Electronic wristbands that count footsteps and calories produce imprecise data. But that’s beside the point. The devices are motivational tools to promote fitness, and the numbers they generate are good enough for that.
Initially embraced by fitness fanatics, activity trackers, such as the Fitbit, Jawbone and FuelBand, have become popular among mainstream users, many of whom are wearing the accouterments as part of New Year’s resolutions to get in shape.
How well do they work? Researchers at Iowa State University recently tested the accuracy of eight activity trackers in measuring energy expenditures and found that most were off by 10% to 15% compared to a precise laboratory measurement. The worst was off by 23.5%.
A newer study of the latest wristbands conducted by the university and featuring more realistic conditions found larger margins of error—15% to 18% for most of the devices with two scoring substantially worse.
You wouldn’t want to plan a diet around those figures. But even if the metric is imperfect, users who wear a tracker religiously will accumulate a consistent log that is valuable for quantifying changes in activity.
“Having a device that is incredibly accurate is less important than developing a device that is relatively accurate but can be worn for long periods,” said Matthew P. Buman, a professor in the School of Nutrition and Health Promotion at Arizona State University who studies activity trackers. “The purpose is to give feedback about what consumers are doing and how they can improve.”
Typically, activity trackers allow users to store and view data online or with a smartphone, track trends and set goals. But the trackers don’t actually count steps or calories. Instead, they measure acceleration, and algorithms devised by the manufacturers convert those measurements into estimates of the number of steps taken and calories burned.
The component in the devices that takes the measurement is an accelerometer, which records movement in three dimensions: up and down, side to side, and forward and backward. Among other things, accelerometers are used to detect earthquakes, guide spacecraft and adjust the orientation of images on smartphones so they remain upright no matter how the phone is held.
“When you put an accelerometer on the wrist, all it is measuring is the swinging motion of the arms,” said Ken Fyfe, who developed one of the first activity trackers, a shoe-mounted sensor designed for runners to measure speed, distance and pace. “Of course, because arms move in a rhythmic fashion with feet, it’s a good step counter.”
Activity trackers worn on the hip or foot are more accurate than wristbands, but consumers seem to prefer wrist-mounted devices, trading accuracy for convenience.
“It may not be the best place, but it’s where people are willing to wear them,” said Gregory J. Welk, a professor of kinesiology at Iowa State University and co-author of the activity tracker study with then doctoral students Jung-Min Lee and Youngwon Kim ; both have since graduated.
The researchers, whose work was published in Medicine & Science in Sports & Exercise, a journal of the American College of Sports Medicine, examined 60 healthy individuals, half men and half women, ages 18 to 43 who wore eight different devices while performing 13 activities over a period of 69 minutes.
The activities included writing at a computer, playing basketball and walking and running at different speeds.
The accuracy of the measurements varies by activity. The experiment considered the activities together in an effort to gauge the accuracy of the devices for “normal adult behavior” rather than a single task.
But most consumers purchase fitness bands for two reasons: to count footsteps taken and calories burned, according to NPD Group, a market-research firm that conducts a consumers and wearable-tech survey twice a year.
The fitness-tracker market has exploded in recent years—for the 12 months ending November 2014, retail sales more than doubled from the same period the previous year to $489.4 million—according to NPD’s retail tracking service, but the market has shifted away from the early adopters, who tended to be fitness fanatics and ultimately found the devices too basic.
“The devices are better, but the type of person buying the devices has changed,” said Weston Henderek, director for connected intelligence for NPD Group. “They’re getting people who don’t exercise and want to start, or people who walk and want do to more.”
Activity trackers are best at detecting brisk walking, jogging or running—activities that produce strong arm movements.
Steps taken around an office or while puttering around a home are difficult to pick up, according to Ray Browning, a professor at Colorado State University who directs the Physical Activity Energetics/Mechanics Laboratory. In addition, trackers mistake some movements for walking, such as waving the arms, while entirely missing physical activities that ideally should be captured.
“Cycling, not great,” Mr. Browning said. “Elliptical training, not great. Yoga, terrible. There are some pretty significant limitations.”
Limits aside, how useful the trackers are ultimately depends on the dedication of the user. Mr. Henderek said about a third discard the devices in a matter of weeks—but more than half stick with them past three months.
So far, no researchers have documented whether the bands actually induce wearers to increase physical activity. The bigger challenge for users may not be documenting their behavior, but taking steps to improve it.
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