If you are committed to keeping your heart in good shape, perhaps you already know how much heart-pumping aerobic exercise you should be getting each week. (For the record, it's at least 150 minutes of moderate-intensity exercise, or 75 minutes of vigorous-intensity exercise, or an equivalent combination).
But just how high should your heart rate rise during exercise? Should you aim for a specific target — and is it dangerous to go above your "maximum" heart rate? For answers to these and related questions, we asked Dr. Sawalla Guseh, director of the Cardiovascular Performance Program at Harvard-affiliated Massachusetts General Hospital.
Maximum and target heart rate, explained
The term maximum (or peak) heart rate refers to the upper limit of what your cardiovascular system can handle during physical activity, measured in beats per minute (bpm). Accurately determining this number requires a cardiopulmonary exercise test, which tracks how your lungs, heart, blood vessels, and muscles react during an exercise challenge. However, it's more practical to estimate your maximum heart rate. The usual formula is 220 minus your age.
Be aware, though, that this just gives you a ballpark figure. "We know the ability to raise your heart rate tends to decrease with age. But using age alone really isn't the best metric for determining your maximum heart rate," says Dr. Guseh. Created in the early 1970s, the formula was based on testing done on men and tends to overestimate peak heart rates in women. And estimates for all individuals may be off by as much as 15 bpm, he adds.
Target heart rates are expressed as a percentage of your maximum heart rate. For example, if you're 65, your estimated maximum heart rate is 220 minus 65, or 155 bpm. For moderate-intensity exercise, your target heart rate range is 64% to 76% of that (99 to 118 bpm). For vigorous-intensity exercise, your target range is around 77% to 93% (119 to 144 bpm). Of course, popular wrist-worn devices can do these calculations for you. But these gadgets have limitations (see "Heart rate tracking: The good, the bad, and the uncertain") and aren't really necessary.
Heart rate tracking: The good, the bad, and the uncertain
Most smart watches and wearable fitness trackers use your age to estimate your target heart rate zones (see main story). These devices periodically measure your heartbeat throughout the day, using optical sensors that detect light bouncing back from the blood flowing beneath the skin of your inner wrist.
For some people, working hard to reach their target heart rate motivates them to exercise more and further boosts their fitness levels, says Harvard cardiologist Dr. Sawalla Guseh. But if you're worried about your heart rate for whatever reason, checking the number too often might make you even more anxious, he says.
In addition, some studies suggest that the heart rate estimates from wrist-worn devices may be less accurate in people with darker skin compared with people whose skin is lighter. If you have one of these devices, you can check its accuracy by taking your pulse manually, ideally at rest and during activities of different levels of intensity. Press your index and middle fingers together on your wrist, just below the base of your thumb. Count the number of beats in 15 seconds and multiply by four to get beats per minute.
Try the talk test
Remember that heart rate targets are simply guides, and there's no medical evidence that healthy people need to exercise at specific heart rates. But knowing the range that's considered moderate or vigorous for you can help you determine whether you're meeting the recommended exercise goals and can be used to guide training to enhance your fitness or performance, says Dr. Guseh. Another option is to simply use the "talk test." If you're breathing faster than usual but can still carry on a conversation, you're exercising at moderate intensity. If you have trouble finishing a sentence, you're doing vigorous-intensity exercise.
If you're new to exercise, start slowly and build up to more vigorous exercise gradually. If you're already an avid exerciser, it's usually not a problem to reach or even exceed your maximum heart rate for short periods of time. However, people who have or are at risk for heart disease should be more cautious and check with their clinician about how to exercise safely.
What else affects heart rate?
A normal resting heart rate is between 60 and 100 bpm. The best time to check yours is first thing in the morning, before you even get out of bed. Other than exercise, factors that can elevate your heart rate include hot weather, dehydration, being at a high altitude, or feeling nervous or excited. Certain drugs (such as stimulants) and some medical conditions (anemia, asthma, infections, and fevers) can also cause your heart rate to rise.
Your heart rate usually dips during sleep, sometimes dropping as slow as 40 to 50 bpm, especially if your resting heart rate is on the low end. Medical problems that may lower your heart rate include heart disease, low thyroid function, and high blood levels of potassium. Beta blockers, a class of medications used to treat high blood pressure and other heart conditions, also slow down the heart; examples include metoprolol (Lopressor, Toprol) and carvedilol (Coreg).
Highly trained athletes often have low resting heart rates, sometimes in the 40s or even 30s, says Dr. Guseh. "After years of training, their hearts undergo what's known as exercise-induced cardiac remodeling. Because their hearts are bigger, each beat pumps more blood, so the heart doesn't have to beat as often to supply the body with blood," he says.
Image: © Cultura RM Exclusive/yellowdog/Getty Images