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In this March 1, 2017 photo, Fitbit's new Alta HR device is displayed in New York. The company known for encouraging people to walk 10,000 steps each day, now wants to put them to sleep as well. (AP Photo/Mark Lennihan)

Your Fitbit may be lying to you. Here's why.


Wrist-worn activity trackers like Fitbit may not be the most reliable assessment of your heart rate, a new study found. After evaluating the accuracy of wearable devices, including Fitbit, Basis and Mio, researchers compared results to those collected from an electrocardiograph (EKG). Turns out, the activity trackers were much less accurate during exercise than at rest. 

The activity devices could overestimate heart rate by as much as 39 beats per minute, or underestimate it by 41 beats per minute.

Dr. Mitesh Patel, an assistant professor of medicine and health care management at the University of Pennsylvania, told CBS News, "These devices are probably good enough to inform consumers of general trends in their heart rate -- high or low -- [but] it’s important to have more accurate information when physicians are relying on this data to make decisions on medications or other tests and treatments."

But Lisa Cadmus-Bertram, an assistant professor of kinesiology at the University of Wisconsin at Madison and also the leader author of the study, downplayed the discrepancies. 

“At any moment, the tracker could be off by a fair bit. But at most moments, it won’t be,” she said. 

Patel offered an explanation for the devices' inaccurate measurements, claiming that the wrist-worn watches tend to move during exercise--thereby compromising results.

Fitbit released a statement in response to the study. It said, “We conducted extensive internal studies which show that Fitbit’s PurePulse technology performs to industry standard expectations for optical heart rate on the wrist. Fitbit devices were tested against properly calibrated industry standard devices like an EKG chest strap across the most popular activities performed worldwide -- including walking, running, biking, elliptical and more.”

The results of the recent study align with those of another study published last month at the American College of Cardiology annual meeting. Those researchers revealed that, depending on the type of activity, the devices were up to 34 beats per minute off.

Your Fitbit may be lying to you. Here's why.

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