Many wearable fitness trackers are failing to accurately measure the number of calories people are burning, according to a new study by Stanford University.
While the fitness trackers are good at measuring heart rate, the researchers were unimpressed by their inability to accurately determine the amount of calories being burned by individuals.
A total of 60 volunteers (29 men and 31 women) were asked to walk, run and cycle while wearing multiple fitness trackers on their wrists. The team tested the accuracy of seven different fitness trackers: the Apple Watch, Samsung Gear S2, Fitbit Surge, Basis Peak, Microsoft Band, PulseOn and Mio Alpha 2.
All but one recorded error rates under 5 per cent for heart rate accuracy, with the Gear S2 having an error rate of 6.8 per cent.
For energy expenditure, the PulseOn fitness tracker was the most inaccurate with a 92.6 per cent error rate, while the Fitbit Surge was lowest with an error rate of 27.4 per cent. No data was available for the Gear S2.
Euan Ashley, co-author of the research, 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.”
The researchers urged caution over people making diet choices based on the amount of calories their fitness tracker says they have burned.
“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.
“When you consider that people are using these estimates to essentially make lifestyle decisions like what they are going to eat for lunch then I think that is something that is worth knowing, and people should know to take these estimates with more than a pinch of salt.”
The number of calories an individual burns during exercise can differ greatly from person to person due to a range of factors. An example provided by the study team was that when walking 10,000 steps, people may burn anything from 400 to 800 calories simply as a result of their weight and height.
Calculating the number of calories burned by an individual is a complex affair. Weight, height, body fat percentage, age, gender, heart rate and the exercise conducted can all affect energy expenditure, as can human error when entering personal details into health apps and trackers.
The findings were published in the Journal of Personalised Medicine.