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How Accurate Is Your Fitness Tracker, Really?

Participants in a new study from Stanford University wore up to four fitness trackers at a time to test their accuracy.
Participants in a new study from Stanford University wore up to four fitness trackers at a time to test their accuracy. (Image credit: Paul Sakuma/Stanford University School of Medicine)

Many fitness trackers measure health data such as your heart rate and the number of calories you burn, but can you really trust the information these devices provide? In a new study, researchers sought to find out.

The results? Your fitness tracker likely measures your heart rate pretty well, but you may not want to trust its readings on how many calories you've burned.

In the study, researchers tested the heart rate and calorie measurements from seven popular fitness trackers: the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and Samsung Gear S2. They analyzed information from 60 volunteers who wore up to four fitness trackers at once while they walked or ran on a treadmill, or cycled on a stationary bicycle. Then, they compared the data from the trackers with data from the standard medical equipment that scientists use to measure people's heart rate and calorie burning in health research.

The fitness trackers' heart rate measurements were pretty accurate: Six of the seven fitness trackers were less than 5 percent off, most of the time, in their measurements of people's heart rates as they rode a stationary bicycle, when compared with a medical-grade electrocardiograph, the "gold standard" for measuring heart rate.

The heart rate measurements that were taken while people were walking were slightly less accurate than those taken while people were doing other activities, but the readings were still acceptable, the researchers said. Three of the trackers were less than 5 percent off for the heart rate measurements, and the rest were less than 9 percent off, the researchers said. [Top 10 Amazing Facts About Your Heart]

In contrast, the trackers' measurements of the number of calories that people burned were way off. The most accurate device was off by about 30 percent, when compared with the gold standard instrument for measuring calories burned, and the least accurate device was off by 93 percent, the researchers said.

"The heart rate measurements performed far better than we expected," study researcher Euan Ashley, a professor of cardiovascular medicine, genetics and biomedical data science at Stanford University, said in a statement. "But the energy expenditure measures were way off the mark. The magnitude of just how bad they were surprised me."

Stanford University cardiology researcher Euan Ashley and his team used special equipment to test the accuracy of fitness trackers. (Image credit: Paul Sakuma/Stanford University School of Medicine)

Overall, one device stood out as being the most accurate — the Apple Watch tended to have the lowest error in its measurements, compared with the other six devices tested.

The researchers concluded that people who use fitness trackers can rely on the heart rate measurements that they read on their devices, but not on the estimates of calories burned.

The researchers aren't sure why the estimates for calories burned were so inaccurate. But it's likely that fitness trackers' algorithms for calculating energy expenditure make assumptions that don't necessarily apply to every person, they said.

"It's very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable," based on a person's fitness level, height, weight and other factors, said study researcher Anna Shcherbina, a graduate student at Stanford.

The researchers plan to conduct another study to examine the accuracy of fitness tracker measurements in conditions outside a laboratory; they will have participants wear the trackers as they go about their usual day.

The study is published online today (May 24) in the Journal of Personalized Medicine.

Original article on Live Science.

Rachael Rettner
Rachael Rettner

Rachael has been with Live Science since 2010. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.