Here's how to tell if your wearable tech is giving you the information you truly need.
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woman checking heart rate on fitness watch
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Wearable exercise trackers and watches offer heart rate information to help you target your peak heart rate zone when you're working out and to monitor your average resting heart rate, but they can also alert you to cardiac issues. Dr. Raj Khandwalla, assistant professor and cardiologist in the Smidt Heart Institute at Cedars-Sinai, answers five important questions to help you understand what your fitness tracker is telling you—and what to do about it.

How does my watch figure out my heart rate?

As your heart pumps faster, the amount of hemoglobin—a protein that ferries oxygen—in your bloodstream increases. Fitness trackers emit red LED light, which is absorbed through your skin and reflected back to a small camera on the back of the tracker. "As you have more blood flow, which occurs with every heartbeat, you have a change in how much light is reflected," says Dr. Khandwalla, since the increased hemoglobin reflects light differently. "A photo diode picks up these changes in absorption. All of that is processed by the watch, and the changes in reflection are converted into heart rate." (This is different from how your tracker measures your steps—an internal gyroscope that tracks movement up, down, and side to side is responsible for that number.)

How accurate is it?

While the most accurate way to read a heart rate is via an echocardiogram, fitness watches do provide "a fairly accurate" picture, says Dr. Khandwalla. In general, trackers measure your heart rate equally well during any type of exercise, but external factors typical of aerobic workouts can interfere with the reading: "As you get your heart rate up, the tracker is making less contact with your skin because you're moving around, there's sweat," notes Dr. Khandwalla. "There's probably more accuracy on a stationary bike than in an HIIT workout."

How can that information help my workouts?

Fitness trackers combine data from the movement-tracking gyroscope with assessments of your heart rate to determine whether or not you're exercising. Some trackers also suggest different target heart rate zones to maximize your fitness results, but those are broad guidelines. "When exercising, your peak heart rate should be 220 minus your age," explains Dr. Khandwalla. "A specific heart rate goal has to do with your workout goals: If you're looking to do high intensity, then you want short spurts of a really high heart rate and then it goes down; that's a good way to build your exercise fitness and improve your cardiac performance. Other folks will have sustained higher heart rates for a longer period of time if they go running or are riding a bike."

Why is my tracker alerting me to a raised heart rate?

Some fitness trackers notify you when your heart rate doesn't seem to match your level of activity—especially when it's clocking a raised heart rate without accompanying movement, or if the rhythm of your heartbeat is irregular. If an alert from your watch occurs alongside other symptoms—chest pain, lightheadedness, heart palpitations—that's cause for immediate medical attention, says Dr. Khandwalla. If your heart rate seems fast but its rhythm is normal, and you feel anxious, or you're sick, then you can monitor your symptoms before rushing to the emergency room.

How much should I depend on the heart rate tracker?

While tracking your heart rate can help you make the most of your workouts, it shouldn't overpower how you feel while exercising; listening to your body's cues about how hard you should be working is more important than reaching a specific heart rate, shares Dr. Khandwalla. "I wouldn't get overly fixated on heart rate—I think it's more important to see trends over time," he says. "It's important that you start slow. If you're inactive and starting for the first time, you don't have to set any records—you can slowly increase the amount that you exercise." As you become more fit, you should also see a lower average resting heart rate—which means your efforts are paying off. "If you are able to bring your resting heart rate down through exercise," says Khandwalla, "that is a sign that you are getting healthier and your heart is getting healthier."

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