The Fitbit has been found to overestimate calories burned, in a US study.
The Fitbit has been found to overestimate calories burned, in a US study.

Fitness trackers 'way off mark', study finds

FITNESS trackers worn on the wrist to record physical activity significantly overestimate calories burned, a new study today suggests.

The least accurate device reported users had exerted up to twice as much energy as they really had.

But all seven models tested - including the Apple Watch, Samsung Gear S2 and Fitbit Surge - were "way off the mark",  the study found.

It means users may fail to achieve their target levels of exercise and struggle to lose weight as quickly as they expect.

Researchers from Stanford University, California, tested the trackers on 60 volunteers as they sat still, walked, ran and cycled.

The results were compared with a "gold standard", obtained using medical-grade equipment.

Six of the devices measured heart rate with an error rate of less than 5%, which the academics considered acceptable.

But none measured energy expenditure accurately, with even the best performer being off by an average of 27%.

The worst,  the PulseOn, was off by an average of 93%.

Apple Watch
Basis Peak
Fitbit Surge
Microsoft Band
Mio Alpha 2
Samsung Gear S2

Devices reported the lowest error for cycling and the highest for walking, the Journal of Personalised Medicine reports.

Errors were also greater for men, people with a higher body mass index, and those with darker skin.

Anna Shcherbina, who worked on the study, 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,"  Ms Shcherbina said.

The devices measure heart rate directly but energy expenditure must be calculated using a combination of factors.

Each device uses its own algorithm to estimate calories burned.

Ms Shcherbina said: "It's very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable based on someone's fitness level, height and weight, etc."

The researchers "suggest caution" in the use of the devices as part of health improvement programs and called for manufacturers to routinely test them against a set standard.