Is your expensive activity tracker fully reliable?

Your activity tracker may provide you a good overall estimate of calories burned, but it is less accurate when measuring certain activities such as strength training, a study says.

Researchers from the Iowa State University (ISU) tested four popular consumer fitness trackers - Fitbit Flex, Nike+ FuelBand SE, Jawbone UP 24 and Misfit Shine - to see how well they measured sedentary, aerobic and resistance activity.

Two research monitors - the BodyMedia Core and Actigraph GT3X+ - were also included in the study.

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

The team 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 five 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.

"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 explained.

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

The results were published in the journal Medicine & Science in Sports & Exercise.