The five trackers used in the study.

A new study from the American Council on Exercise (ACE) looked at five popular consumer activity trackers and evaluated them for accuracy. The study found that five devices -- Nike+ FuelBand, Fitbit Ultra, Jawbone UP, BodyMedia FitCore and Adidas MiCoach -- all tracked steps relatively accurately, but many were less accurate when it comes to tracking caloric expenditure, especially during activities beyond walking or running.

“Predicting caloric expenditure is a relatively complicated process,” ACE Chief Science Officer Cedric X. Bryant said in a statement. “There are certain assumptions that are made when developing the algorithms that translate movement activity detected by the devices into calories burned. Even devices with the best prediction equations will have some margin of error due to natural biological variability.”

The research was conducted at the University of Wisconsin, La Crosse, with 20 subjects -- 10 male and 10 female. Participants tested the five trackers under four conditions: walking on a treadmill, running on a treadmill, using an elliptical machine, and agility activities like ladder drills, free throws and T-drills. In addition to the tracker, they wore an NL-2000i pedometer and portable metabolic analyzers.

When participants were walking, running, or using the elliptical, all five devices recorded step counts within 10 percent accuracy as compared to the research-grade pedometer. Many of them underestimated steps during the agility drills, though, owing to "the variety of more complex movements" according to researchers. Jawbone UP was the most accurate across the four categories.

For energy expenditure though, all the devices exhibited significant differences from the metabolic analyzer on at least one exercise. Some over-calculated expenditure while others under-calculated it. Fitbit and Nike were the closest to accurate for walking, Fitbit for running, Jawbone for the elliptical, while none got particularly close on the agility exercises.

Researcher John Porcari said that calorie tracking is complicated because of biological variations between individuals.

"Predicting calorie burn is a complicated thing,” he said in a statement. “People vary how they move their arms, for example. Some are more efficient and some are more variable. Most devices probably won’t get within 10 to 15 percent accuracy because there is simply too much biological variability.”

It's worth noting that the research, published in January, was conducted with devices that are all now at least one generation out of date. Newer devices might be more accurate in tracking both steps and calories. Researchers pointed out in their conclusion that perfect accuracy isn't the be-all, end-all of value for activity trackers anyway.

“Although the devices evaluated in the study aren’t ideal for measuring the number of calories burned, they can provide consumers with a reasonable estimate of how much physical activity they are incorporating into their daily routines,” Bryant said. “Having access to that information can be a valuable motivational and informational tool for people beginning a fitness journey, as well as those trying to increase their level of daily physical activity.”