Fitness trackers measure heart rate well but off track on calories burnt

The most accurate device was off by 27% on average, the least accurate was off by 93% claims study

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

A team of researchers from the Stanford University in California evaluated the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2.

Sixty 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 per cent, none of the seven devices measured energy expenditure accurately.

Even the most accurate device was off by an average of 27 per cent and the least accurate was off by 93 per cent, 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 devices, 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.