Your activity tracker may not be as accurate as you think!

Activity trackers can provide a good overall estimate of calories burned, but they may be less accurate when measuring certain activities, such as strength training, scientists say.

A team of researchers from Iowa State University's Department of Kinesiology put some of the more popular activity trackers to the test.

They tested four consumer fitness trackers - Fitbit Flex, Nike+ FuelBand SE, Jawbone UP 24 and Misfit Shine - as well as two monitors normally used in research - the BodyMedia Core and Actigraph GT3X+ - to see how well they measured sedentary, aerobic and resistance activity.

Overall, the BodyMedia Core was the top performer with a rate of error of 15.3 per cent. The Misfit Shine was the least accurate with a 30.4 per cent error rate.
 

The researchers designed the study to mimic real daily living activities. The 56 participants were asked to complete 20 minutes of sedentary activity, such as reading a book, working at the computer or watching a video.

That was followed by 25 minutes of their choice of aerobic activity and 25 minutes of resistance exercise, with 5 minutes of rest between each activity.

"By looking at the most commonly performed activities in exercise and daily living settings, we can examine where the errors occur," said lead author Yang Bai, a graduate research assistant in kinesiology at Iowa State University in US.

"As expected, some monitors overestimate or underestimate all three activities, but some monitors overestimate one type and underestimate the other two categories, which can cancel out if we don't measure them separately," Bai said.

Researchers said accuracy is important, but it is only part of the equation in terms of improving physical activity levels.

"I think the key to a consumer is not so much if the activity monitor is accurate in terms of calories, but whether it's motivational for them and keeps them accountable for activity in a day," said Greg Welk, professor of kinesiology at Iowa State University.

The study was published in the journal Medicine and Science.