Weighing Weight: Thinking Critically about Wheelchair Optimization

What is your initial reaction when asked the question, "Does weight matter in terms of ultra lightweight wheelchairs (ULWC)?" This is a nuanced, even controversial, topic among experienced wheelchair prescribers. If you have a moment, make a few notes with your reaction before reading further.

Configuration

One easy point of agreement is that weight alone is not the only consideration. The RESNA position states that optimal ULWC configuration is essential to maximizing functional potential, propulsion efficiency, and wheelchair performance.⁵ So how is optimal ULWC configuration accomplished? To improve propulsion efficiency, Brubaker stated that a forward axle position, allowing for a greater distribution of mass over the rear wheel, decreased rolling resistance.¹ Similarly, the PVA Upper Limb Preservation Guide recommends that the rear axle be adjusted as far forward as possible without compromising a rider's stability.⁴

To achieve an optimal configuration, clinicians and ATPs need to focus on balancing the rider's center of mass (CoM) over the wheelchair's center of gravity (CoG). Balancing CoM over CoG provides the rider with the stability necessary to reach overhead and complete functional activities while increasing the ease of wheelchair propulsion and wheelchair responsiveness. Seat depth, seat-to-floor height, seat slope, back support height & angle, and horizontal & vertical axle position are configured to provide a rider stability within the system.⁵ Additionally, it is essential to understand the impact of seat depth, frame length, and front frame angle on the wheelchair's wheelbase and caster position. All of these elements combine to impact weight distribution and rolling dynamics. These critical frame measurements are explained in detail within the Clinical Considerations for Rigid Wheelchair Prescription course.

Once an optimal configuration has been determined for the rider, it is important to understand how wheelchair components and material selection can impact wheelchair performance. Generally, it is important to minimize the overall weight, as an increase in weight increases rolling resistance.⁵ Rolling resistance (F_rr) being the frictional force that is calculated using the coefficient of friction (μ_RR) multiplied by the weight (W) on the wheel³: F_rr = μ_RR x W.

Components

In relation to components, clinicians and ATPs must review the pros and cons of specific components with the rider in order to determine which components are essential to meet the rider's individual needs. For example, Ott et al. conducted a study to evaluate rolling resistance of different tires and casters.³ To mitigate rolling resistance, the study found that rear wheels with pneumatic tires were preferred over solid airless inserts, and 4-5" casters were preferred over 8" casters. In fact, compared to high pressure pneumatic tires, the perceived weight equivalent for tires with airless inserts was an increase of 96 lbs., while the perceived weight equivalent for tires with 40% of maximum tire pressure was an increase of 16 lbs. This indicates that an airless or "flat-free" insert is less favorable than a pneumatic tire that has lost pressure.³

Materials

In relation to materials, manufacturers design ULWC frames and components from aluminum, titanium, and carbon fiber with the goal being to reduce weight while increasing strength, durability, and performance. However, due to improved design and engineering, the weight differential between materials used in modern ULWC frames is minimal and there is no consensus as to which material is optimal. Ott suggests that device weight should not be heavily considered since the differences in weight are not enough to have a statistically significant impact on rolling resistance.³

So if you were to compare wheelchairs with identical frame geometry, configuration, and component selection, how would you weight the impact of weight on the rider? Aside from wheelchair performance, we must also consider how weight may impact a rider's functional potential. While there is no consensus as to how much weight is statistically significant in terms of shoulder loading during propulsion, we know that riders who self-propel an ULWC are at risk for repetitive strain injury (RSI), with risk factors including the frequency and force required for propulsion.⁵ Therefore, I believe the answer for how to weigh weight exists with the rider.

Rider Input

Though we often seek research to inform practice, research is not always available and not all research fits the realities of practice.² In this case, it is important to recognize that the riders' experience and clinical expertise also exist as evidence within the evidence-based practice (EBP) model, and should not be disqualified.⁶ In my career, I have asked many riders whether weight matters, and the vast majority respond with an unequivocal "Yes." For those riders who have already optimized the configuration of their wheelchair and are still looking to improve the performance of their wheelchair, weight is a consideration. For the rider who is routinely folding and loading their wheelchair into a vehicle, weight may impact their independence.⁵ And for the rider who is unable to achieve optimal weight distribution within their wheelchair due to physical impairment or functional limitations, the weight of the wheelchair may be the only factor left to improve rolling resistance.

Ultimately, I don't weigh weight. I understand its application and its implications and have a responsibility to educate consumers. The rider determines if weight impacts their function, their independence, and their quality of life.