You know the display of calories burned on a treadmill or elliptical tends to be false, so you bought a fitness tracker years ago to monitor your workout efforts. Well, that device you swear by may be just as inaccurate-by up to 23 percent!-a new study reports.

Iowa State University researchers had 30 men and 30 women wear eight different fitness monitors for a 69-minute workout that included 13 different activities, from typing at a computer and playing Wii tennis to running and playing basketball. Participants also wore a portable metabolic analyzer that provided a true indicator of calorie burn.

Researchers found that the BodyMedia FIT device was the most accurate and comparable to research models, with a 9.3-percent error rating. After that came the Fitbit Zip (10.1 percent), Fitbit One (10.4), Jawbone Up (12.2) Actigraph (12.6), Directlife (12.8), Nike Fuel Band (13.0), and Basis Band (23.5).

"People buy these activity monitors assuming they work, but some of them are not that accurate or have never been tested before," said kinesiology professor Gregory Welk in a press release. "These companies just produce a nice-looking device with a fancy display and people buy it."

Most devices are better suited for capturing "locomotor movements and general aerobic activity, as opposed to resistance exercise," he says, adding that newer monitors on the market may be better at tracking strength training.

Accuracy is important in general and because research has shown that people-especially those overweight-tend to overestimate the amount of activity they've done or the number of calories they've burned on any given day. As long as a device is giving a reasonably accurate measure (with error rates of 10 to 15 percent), Welk says, it can help correct for this common miscalculation.

"The validity is probably not as important as the usability and utility for an individual user," he says. "If the device helps to prompt and nudge behavior, then it is doing its job."

He adds, however, that fitness trackers won't magically change a sedentary person's behavior or keep us from pigging out on dinner-and that the data they provide should only be one part of a larger weight-loss or wellness strategy. Overall, he says, our bodies are still pretty good at telling us what they need.

"A person shouldn't only worry about counting calorie intake or expenditure since it becomes too difficult and cumbersome," he says. "Learning to pay attention to cues such as hunger can help to avoid overeating, and trying to get some physical activity is a simple strategy for energy expenditure."