An estimated 19 million activity trackers were used in 2014, but when it comes to tracking steps and calories, how many of those were actually accurate? A new study commissioned by the American Council on Exercise (ACE) evaluated five popular activity trackers to determine which ones delivered as advertised.

The results were clear - although many of the devices tested could effectively measure steps taken when walking, running and using an elliptical trainer within 10 percent accuracy, they were ineffective at tracking steps for exercises involving complex movements such as sports, weight lifting or cross training. Of all the trackers evaluated, the Jawbone UP provided the most accurate step count.

Monitoring caloric expenditure posed a greater challenge, as the trackers tested resulted in ranging from 13 to 60 percent of the true values.

"Predicting caloric expenditure is a relatively complicated process," said ACE Chief Science Officer Cedric X. Bryant, Ph.D. "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."

In the study, researchers at the University of Wisconsin, LaCrosse, recruited 10 men and women to help evaluate the Nike+ Fuelband, Fitbit Ultra, Jawbone UP, BodyMedia FitCore, and the Adidas MiCoach*. Participants wore each of the trackers while walking and running on a level treadmill, completing an elliptical workout, and performing sports-related exercises including ladder drills, free throws and T-drills.

To assess accuracy, all activity trackers evaluated in the study were compared to portable metabolic analyzers worn by all participants and the NL-2000i pedometer, which has been shown to be accurate in previous research studies.

Despite the shortcomings of devices tested, however, ACE recognizes that activity trackers still can serve as important tools for people seeking to incorporate more physical activity into their lifestyles.

"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."

"It is important for consumers to understand that while activity trackers may not precisely estimate caloric expenditure," Bryant said, "they seem to accurately measure step counts during certain activities and can be used to effectively quantify and track changes in an individual's physical-activity habits."

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