Activity Trackers Are Better at Counting Steps
Than Measuring Sleep

Wearable activity trackers that promise to monitor physical activity, sleep, and more are becoming increasingly popular with health-conscious consumers. A recent study led by researchers from the University of North Carolina Gillings School of Global Public Health and research institute RTI International found that the trackers are better at measuring some metrics than others.

Kelly Evenson, PhD, a research professor of epidemiology at the Gillings School and an RTI University Scholar, is the lead author of a study published online in December 2015 by the International Journal of Behavioral Nutrition and Physical Activity.

Evenson and coauthors conducted a systematic review of 22 published articles researching the ability of Fitbit and Jawbone—two popular activity trackers—to measure steps, distance, physical activity, calories, and sleep.

"Wearable devices that track physical activity, sleep, and other behaviors are growing significantly in popularity," says Robert Furberg, PhD, MBA, senior clinical informaticist at RTI International and coauthor of the study. "We conducted this review to understand how accurate these devices are."

Several studies indicated that the step counting feature was accurate both in the lab and in the field. Only one study assessed distance tracking for the Fitbit, finding that the device tends to overestimate at slower speeds and underestimate at faster speeds.

Two field-based studies compared accelerometry-assessed physical activity to results from the trackers, with one study finding high correlation (in Fitbits), while another study noted a wide range in correlation (in both Fitbit and Jawbone brands).

Using several different comparison measures, other researchers found that both tracker brands over- and underestimated calories used, and overestimated total sleep time.

Overall, the systematic review indicated higher validity of step counting, inconclusive findings (based on few studies) for distance and physical activity, and lower validity for calories (energy expenditure) and sleep.

To make trackers as accurate as possible, the authors suggest some strategies for device wearers.

"When researching information on the trackers, we learned several tips users may be able to implement to make their tracker more accurate," Evenson says.

Helpful recommendations include the following:
• wear the tracker in the same position each day;
• enter personal details like height and weight correctly at initial setup, and update if there's significant change in weight;
• correctly calibrate the length of a walking stride;
• add more information via the device's journal function; and
• interact with the sleep mode settings.

— Source: RTI International