Watches that count your steps, monitor your workouts and record your sleep cycles are not new. In fact, the first mass-market consumer fitness tracker came out in 2009. But after nearly two decades, researchers are still learning more about the devices’ capabilities — or, more specifically, the lack thereof.
In a new study released this month, researchers at Florida International University examined popular smartwatches to learn how accurately they count calories during a workout. It found that some of the most popular watches today actually overstate how many calories people burn, with some brands being more off base than others.
The study suggests that the millions of Americans who rely on their devices to keep track of their fitness goals may have been led to believe they were farther along toward physical health than they really were.
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An overstating calorie counter
The researchers tested four smartwatches — the Apple Watch, Garmin, Samsung Galaxy Watch and Fitbit — against clinical lab equipment. They had 58 adults ride a recumbent bike at different intensities while wearing the different watches.
They found the watches consistently overstated how many calories people burned while working out. Meaning, you may hit the treadmill for 40 minutes and think you burned 400 calories when, in reality, it may have been only 350 or even 300.
“You can’t treat the calories burned number it gives you as an accurate number, because it’s not,” Jason Kostrna, the study’s lead author, told FIU. “You could be off hundreds of calories each week and easily end up in a calorie surplus when you think you’re in a calorie deficit.”
For the most part, the calorie counts landed between 15% and 25% off. However, numbers were much higher for some of the worst-performing watches. The winner of the group? The Apple Watch. Meanwhile, Garmin and Samsung overestimated the calorie counts the most.
Higher body fat = more inaccurate results
The study also found that some users may receive more inaccurate readings than others.
The higher a person’s body fat, the greater the watch’s error tended to be, the researchers found. This means that the people the study said are most likely to track calories for weight management may be getting some of the most inaccurate results.
Why? Kostrna said researchers don’t have a definitive answer, but said algorithms may come into play.
He also said the watches should not be disregarded altogether.
“They may be imperfect, but these devices can still have an important role to play,” Kostrna said. “Track your own trends over time. If tracking helps, enjoy it. If it becomes too much, don’t fixate on it. Focus on how you feel and use the data as one tool to help you improve over time.”
Now, Kostrna and other researchers are turning to strength training exercises to see how well watches track the short, intense bursts of effort that can be difficult for algorithms to read.
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