If you’re trying to lose weight, an activity monitor can help by telling you how many calories you’re burning.

Some are more accurate than others, but in general the monitors work pretty well for walking and running.

If you want to compare activities on the other hand — say weight lifting versus running — you may be out of luck.

That’s the finding of a team of researchers from Iowa State University.

The team had volunteers wear various monitors while breathing through masks that measured how much oxygen they consumed. This is a standard technique for estimating how many calories someone burns.

The researchers compared four consumer monitors to see how well they measured calories burned in sedentary, aerobic and resistance activity: the Fitbit Flex, Nike+ FuelBand SE, Jawbone UP 24 and Misfit Shine.

They also compared two monitors used by researchers – the BodyMedia Core and Actigraph GT3X+

Most of the devices did well in measuring aerobic activity. But they had a high error rate when measuring resistance activity. That’s where you lift weights (including your own), pull on elastic bands, or do something similar to build your strength.

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 results are published in Medicine & Science in Sports & Exercise. The following is a breakdown of error rates for each monitor based on activity:

A test of real conditions

ISU 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 Yang Bai, lead author, in a press release. “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.”

Accuracy is important, but it is only part of the equation in terms of improving physical activity levels, said Greg Welk, a professor of kinesiology in the press release.

“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.”

Photo by Iowa State University

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