Smartwatches have become an increasingly popular way for people to monitor their health and fitness. About 100 million – nearly 4 in 10 – U.S. adults own a smartwatch. Worldwide, market research estimates approximately 560 million smartwatch wearers.

Yet a growing body of research demonstrates that one of the most commonly used smartwatch metrics – number of calories burned – is often inaccurate.

I’m an exercise scientist, and I study physical activity behavior and the integration of wearable technology. I recently worked with a team of colleagues at Florida International University’s Medical Photonics Laboratory on a study examining how well commonly used smartwatches measure physiological responses during exercise, and whether that accuracy differs across individuals.

Our team found that not only can these devices err substantially in their measurements of calories burned, but those errors also increase as the wearer’s body fat percentage rises.

Most smartwatches contain accelerometers that measure movement and use a light-based sensor to estimate heart rate. With each heartbeat, the amount of blood flowing through the vessels in your wrist changes. Blood absorbs light, allowing the watch to track changes in the light reflected back to the sensor. That pattern allows the watch to estimate your heart rate.

The watch combines the data from these sensors with information about the user, such as age, sex, height and weight. It then uses an algorithm to estimate how much energy that person expended.

Heart rate and movement are relatively accessible signals for a wrist-worn device, but caloric expenditure is much more complex. It requires the watch to use several pieces of information that it can measure reasonably well to predict something it cannot measure directly. But differences in the wearers’ body composition, fitness, movement efficiency and physiological responses to exercise can all affect that prediction.

For our study, we........

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