(NEW YORK) -- Activity trackers -- wearable devices that count steps and measure calorie burn -- are going through a boom. Sales of the devices last year topped $330 million, according the market research group NPD, and consumers have more than two dozen brands and styles to choose from, including shoe chips, bracelets, anklets, pendants and clip-ons. To think it all started with a pair of "magic underwear."
Dr. James Levine, a professor of medicine at the Mayo Clinic in Arizona, studies the relationship between movement performed outside the gym and obesity. About a decade ago, he rigged up an undergarment with sensors designed to catch the body's every little shift in movement.
"I figured that most people wear underwear most of the time so this was a good way to figure out what most people do for activity throughout the day," Levine recalled. By having his subjects wear what he called the magic underwear or "fidget pants," Levine discovered that thin people tap their feet, jiggle their pencils and shift in their seats about two and half hours more each day than obese people do. Sensing he was onto something that could help people control their weight, he said his next goal was to turn his experimental undies into a gadget that could help motivate the average person to get up and move more.
Levine realized he couldn't ask Americans to wear complex panties to measure their movement. His studies showed that wrapping a sensor around the spine would also provide good data. But that too was a tough sell, he said. So he and his team created a chip that inserted into the heel of a shoe and then later, a motion-tracking bracelet.
The idea of a simple, stylish tracker has caught on. Many consumers are now obsessed with tracking their every move. But some experts say the tradeoff for style and comfort has been accuracy.
"Most brands of trackers use proprietary algorithms and formulas that haven't been independently tested in the lab and they aren't available to the public so it's impossible to know how they are capturing their information," said John Jakicic, director of the physical activity and weight management research center at the University of Pittsburgh.
As Jakicic explained it, most trackers rely on a piece of hardware known as a three-axis accelerometer to gather data. Using a technology similar to what a cellphone uses to change the orientation of its screen when you shift its position, accelerometers combine hip and foot movement with speed of movement and changes in direction to come up with an estimation of steps, mileage and calorie count.
If a tracker relies on is accelerometer alone, Jakicic said, the information it provides may not be completely reliable.
For example, a bracelet gadget will do a good job detecting steps from walking and running because the wrist moves during this kind of activity. But it won't give you any credit for pedaling away on a stationary bike with your hands planted on the handlebars.
In one small study involving 17 subjects published in the journal Medicine & Science in Sports & Exercise, a Colorado State University team compared the readings from sophisticated lab equipment and a popular first-generation tracker, a tiny user-friendly gadget that clips onto a waistband. The tracker underestimated calorie burn by nearly 30 percent, researchers found. And in another small study from the same researchers, mileage readings among popular trackers varied by as much as 39 percent over the same distance, though the step counts were accurate to within 1 percent of the lab equipment.
Jakicic said some trackers improve their precision by incorporating other types of sensors. A device that incorporates information about say, GPS, body heat and heart rate, is probably smart enough to know the difference between sitting in a chair and cranking away on a cycle, he said. And Ray Browning, the lead Colorado State researcher said as the technology evolves, he expects trackers will get better and better. Browning's team is also working on a fitness tracker of their own.
"Our studies looked at early versions" of commercially available trackers, he pointed out. "I suspect if we looked at some of the equipment that's available today the readings would be vastly improved."
Jakicic agreed. He also said that precision may be beside the point.
"What's important is consistency," he noted. "Even if step tracking is off by 10 to 20 percent, as long as it's off by the same amount every day, the consumer can still track movement from day to day and that should be enough to help people increase their activity levels."
Jakicic does worry about calorie burn numbers being way off for anyone trying to balance calories eaten with calories burned. But he still thinks trackers can be a powerful motivator for someone focused on living a more active lifestyle. A recent Indiana University study found that subjects wearing fitness trackers walked 16 percent more steps than a control group, and they lost weight.
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