At first glance, a consumer propelling an ultralightweight wheelchair seems like a pretty straightforward activity. And it somewhat can be, if the consumer is literally moving straight forward, on a smooth, level surface.
Of course, that’s rarely the case for any length of space or time. Thus, understanding more precisely how self propulsion works, what energy it requires, and how to make every push on a handrim as efficient as possible is critical.
Stephen Sprigle, Ph.D., PT, is a professor at the School of Applied Physiology & Industrial Design at the Georgia Institute of Technology. He is also director of the Center for Assistive Technology & Environmental Access and principal investigator of the Rehabilitation Engineering Research Center on Wheeled Mobility.
“Moving about in a wheelchair is really about maneuverability because you don’t go straight,” Sprigle tells Mobility Management. “We don’t walk in a straight manner, and people in wheelchairs don’t move about in a straight manner because of environmental constraints. So the data is pretty clear that ambulators and people in wheelchairs move around in short bursts of movement.”
Those “short bursts,” Sprigle says, mean “we’re always starting, stopping and turning. Starting, stopping and turning are really changes in momentum, and it’s how you maneuver. Maneuvering a wheelchair, just like maneuvering a body, is impacted by two principles, friction and inertia. And we have control from a design standpoint and a prescription perspective of both of these.”
In laymen’s terms, Sprigle and his team are studying not just the seemingly simple activity of wheelchair propulsion, but also the factors that make that propulsion less efficient. Efficiency loss results from many different things — for example, he points out: “When you steer a wheelchair, you’re braking one side and driving the other. You’re wasting gas.” Changing the mass of the system (i.e., the wheelchair + the person in the wheelchair) by adding armrests, anti-tippers or other equipment can also impact efficiency.
“Mass is very specific to the configuration, and we choose configurations for different reasons,” he says. “There’s a reason why you want to have armrests. There’s a reason you want to have anti-tip bars. They’re not evil; they shouldn’t be treated as some kind of stick, like, ‘If you add this, you’re going to get a bad performance.’ There’s a reason people have solid tires. But they also pay a price.”
One of the key tools to the research is a robotic system capable of propelling a wheelchair consistently for long periods of time — with a precision that human wheelchair users cannot replicate due to eventual fatigue or any of the other reasons our bodies feel and perform a bit differently from day to day. By using the robotic system with the same wheelchair setup, Sprigle and his team are able to track how energy is invested and used and lost, and when those losses, those inefficient moments, happen.
We’ll have more on this ongoing research in future Mobility Management issues.