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Stanford University School of Medicine

Fitness trackers accurately measure heart rate but not calories burned, Stanford study shows

Your fitness tracker knows how fast your heart beats when you bicycle to work and how your heart flip-flops when your sweetie surprises you with a kiss.

But, new research shows, the device doesn't have an accurate idea of how many calories you burned taking the stairs instead of the elevator, let alone whether you should take a second helping of ribs after your weekend 5K run.

Stanford Medicine researchers who tested seven commercial fitness tracking devices under a variety of conditions concluded that all seven were fairly accurate when measuring heart rate but quite off at measuring energy expenditure, or calories.

The wearable trackers were chosen for being able to measure both heart rate and energy expenditure and tested in a diverse group of 60 volunteers who sat, walked, ran and bicycled. The research appears in the Journal of Personalized Medicine today.

As I reported in a news release:

None of the seven devices measured energy expenditure accurately. Even the most accurate device was off by an average of 27 percent. And the least accurate was off by 93 percent.

'People are basing life decisions on the data provided by these devices,' said Euan Ashley, DPhil, FRCP, professor of cardiovascular medicine, of genetics and of biomedical data science at Stanford [shown in the photo above]. But consumer devices aren't held to the same standards as medical-grade devices, and it's hard for doctors to know what to make of heart-rate data and other data from a patient's wearable device, he said.

Increasingly, Ashley told me, patients are sharing fitness-tracker data with their physicians. And both doctors and consumers need to know what those readings mean. In fact, other research has shown that wearable-device data such as skin temperature and heart rate can reveal whether someone is coming down with an infection.

Yet gaps in the devices' abilities remain. For example, the team also found that the presence of tattoos and body mass index could affect the measurements. To learn more, the researchers are eager to share their data -- and to collect other device data at

Previously: Stanford study shows wearable sensors can tell when you're getting sickWearable sweat sensor can diagnose disease, Stanford-led study finds and Harnessing mobile health technologies to transform human health
Photo by Paul Sakuma

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