Staying on track with your #fitspo goals has never been easier with the advent of such fitness trackers as Fitbit or Jawbone. Wheelchair users, however, have few options to turn to — up until now, that is. When Apple rolls out its new watchOS 3 this fall, the updated fitness and health capabilities in the Activity app will be calibrated to track the unique fitness metrics of wheelchair users. Below, Ron Huang, Apple's Director of Software Engineering for Location and Motion Technologies, lets us in on the mechanics of this innovation for good. 

Amassing data from scratch

We tried to dig into existing literature and studies to find out more about what might be out there for us to use as a baseline to start out with. There's been very few studies done on wheelchair energy expenditure, and existing ones tend to be based on smaller subject numbers. If we tried to build a feature for a huge customer base based on a small number of studies, we could easily incur a lot of errors on that.

In addition, a lot of the basic formulas used to calculate energy expenditure and calories don't match very well to wheelchair users because they depend on a weight metric. Due to body composition differences, weight is not necessarily the best quality to focus on for wheelchair users. 

apple watch wheelchair

As such, we quickly realised we had to start our research from scratch. So we conducted studies with Apple employees who are wheelchair users; the Challenged Athletes Foundation down in San Diego, California, which has a wide membership network of wheelchair users who could do studies with us; and partnered with the Lakeshore Foundation to test the features. 

All in all, we collected over 3,500 hours of data across 700 sessions with 300 subjects. From there, we were able to amass a large amount of data to develop our algorithms. 

Tracking pushes, not steps 

We wanted to measure the true motion and trajectory taken by these users, so we utilised chest straps to capture accurate heart rate measurements, mounted wheel sensors on the wheels to count actual revolutions, and GPS devices on their chairs so we could track how fast they were going and the routes they were taking. 

Pushes, like steps, are a fundamental element we can use to track their activity levels. However, it's a lot harder to track than steps because people push differently. The most common push is the semi-circular push, where — if you're looking at the chair from the side — you're pushing from 10 o'clock to three o'clock. Secondly, when you're trying to go uphill or downhill, you need to have quicker hands so your chair doesn't just roll back. With quicker hands and shorter strokes, you get an arc push. Finally, if users are racing or playing sports and essentially trying to go faster, they would lean back, push forward with their entire body, then throw their body back. This is what we call the single loop over push. These pushes have drastically different frequencies, energies, and amplitudes. We needed to ensure we could count these different pushes correctly.

apple watch wheelchair

New dedicated workout modes

Within the app, where users are invited to enter their weight and height, wheelchair users will toggle one additional bit to indicate they are a wheelchair user and the rest happens automatically. So instead of steps, the app will track pushes. We also added two new dedicated workout modes for wheelchair users. Even before we announced this feature at the Apple Worldwide Developers Conference back in early June, we had partnered with the Lakeshore Foundation in Alabama to have their users try the software and give us feedback. We had your average users as well as paralympic athletes trial the app. With their experience and feedback, we managed to fine-tune the features in the app.