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Although she thinks about hearts all day, Dr. Allison Zielinski, a cardiologist at Northwestern Medicine and co-director of the sports cardiology program at Northwestern, in Chicago, admits that she doesn’t use heart rate zones in her own training—she’s completed six marathons, and is currently training for an Ironman 70.3. Often, patients ask her “What should my heart rate be while running?” She can’t give one solid answer. “It really depends on what your goals are,” she says.
Some coaches and training plans prescribe training based solely on heart rate, so instead of telling you to run at a specific pace (such as 9:30 per mile or the pace you could run for a 5K), they’ll give you a target heart rate zone to stick to. Heather Hart, an ACSM-certified exercise physiologist and co-founder of Hart Strength and Endurance Coaching in Myrtle Beach, South Carolina, has trained this way before herself. She recommends it to newer runners who are learning how to tune into their own levels of effort and exertion, or those who are moving up in distance. Athletes who’ve dealt with injury and burnout, or who have plateaued in their performance and aren’t sure how to make progress, also may benefit.
Athena Farias, an exercise physiologist and running coach at Get Fit SATX in San Antonio, has sometimes found new runners get overwhelmed by the amount of data involved in heart rate training. In her experience, the method works better for athletes who are more advanced, love crunching numbers, and have a nuanced understanding of the factors that can influence heart rate.
How Heart Rate Training Works
Most heart rate-based plans use three, four, or five zones, calculated either as a percentage of your maximum heart rate or your lactate threshold heart rate (see how to estimate these, above). You’ll receive specific instructions on which zones to target for each workout.
Easy runs boost blood flow through your body, and also enhance your aerobic fitness—prompting your body to grow new blood vessels and mitochondria, the tiny power factories inside your muscle cells that produce energy. Fast runs, meanwhile, are where you improve your lactate threshold—and super-hard ones, near your maximum heart rate, recruit and enhance your fast-twitch muscle fibers.
Generally, Hart says, heart rate training plans instruct you to spend at least 80 percent of your time in easy zones, and only 20 percent in harder zones. It often takes a little time, about four to six weeks, for runners to adapt to measuring their intensity by heart rate. Typically, the biggest struggle is actually slowing down enough on easy runs to keep your heart rate low, something Hart found herself. But once things click, you may find you recover better, can go even faster on hard efforts, and make progress toward your goals—and stay happier while doing it.
“Once people experience it, they realize how good they feel—training doesn’t have to hurt all the time, and you don’t have to feel burnt out or exhausted or sore seven days a week,” Hart says. “My personal number-one goal is to help clients get to their starting line injury-free and healthy. To me, heart rate training is a fantastic way to keep clients honest with themselves about their efforts so that we do get there healthy.”
That said, there are some runners who should probably steer clear of heart rate training. This includes athletes:
- With a pre-existing cardiac issue that can affect heart rate
- Taking medicines that affect heart rate, such as asthma inhalers or beta blockers
- Who are going through a very stressful period of life, or a time like perimenopause, when heart rate fluctuates more
- Whose heart rates are so high that any running kicks them into a high-intensity zone
It’s also tougher to do if you’re in a hot, humid climate—like Farias is in San Antonio—or if you’re training on trails, where uneven terrain makes it challenging to keep a steady effort. And if you live at altitude, you’ll need to adjust your calculations—between 10 and 30 percent, based on the exact height you’re at and your body’s response to the difference.
Make Heart Rate Training Easier
Even if you don’t do all your training in prescribed zones, you can still use these numbers to guide your efforts. Neely Spence Gracey, an elite runner and coach, for instance, watches her heart rate instead of her pace on easy runs, just to make sure she’s not going too fast. She recommends her athletes do this too, especially when they’re coming back from pregnancy or injury, ramping up their mileage or intensity, or struggling to hit their paces in faster workouts.
Whether you’re doing every run by heart rate or just checking in every once in a while, try these tips to make the process simpler and less burdensome:
- Flip your screen. Set up your watch to give you only heart rate, rather than distance or pace. That way, you’re not worrying about details that don’t matter.
- That said, don’t obsess. Staring at your watch to make sure you’re in the right zone can actually increase your heart rate on its own, Hart says. Besides, any single reading might not be accurate. Instead, glance down every once in a while to make sure you’re generally in the right zone. And pair it with another measure—such as the “talk test,” or whether you can speak while running—to gauge your efforts.
- Set alerts. Another option: Let your watch check you instead. On many watches, you can set a heart rate goal, and have your device buzz or beep when you move outside your desired zone.
