Singh says. “It certainly would be inaccurate to believe that we all fit nicely into these 10 percent increments.” Heart rate zones should be used only as an approximation, and you should not drive yourself crazy by trying to fit within them, she adds. “Those formulas are really helpful for a guide, but I always hesitate to say ‘train to a specific number’ because day-to-day everything is different.”
Brant Stachel, professional coach and psychotherapist based in Kingston, Ontario, agrees; each zone has to start and end somewhere, so being a few beats off doesn’t mean much. “There is nothing physiologically different between a heart rate of 140 and zone 1, and a heart rate of 141 and zone 2,” he says. “There’s no switch to all of a sudden completely different metabolism.”
There are, of course, different training philosophies out there, some of which, for example, shift all five zones and start at 55 percent of MHR for zone 1, while others follow the 80/20 rules of splitting runs into easy and hard and keeping a heart rate below 75-80 percent of MHR for the easy portion.
But to me, all of them are an extension of a guessing game that still builds rigid borders around my zones. That’s when I learned a better way to approach heart rate zones that doesn’t use any formula at all.
The Focus Shouldn’t Be on Heart Rate—But Rather on Effort
Singh says that, like me, many of her patients come to her with worries about their heart rate being outside of the averages. “What I usually will counsel my patients on is trusting and learning to communicate with your body,” Singh says, “remembering that the relationship you have is between you and running, not between running and a watch.”
Sport Psychology Coach Emily Saul gives her athletes permission to pay a lot less attention to their watches. “I don’t have to know exactly what my heart rate is, for example, in order to know whether it feels comfortable or too high or lower than usual—it sort of gives me indication of my perception of effort,” Saul says. “And if I focus on allowing myself to perceive that information, I then can choose to use my watch as a reference point, but certainly not the only source of information—then the watch becomes a tool rather than a God.”
Stachel likes to remind his athletes that people from the ’80s still ran very fast with none of the technology we have today. “I want to frame it that way first: it isn’t a mandate or a requisite to have this data. Can it be helpful? Yes, absolutely.” In his coaching, he accounts for some of the data but is equally interested in his athletes’ assessments of their runs. “I try to teach my athletes that we want to triangulate across multiple sources: I want to see pace, I want to see heart rate, and I want to hear RPE.”
Rate of perceived exertion, a subjective 1-10 scale of how a workout feels, offers an alternative way to evaluate runs. It correlates with breathing and the ease with which a runner can carry on a conversation.
- 0: No exertion
- 1: Very light exertion
- 2 – 3: Light exertion (natural breath, can talk)
- 4 – 5: Moderate (breathing more labored, can talk in short spurts)
- 6 – 7: High, vigorous exertion
- 8 – 9: Very hard exertion (breathing heavily, can barely talk)
- 10: Maximum, all out (can barely breathe, can’t talk)
This scale can replace heart rate zones and help you learn to trust your body’s signals. That’s exactly what Singh, Saul, and Stachel have been getting at with running by effort and trusting my body. While I never thought of my runs in terms of RPE, I had run by feel before my injury—all I had to do was go back to that and relate my runs on the training plan to the RPE scale.
The Bottom Line? Define Your Own Zone 2
We’re each different, so our bodies and hearts respond differently to exercise—and while my friend, who runs similar paces to me, may have a heart rate in the 130s on our joined easy runs, mine can be much higher. “To focus on it as: because [your heart rate] is higher than other people’s, there is something wrong, that just produces an unhelpful effect—the reaction that your body has is one of needing to protect or guard, rather than simply be engaged in the running,” Saul says. “If you have a history of having a running heart rate in the 160s and if you have the medical information that there’s nothing wrong with your heart, then we’ve got a big body of evidence that says that’s pretty normal for your body.”
From the psychological point of view, stressing over not fitting neatly into running zones can create all kinds of anxiety. “If we can reframe that… there isn’t something wrong with you, we can then focus on a belief that really amplifies your ability: your body doing what it is able to do in a healthy way so that we get a different response… which is one of feeling capable, empowered, and good,” Saul adds.
Just because my personality type requires examining all accessible layers of validation, I ended up going as far back as Strava would allow, to my pre-Garmin era of wearing an Apple Watch, comparing paces and heart rates at various parts of my previous trainings. It turns out my heart rate was simply always high. So I adjusted my Garmin heart rate zones manually, for my mental peace, and returned to running by feel—or RPE—trusting that if my run feels easy, it doesn’t matter what any tech says about my heart rate.
Pavlína Černá, an RRCA-certified run coach and cycling enthusiast, has been with Runner’s World, Bicycling, and Popular Mechanics since August 2021. When she doesn’t edit, she writes; when she doesn’t write, she reads or translates. In whatever time she has left, you can find her outside running, riding, or roller-skating to the beat of one of the many audiobooks on her TBL list.