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How Accurate Are Fitness Tracker Heart Rate Monitors?
A man running with a heart rate monitor watch.
Credit: Maridav, Shutterstock

Fitness trackers with heart-rate monitors are popular these days, but how accurate are they, really?

A new study put these devices to the test, and the heart-rate monitors' scores were not stellar — some of these wrist-worn heart-rate monitors were more accurate than others, but not one was as accurate as a chest strap monitor, the researchers found.

The results don't necessarily mean you should ditch your tracker. But if getting an accurate heart-rate measurement is very important to you, then you might want to consider a chest strap monitor, said study author Dr. Marc Gillinov, a cardiac surgeon at Cleveland Clinic.

"If you really must know your heart rate accurately — whether that's for health or training — a chest strap with an electrode is best," Gillinov told Live Science. [Who Has the Most Accurate Heart Rate Monitor?]

People who already have a wrist-worn heart-rate monitor may be aware that readings aren't always accurate. Gillinov advised users not to panic if they get a heart-rate reading that seems to be way too high or too low. "It may very well be incorrect," Gillinov said. If this happens, wait a moment and check the reading again, Gillinov said.

Commercial heart-rate monitors have traditionally used chest straps, which measure the heart's electrical activity with electrodes. But in recent years, many fitness tracker companies have added heart-rate monitors to their wrist-worn devices, and these use optical sensors, which detect light bouncing back from blood flow beneath the skin, to measure your pulse.

Although many studies have tested and confirmed the accuracy of chest-strap heart-rate monitors, there's been little study of wrist-worn monitors.

In the new study, Gillinov and colleagues tested the wrist-worn heart-rate monitors on four devices: the Apple Watch, the Fitbit Charge HR, the Mio Fuse and the Basis Peak.

Each of the 50 healthy adults wore two trackers at a time (one on each wrist) while walking on a treadmill at different speeds, from 2 mph up to 6 mph (3.2 to 9.7 km/h). The participants also wore a device from fitness tracker company Polar, called the H7 chest strap monitor, as well as electrodes used for a standard electrocardiogram (EKG) test, which also monitors the heart's electrical activity.

The measurements from the Polar chest strap and EKG were nearly identical, but the wrist-worn heart-rate monitors were not as accurate.

Of the four wrist-worn monitors, the Apple Watch and the Mio Fuse did the best. Most of their measurements fell within a range of 29 beats per minute (BPM) under the measurements from the EKG to 27 BPM above it. In contrast, the Fitbit Charge HR had measurements that ranged from 39 BPM under to 34 BPM above the measurements from the EKG, and the Basis Peak had measurements ranging from 33 BPM under to 39 BPM above the measurements from the EKG.

The Charge tended to underestimate heart rate, while the Peak tended to overestimate heart rate, the researchers noted. In general, the wrist-worn monitors were most accurate when the person was at rest, and their accuracy diminished as the wearer's activity level increased, the researchers said.

Wrist-worn heart-rate monitors use light to measure your pulse. They shine a light into the blood vessels in your wrist, and then detect the changes in blood volume that occur each time your heart beats and pushes blood through your body. This way of measuring heart rate can become challenging when people move around a lot (as they do when they exercise), because factors such as ambient light and the movement of a person's muscles can interfere with the measurements. Although companies have developed algorithms that cancel out a lot of the "noise" generated by people's movements, this noise has the potential to be a bigger problem for wrist-worn monitors than for chest straps that use electrodes, Gillinov said.

Gillinov said that right now, it's too early to use wrist-worn heart-rate monitors to guide therapy for patients with heart disease. People may go to their doctor with data from a wrist-worn heart-rate monitor, but Gillinov said he would still use an EKG or Holter monitor (a portable medical device that measures the heart's electrical activity) to investigate any potential heart problems further.

The new study was limited because it tested only four wrist-worn heart-rate monitors, and it only tracked patients when they were exercising on a treadmill. For an upcoming study, Gillinov and colleagues are studying the accuracy of more fitness trackers while participants perform other activities, such as biking, he said.

In a statement, Fitbit said that its trackers are not intended to be medical devices. The company's own tests showed that their heart rate monitor "performs to industry standard expectations" for optical heart rate monitoring on the wrist, and has an error of less than 6 bpm, on average, when compared to a chest strap.

"We believe optimal heart rate monitoring for those striving to reach their health and fitness goals is best achieved by taking heart rate across an extended time frame, such as during the entire task of interest, to get more reliable and meaningful outcomes," the statement said. "Instantaneous heart rate readings at a pre-determined time point can be more subject to error."

The Basis Peak was recalled in August because of the device's potential to cause blisters and burns on the skin.

The new study is published online today (Oct. 12) in the journal JAMA Cardiology.

Original article on Live Science.