- Allow wiggle room. Even with the most precise chest strap, your heart rate may vary based on external and internal factors. And on shorter, faster runs, it’s nearly impossible to get a good reading quickly enough to gauge and adjust your effort in the moment. To get the most out of your data, looking at it afterward instead of worrying too much in the moment, and work to sync up your heart rate with a measure like your rate of perceived exertion (how hard are you running, from one to 10).
Gracey also uses resting heart rate to guide her progress and keep an eye out for burnout. Check yours every morning—if you don’t have a smart watch, simply take your pulse as soon as possible after you wake up (or after you’ve gone to the bathroom, since a full bladder can be a stressor). If it’s 10 beats per minute higher than normal, stick to easy running for that day. If it’s elevated by 20 (or more), consider a complete rest day.
Heart rate variability can be used for similar purposes. Devices like the Whoop use this measure to assess your recovery, then tell you if you’re ready to train hard again or need more rest, advice that can help you stay healthy and prevent overtraining. That’s, of course, assuming you’re committed to following it, and not powering through when it’s better to skip a run or workout.
How Accurate are Heart Rate Monitors?
True, you can always check your heart rate the old-fashioned way: By putting your finger on your neck or your wrist and counting. But since that’s not exactly practical to do on a run, there’s an ever-expanding array of wearables and technology to track it for you.
The most ubiquitous are the wrist-based monitors in Apple Watches, Garmin GPS devices, and Fitbit fitness trackers, among others. Most use a technology called photoplethysmography, or PPG, to track your heart rate, says Ben Nelson, a clinical research scientist at Meru Health who’s studied the accuracy of wearables in tracking and improving physical and mental health.
“It’s basically shining a light into your wrist, and then reading how much light is refracted back up,” he says. The light patterns change as blood flows through your veins with each pulse. From there, the device uses an algorithm to convert the raw signals from the light into a number you can understand, like beats per minute.
While studies suggest many of these products are relatively accurate at calculating heart rate and continuously improving, Nelson offers a few caveats. For one thing, the more you move, the greater the amount of “motion artifacts”—flicks of your wrist or other random movements that the algorithms have to strip out to get an accurate reading. The problem can be worse if you wear the device on your dominant wrist, which you tend to move even more. You can usually mark this in your settings, so companies’ calculations account for it, but it still opens up more room for error.
Sweat and humidity, meanwhile, can cause the watch to slip or interfere with the readings. So can bright light, especially if the fit of your watch is a little bit loose. One other issue—it’s a big one—is that research suggests PPG technology doesn’t work as well for Black people, or anyone whose skin contains more of the pigment melanin. “As the green light is reflected into our skin, more of it is absorbed by darker skin tones,” Nelson says. “So there’s less that’s actually refracted back up to the sensor.” Tattoos and arm hair, too, can interfere with accurate readings.
So while a watch or wrist strap represents a good-enough way to track resting heart rate for most people—and Dr. Zielinski often looks at such data when she’s treating patients—it’s not nearly as accurate when you’re running, Nelson says. Chest straps actually read the heart’s electrical signals, so they’re far more precise, even in motion. And while dampness and sweat can derail a wrist monitor, wetness is actually essential for chest monitors to properly conduct electricity, Davenport says.
“Bottom line, if you can use a chest strap, you’re going to get better results with your heart rate readings,” Hart says. However, she knows they can chafe or cause problems with the fit of your sports bra. “I totally understand why people might not want to wear them.”
If you are going to use a wrist-based monitor, make sure it fits properly. “Find that sweet spot where it’s tight enough that it can read, but not uncomfortable,” says Kimberly Gandler, human performance manager at the Memorial Hermann IRONMAN Sports Medicine Institute in Houston. “It shouldn’t be sliding around too much, and in a consistent place.” Super-sweaty? Take a few minutes to wipe it down, especially if your readings don’t fit your general pattern.
Over time, Nelson expects even more options. Some devices already use other colors of light besides green, such as red—and while it’s not clear yet, it’s possible those hues will work better on darker skin, he says. And companies are experimenting with different positioning, from smart rings to earbuds with heart rate monitors built in, like the Jabra Sport Pulse. Although the earbuds often use PPG technology, your head tends to move less than your wrist does, so the readings might be more consistent.
“I think these wearables are phenomenal as they are right now, and I think they’ll just continue getting more and more accurate,” he says. “I use these devices. I love them. And I think they’re going to play a larger and larger role in sports and healthcare.