The Good News and Bad News on Fitness Trackers

It looks like your Fitbit holds the key to your heart but not your stomach.

New research says that while fitness trackers accurately measure heart rates, their calorie estimates are far less reliable.

In a recent study, researchers at Stanford University evaluated the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear. The Stanford team found that while the gadgets could accurately record the heartbeats but were off the mark on calories.

Studying a control group of 60 volunteers, the Stanford team determined that the amount of calories burned, displayed by fitness trackers, was off by an average of 27 percent.

And that was just the average. The least accurate measurements were off by an alarming 93 percent. Conversely, the heart rate measurements had an error rate of less than five percent.

Researchers attributed the errors to factors ranging from movement to physiology.

“When someone is exercising, the watch may shift along the wrist or lost contact with the wearer’s skin,” Stanford researcher Anna Shcherbina told BTRtoday. “Perspiration, tattoos, skin tone and body hair may all factor into errors in heart rate measurement.”

Several factors influenced the accuracy of the readings. Wearing them on different arms had a significant statistical impact on recording heart rates and energy expenditure. The placement of the device also mattered. Readings changed when the device was placed on the wrist versus further up on the arm.

The researchers found that the devices could be biased consistently underestimated or overestimated energy expenditure, depending on the situation.

“The devices tend to underestimate energy expenditure when a person is stationary or engaged in moderate activity, but overestimate energy expenditure when the person is engaged in vigorous physical activity,” Shcherbina says.

In Phase 2 of the study, the Stanford team will test the latest version of each fitness tracker from Phase 1, along with several devices not included in the original study.
Users can suggest devices to test on the team’s website.
The team will monitor the subjects with a portable devices to measure of subjects in their daily life instead of a clinical setting.
In light of the Stanford team’s data, consumers are faced with difficult choices when they search out accurate wearable devices. With the calorie counts so far off, how can people looking to lose weight trust their devices?

Perhaps they can’t. However, the heart rate measurements were accurate. That means devices could help Americans suffering from heart-related health problems.

“Consumers concerned about their cardiac health might consider using a smart watch for continuous monitoring of heart rate,” Shcherbina says “In contrast, energy expenditure error was quite high across devices, so consumers should not trust these numbers as being 100 percent accurate.”

The original study tracked subjects wearing the devices for a single workout. They hope to get a broader timeline of data in the next study by tracking subjects for longer periods of time.