Your fitness tracker is 27% to 93% inaccurate: Stanford

New study finds none of the popular devices count calories accurately

NEW YORK: Your fitness tracker may accurately measure heart rate but misguide you on the number of calories you burnt, researchers say.

A team of researchers from Stanford University in California evaluated the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2. 60 volunteers, including 31 women, wore the seven devices while walking or running on treadmills or using stationary bicycles. Each volunteer's heart was measured with a medical-grade electrocardiograph.



Results from the wearable devices were then compared with a medical-grade electrocardiograph - the "gold standard" for measuring heart rate. While these devices measured heart rate with an error rate of less than 5%, none of the seven devices measured energy expenditure accurately.
Even the most accurate device was off by an average of 27% and the least accurate was off by 93%, the researchers said.

"The heart rate measurements performed far better than we expected, but the energy expenditure measures were way off the mark. The magnitude of just how bad they were surprised me," said Euan Ashley, Professor at Stanford University.

As these devices are not up to the same standards as medical-grade ones, it's hard for doctors to know what to make of heart-rate data and other data from a patient's wearable device, Ashley said.



The findings were published in the Journal of Personalized Medicine. Although manufacturers test the accuracy of activity devices extensively, it's hard for consumers to know how accurate such information is or the process that the manufacturers used in testing the devices.

The take-home message is that a user can pretty much rely on a fitness tracker's heart rate measurements. But basing the number of doughnuts you eat on how many calories your device says you burned is a really bad idea, Ashley said.

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