You may not get an accurate heart rate from your wrist-worn fitness tracker, a new study suggess.
Though these devices may have some legitimate benefits, don’t rely on them for medical purposes.
Researchers evaluated four fitness trackers from Fitbit, Basis and Mio, and compared results to those from an electrocardiograph. They found results varied among the different models, and all of them were much less accurate during exercise than when wearers were resting.
“At any moment, the tracker could be off by a fair bit. But at most moments, it won’t be,” said Lisa Cadmus-Bertram, an assistant professor of kinesiology at the University of Wisconsin at Madison and the study’s lead author. “The heart-rate feature performed better at rest. They’re not as precise during exercise.”
For the study, 40 healthy adults, aged 30 to 65, tested the Fitbit Surge, Fitbit Charge, Basis Peak and Mio Fuse. At rest, the Fitbit Surge was most accurate, while the Basis Peak was least accurate.
During exercise on a treadmill at 65 percent of maximum heart rate—220 beats per minute minus age—the trackers were the least accurate, overestimating heart rate by as much as 39 beats per minute or underestimating it by as much as 41 beats per minute.
The findings back up the findings of a different study released last month. In that study, the devices were off by up to 34 beats per minute and were least accurate while participants exercised.
Wrist-worn activity trackers generally use a light-emitting diode, or LED, that measures heart rate by detecting changes in the amount of blood in the skin. Experts believe accuracy may be compromised when the devices move around on the arm, especially during exercise.
Fitbit’s maker issued a statement saying its fitness trackers aren’t intended to be medical devices. ”We conducted extensive internal studies which show that Fitbit’s PurePulse technology performs to industry standard expectations for optical heart rate on the wrist,” the statement said. “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.”
One caution: The data for the most recent study were collected about a year ago.
“Not only have newer models since been released, but the algorithms behind the data are presumably being updated and improved on a regular basis,” Cadmus-Bertram said. “So the results we found might be different if we did the study again now.”
And she’s not suggesting you give up your Fitbit. “On the whole, fitness trackers still provide a tremendous amount of useful information to the average user who just wants some feedback to help them to increase their exercise level.”
About 20 percent of American adults own a wearable activity tracker.
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