Fitness trackers works for people with disabilities

While the armbands measure how the body is mechanically moving, the metabolic carts measure oxygen inhaled and carbon dioxide exhaled, which can be used to calculate energy expenditure.

Once Ding has the accelerometer data, she can match it with biometric data from the metabolic cart and create models that predict how many calories study subjects are burning during either formal exercise or just by going about their daily lives.

Fitness tracking could be a game changer for Americans with disabilities, a group that’s nearly twice as likely to be obese as the general population. But simply adding extra algorithms to activity trackers already on the consumer market isn’t as easy as it sounds, says Ding.

Trackers use their own algorithms to automatically convert data into metrics consumers can understand, like steps taken, calories burned, or stairs climbed. When these conversions are done in the hardware portion of a device, tacking on an additional calculation for wheelchair users requires a major device redesign. "It’s basically creating a new line of product," Ding says.

That’s a hard sell in the increasingly competitive fitness tracking market, especially when the number of potential users is so much smaller than the general population. The U.S. Census Bureau estimates that there are currently 3.3 million wheelchair users in the US, though it’s hard to estimate how many would be interested in owning a fitness tracker.

Ding is currently working to find open source sensors that would allow her to build her own fitness-tracking armband, that could give wheelchair users information through a smartphone app.

Next door to Dan Ding, Jon Pearlman is working to solve an even more basic problem facing the mobility-impaired. "Most wheelchair users don’t know their weight," says Pearlman, the Human Engineering Research Lab’s associate director for product innovation and translation.

For those who can’t stand on a typical bathroom scale, just getting a regular weight measurement often requires traveling to a doctor’s office to use a roll-on scale. While some have come up with crafty solutions—Pearlman says that he’s spoken with people who place a bathroom scale on a chair to get a weight measurement or go to local pet stores to use animal scales—affordable assistive scales are few and far between.

"If you want to get [an assistive scale] at home, it’s very difficult to find an insurance company that will pay for it, even though weight is a critical factor and obesity is an epidemic in the U.S.," Pearlman says. "There’s really strong research evidence that says the more feedback you get about activity, it drives behavioral changes. ... There’s a significant difference in how much weight you’ll lose if you have daily updates."

Roll-on scales can range from about $600 to more than $3,500. Scales built into chairs or lifts users can sit in are often cheaper, but require someone to help transfer the user onto the scale. Pearlman’s team has built their own scale—a series of four hockey puck-like discs that slide under the feet of a bed.

The discs measure the weight of the bed and subtract that number out when someone lays down on top. Users can check their weight on their smartphone or by looking at a bedside display. Pearlman’s scale is currently in clinical evaluations. He hopes to commercialise it for $200 or less within the next two years.

Until better and more affordable assistive fitness and weight monitoring options are available and access to cheap, healthy foods is increased, people like Sebastian Williams will continue to fight a constant, and often losing, battle against their own waistlines.

"Because you can’t move around a bunch, no matter how you diet you still put on weight," Williams says. "To be perfectly honest, after a while you say the heck with it."