SCI Roundup: Ultralights & Propulsion

Putting the Push in Propulsion

When aiming for efficient ultralightweight propulsion, what ultimate goals should be kept in mind? Long-term ability to self-propel? Improved daily functioning? Reduced risk of injury or pain?

David Boninger, Ph.D., VP of Out-Front, says the ultimate long-term goals are “all of the above — meaning that each of the goals is critical. One goal is to enable an individual to remain in a manual chair longer because he or she remains able to self-propel. Another central goal is to reduce the risk of pain and injury. If we can successfully reduce pain and injury, then we’ll be assured of achieving another critical goal — improved daily functioning and independence. These goals are closely related to one another: Improved daily function and continued independence rests on minimizing pain and injury.”

Despite the large scope of the goals, Boninger adds that the formula for achieving is less complicated.

“One key part of the formula is to reduce the stress and strain of each push on the handrim,” he says.

Thousands of Times Per Day

“In any given day, a person propelling their chair pushes on their handrim 2,000 to 3,000 times,” Boninger says. “So, anything that improves that interface — that makes the push less stressful — can make a huge difference… 2,000 to 3,000 times a day!”

An effective way to reduce the strain of propulsion is to significantly reduce the number of pushes.

“This is achieved by using longer, smoother pushes on the handrim, which will help to reduce push frequency and minimize forces,” Boninger says. “Fewer pushes means less repetitive stress. This formula stems directly from recommendations in the clinical practice guideline for the Preservation of Upper Limb Function Following Spinal Cord Injury.”

Evaluating the Trade-Offs

Using ergonomic components can help tip the push/strain equation in favor of consumers — but as is often the case with mobility systems, there might be trade-offs to think about. For example, will changing components add weight to the chair, thereby making it heavier to push? Will new components make the chair wider, which could lead to maneuverability or accessibility problems?

What give-and-take considerations need to be considered?

“When we think about aftermarket components that can impact propulsion techniques and ultimately help us achieve the goals we discussed earlier, we definitely want to be aware of what other aspects of the chair are affected when these components are added,” Boninger says. “With ergonomic handrims, for example, people are typically most interested in whether or not there are any meaningful changes in the width and weight of the chair.”

Fortunately, Boninger adds that today’s ergonomic handrims frequently don’t add width to the chair, depending on consumer preference and how the handrims and wheels are configured.

And when considering weight that ergonomic components can add, Boninger says it’s important to consider the overall picture.

“It is really important to keep in mind that the small changes in weight are more than off set by the benefits of ergonomic handrims,” he notes. In the case of his own Out-Front products, “These handrims have been heavily researched and tested, and time and time again, we find that the ergonomic benefits — like not needing to grip the rim as hard and not needing to grab the tire — far outweigh any small differences in weight.”

Boninger adds that using ergonomic pushrims aren’t the only factor in improving propulsion, but can be part of an equation that can make a big difference.

“When it comes to efficient propulsion, many factors come into play, but the primary interface or focal point for propulsion is the hand meeting the handrim,” he says. “So, an ergonomic handrim is certainly a key component capable of making a huge difference in propulsion efficiency. But it’s not just the handrim — it’s also proper posture and positioning, so that one’s arms are in the right position to most eff ectively achieve a long, smooth push on the handrim. When the proper ingredients are in place — an ergonomic handrim, proper positioning, an ultralight, well-fitted chair — then the difference can be huge, and it can light up the face of someone experiencing these ingredients for the first time.”

This article originally appeared in the June 2012 issue of Mobility Management.

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