According to Yahoo, the American Council on Exercise (ACE) estimated that 19 million fitness trackers were in use last year. But, that doesn't mean they're all they're cracked up to be. As Yahoo reports, according to a small, new study from ACE, these trackers may not be accurate at estimating calories burned. Participants in the study did two 50-minute exercise sessions wearing five popular types of trackers. The results showed the devices were excellent at calculating steps taken when participants were walking, running, or using the elliptical (they came within 10% accuracy). However, the trackers underestimated activity during agility drills, likely due to the fact that those workouts involve complex motions which are harder to track than counting steps. And, most of the devices either overestimated or underestimated the number of calories burned. Kelvin Gary, a personal trainer and owner of Body Space Fitness in New York City, told Yahoo that fitness trackers are still useful, despite their flaws. “They don’t always show a true representation of activity level,” he says, “but we always ask our clients to have a goal, and these devices are great for setting activity-based goals.” To see which tracker fared best in the study, read on at Yahoo Health.