How Apple Made The Watch Work For Wheelchair Users

In America alone, there are more than 2.2 million people who depend upon wheelchairs to get around every day. But most wheelchair users aren’t active. They’re more sedentary, on average, than those who have full use of their legsā€”and are consequently at much greater risk of heart attack, stroke, and diabetes.

Amazon is currently selling more than 1,000 different models of fitness trackers online. But how many support wheelchair users, an enormous demographic in desperate need of sophisticated tools to manage and encourage fitness?

None. Zero. Zilch. Up until now.

Two weeks ago, Apple made a seemingly small announcement at its annual Worldwide Developer’s Conference. Starting in September, the Apple Watch will support wheelchair users, allowing them to track their fitness goals the same as anyone else. But this feature is a big deal to the millions of people around the world who live their lives in wheelchairs. It was also an incredible technical challenge to pull off, requiring Apple to mount the most comprehensive study ever on fitness among wheelchair users, as well as a complete overhaul to the design of its fitness tracking algorithms.

We spoke to Apple’s Ron Huang, director of software engineering for location and motion technologies, to get an inside look at how Cupertino made the Apple Watch’s wheelchair tracking features possible.


Whether you’re talking about a Fitbit or something more sophisticated like an Apple Watch, all activity trackers work pretty much the same way. Inside is an accelerometer, a little chip that can detect and record acceleration data. In graphic form, that data looks something like a Joy Division album cover, but in the spikes and troughs of that data stream is a record of every gesture, step, jumping jack, or swimming pool cannonball you’ve made while wearing your tracker. All the device needs is the proper algorithms to decode it.

In the case of the Apple Watch, its algorithms look for two things to measure a step. Since most people who aren’t Molly Shannon on Seinfeld swing their arms when they walk, the Apple Watch tracks arm movement, with the length of the arm swing roughly corresponding to the distance of the stride. But people swing their arms a lot, even when they’re not walking, so the Apple Watch also looks for a telltale data spike of a heel striking the ground, punctuating each step.

In this way, it can tell the difference between someone jogging and someone doing a Cabbage Patch. Multiply the number of steps versus the average number of calories burned per step according to medical consensus adjusted for an individual’s height or weight, and voila! You’ve got a fitness tracker that can convert steps into calories burned.


But this algorithm breaks down for wheelchair users. Most obviously, those who get around on wheels don’t strike their heels against the ground. Even the way wheelchair users move their arms when pushing themselves is different than the way people swing their arms when they walk. Walking is a regular motion; pushing, comparatively, is irregular. Wheelchair users need to start, stop, and adjust their pushes more than walkers do. To make the Apple Watch’s fitness tracking functionality useful to wheelchair users, then, Apple needed to totally reexamine its algorithms.

First, Apple’s software engineers examined the available scientific literature on how wheelchair users burn calories. But this literature was lacking. The existing studies tended to only involve a small number of subjects, and their methodology in translating pushes to calories wasn’t applicable to the real world. For example, the studies might prevent their subjects from using their own wheelchairs, or only track how many calories a wheelchair user was burning on a treadmill, not on their home turf.

None of this was useful data for a general-audience device meant to track wheelchair users outside of a lab setting. Apple found the existing studies so lacking that it ended up conducting the most comprehensive survey of wheelchair fitness to date. They teamed up with the Lakeshore Foundation and the Challenged Athletes Foundation, two organizations dedicated to promoting fitness among people with disabilities.

To read the rest of this article, published in Fast Co Design, please click here.