PALO ALTO, Calif. — That brand-new Fitbit Surge or Apple Watch you got for Christmas might not be the best way to measure caloric expenditure, a research team at Stanford University claims in a recent study on popular fitness trackers.
Euan Ashley, lead author of the project, found that while fitness trackers are usually accurate in measuring heart rate, energy tracking was an entirely different matter.
“The heart rate measurements performed far better than we expected, but the energy expenditure measures were way off the mark,” says Ashley, a professor of cardiovascular medicine, genetics, and biomedical data science, in a university release. “The magnitude of just how bad they were surprised me.”
The researchers tested seven popular fitness trackers: the Fitbit Surge, Apple Watch, Basis Peak, Microsoft Band, Mio Alpha 2, PulseOn, and the Samsung Gear S2. They found that six of the seven devices that measured heart rate in a diverse group of 60 volunteers using treadmills or stationary bikes produced an error rate of less than five percent.
However, none of the devices tested measured energy expenditure accurately when compared to medical-grade devices designed for testing energy expenditure. The most accurate device that measured caloric expenditure was off by an average of 27 percent, the least accurate was off by a whopping 93 percent.
Consumer devices aren’t held to the same standard as medical-grade devices, and the study authors theorized that the proprietary algorithms used by these devices to measure energy expenditure can’t be replicated accurately among a diverse group of consumers of different sizes, ages, and health levels. While measuring heart rate is done directly and simply, energy expenditure is calculated with a series of variables, including the body mass index of an individual and other factors.
“People are basing life decisions on the data provided by these devices,” says Ashley, who worries that users are basing the amount of junk food they consume off of the number of calories their devices claim they’ve burned.
Ashley and his associates published a paper on their findings in the Journal of Personalized Medicine. They also made their findings open to the public and encouraged people to upload their own results to the dataset.