Fitness trackers perform poorly when calculating calories burned

Fitness trackers

Fitness trackers are able to track heart rate accurately but not the number of calories burned, new research suggests.

A team from Stanford University asked 29 men and 31 women to wear several fitness trackers on their wrists and run, cycle and walk, in order to test out the accuracy of the wearable devices.

Seven fitness trackers were used – the Apple Watch, Samsung Gear S2, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Basis Peak – and apart from the Samsung Gear S2, they all had error rates below 5 per cent for heart rate measurements. The Gear S2 had an error rate of 6.8 per cent.

However, the error rates were drastically different with regards to measuring energy expenditure. The most accurate was the Fitbit Surge (27.4 per cent error rate), while the PulseOn fitness tracker performed worst with an error rate of 92.6 per cent, although there was no data for the Gear S2 on calories burned.

Euan Ashley, study co-author, said: “We were pleasantly surprised at how well the heart rate did. Under many circumstances for most of the devices, they actually did really quite well.

“At the same time we were unpleasantly surprised at how poor the calorie estimates were for the devices. They were really all over the map.”

Ashley and his team stressed that the number of calories fitness trackers say we have burned should be taken “with more than a pinch of salt”.

“People need to know that on energy expenditure, they give rough estimates. If you go to the gym and you think you’ve lost 400 calories, you might feel you’ve got 400 calories to play with.

Energy expenditure differs greatly from person to person, affected by factors such as height, weight, gender, body fat percentage and the type and intensity of exercise.

The findings were published in the Journal of Personalised Medicine.

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