Your Fitness Tracker Isn’t the Best Way to Measure Heart Rate

New research shows accuracy of wrist-worn heart monitors varies

Your Fitness Tracker isn't the Best Way to Measure Heart Rate

More and more fitness trackers that are worn on the wrist feature heart rate monitors in the name of helping you stay healthy. But results of a new study by a team of Cleveland Clinic physicians shows that they are not as accurate as chest-worn heart monitors.

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In a research letter published today in the journal JAMA Cardiology, the team, led by cardiac surgeon Marc Gillinov, MD, also found that some of the wrist-worn monitors were more accurate than others.

How these devices work

The chest-worn monitors, which are used by serious runners, distance cyclists and other elite athletes, work in a similar way to an electrocardiogram (EKG), which your physician uses for diagnostic purposes. Through the skin, both devices detect and measure the electrical signal that the heart transmits when it beats.

When you get an EKG, electrodes are attached to your skin and the EKG device, which records your heart’s electrical activity. A computer draws a picture on graph paper from information supplied by the electrodes.

The chest-worn heart monitor consists of two parts — a transmitter attached to a belt worn around the chest, and a receiver worn on the wrist like a watch. The transmitter picks up the electric signal and then sends an electromagnetic signal containing heart rate data to the wrist receiver, which displays the heart rate.

The fitness tracker heart rate monitors, on the other hand, use optical sensors to detect the blood coursing through your veins. They have some recognized drawbacks.

For example, because they’re on your wrist, optical heart rate sensors read your blood flow when it’s farther from your heart. Accuracy also can be reduced by light hitting the sensor as you move your arm or flex your wrist.

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Comparing the monitors

The researchers recruited 50 healthy adults without cardiovascular disease and randomly assigned each of them to wear two of four wrist-worn fitness devices with heart rate monitors — one on each wrist— while walking or running on a  treadmill. All the participants also wore standard EKG electrodes and a chest-worn monitor.

Heart rate was measured with participants resting and walking or running on the treadmill at various speeds. The participants exercised for three minutes at each setting, with heart rate recorded at the three-minute point. Recovery heart rate was also measured at 30, 60 and 90 seconds at the end of the three minutes.

The researchers found that the most accurate readings — as measured against the EKG — were from the chest-worn monitor, Dr. Gillinov says.

“If you really want to know your heart rate, wear the chest strap heart monitor because that senses electricity,” he says.

The value of fitness trackers

The wrist-worn monitors are fine for recreational use, Dr. Gillinov says. The researchers found that they were most accurate at measuring resting heart rates, but their accuracy diminished when used during exercise.

“These sorts of monitors are more of a consumer-driven device,” Dr. Gillinov says. “In the hospital, of course, we’re not going to rely on a wrist-worn monitor — and especially now that we know that they are not as accurate as an EKG.”

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Dr. Gillinov says that many cardiac patients use the wrist-worn heart monitors to stay within physician-recommended, safe heart rate thresholds during rehabilitation and exercise. The study shows that these patients should use a chest-worn monitor for the most precise information, Dr. Gillinov says.

Many cardiac patients will report to their physicians the readings from their wrist-worn monitors and ask if they are a reliable measurement, Dr. Gillinov says.

“The wrist-worn fitness devices that include heart rate monitors are incredibly popular,” he says. “Patients come in and say, ‘My heart rate was 170,’ and want to know, is that true? Are these devices accurate? Now we can answer that.”

Sometime patients will report getting an unusually high reading from their wrist-worn monitor. When that happens, Dr. Gillinov advises taking several measurements.

“Don’t make too much of a single reading, or even two readings,” he says. “Do several readings because you can’t count on these devices to be accurate every time.”