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Some people who rely on fitness trackers to see how hard they work out may want to rethink this approach, according to a small study that suggests the increasingly popular devices may get more accurate heart rate readings when users are at rest than during exercise.

The study tested four popular wristbands, each of which has a light-emitting diode (LED) that measures heart rate from tiny changes in skin blood volumes by using light reflected from the skin.

Participants in the study – 40 healthy adults – wore two trackers on each wrist and compared resting and exercise heart rate readings on the devices to the gold standard used by doctors: an electrocardiogram (ECG or EKG) test.

At rest, the Fitbit Surge got heart rate measurements that most closely matched the ECG results, and the Basis Peak was furthest off. In tests that also included the Fitbit Charge and Mio Fuse, none of the trackers got exercise heart rate readings that came close to the ECG.

These results suggest that while the trackers may help monitor daily activity, it’s not clear the heart rate readouts would be accurate enough to help patients with certain health problems make medical decisions, the authors note in Annals of Internal Medicine.

“At any moment, the tracker could be off by a fair bit, but at most moments, it won’t be,” said lead study author Lisa Cadmus-Bertram of the University of Wisconsin in Madison.

“This is why our paper doesn’t suggest that the commercial trackers we tested would be sufficient for medical applications where high precision is needed during exercise,” Cadmus-Bertram said by email. “Yet for the typical recreational user, they may still provide feedback that’s useful and motivational.”

To assess the accuracy of the trackers, researchers examined heart rate data for participants who were 49 years old on average and slightly overweight.

First, they looked at the amount of agreement between the readings from the trackers and…



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