Out of Sync: Fitness Trackers Don’t Agree When It Comes to Measuring Heart Rate

Activity trackers worn on the wrist are more popular than ever, but how good are they at actually measuring your heart rate? Apparently not so hot, according to a new study that found significant variability among four popular models on the market. 

For some people, these inaccurate heart-rate readings can cause unnecessary anxiety and trips to the physician, senior author Marc Gillinov, MD (Cleveland Clinic, Cleveland, OH), told TCTMD.

“We see a lot of cardiac patients who have these devices and many of them report erroneous readings and [make them] really worried,” he said. “We set out to look at data validating these wrist-worn monitors and found that there was almost none.”

Led by his Cleveland Clinic colleague Robert Wang, MD, the researchers conducted their own study by comparing fitness trackers with standard ECG limb leads and a chest strap monitor in 50 healthy adults. The four models studied were the Apple Watch (Apple), Mio Fuse (Mio Global), Fitbit Charge HR (Fitbit), and the now-defunct Basis Peak (Basis). Volunteers had their heart rate assessed at rest and while exercising at 2 to 6 mph on a treadmill for 3-minute intervals.

Different Technologies, Different Results

The study, published online October 12, 2016, as a research letter in JAMA Cardiology, found that none of the trackers were as accurate as the ECG. The Apple Watch and Mio Fuse agreed with the ECG 91% of the time, while the Fitbit and Basis were in agreement 84% and 83%, respectively. Trackers were more accurate when a person was at rest than while exercising, likely due to some loss of contact between the skin and the sensor when a person’s arms are moving.

Unlike an ECG, which measures electrical impulses, the activity trackers measure heart rate with light, Gillinov noted. “They shine light into the tissue and determine how much light is absorbed or reflected back and [that] amount of light is related to blood flow, which is related to your heart rate. [An ECG and an activity tracker] are two very different technologies.”

Gillinov said there are two main messages to the study. “The first is that if a person really must know his or her heart rate, use a chest strap,” he said. Examples would be a cardiac patient or someone who has been given a “safe heart-rate range” for exercise. “Message number two is if you use one of these wrist-worn monitors and you do see a value that is out of range, don’t panic. Relax, and check it again because the monitor probably is incorrect. This [variability] is not dangerous for most of us,” Gillinov observed.

For now, the results are only applicable to walking on a treadmill, but Gillinov said his group plans to study more of these devices in different exercise settings such as biking and elliptical training, as well as in different populations with risk factors or established health problems.