Weighing Weight: Thinking Critically about Wheelchair Optimisation

Let’s start with the question - Does weight matter in terms of active user manual self-propel wheelchairs? Often a nuanced, even controversial topic among experienced wheelchair prescribers- what is your initial reaction? If you have a moment, make a few notes before reading further.

One easy point of agreement is that weight alone is not the only consideration. The RESNA position states that optimal wheelchair configuration is essential to maximizing functional potential, propulsion efficiency, and wheelchair performance.⁵ So how is optimal self-propel wheelchair 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 comprising a rider’s stability.⁴ 

To achieve an optimal configuration, clinicians and other providers 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 and angle, and horizontal and 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 these elements combine to impact weight distribution and rolling dynamics.

Once an optimal configuration has been determined, it is important to understand how wheelchair components and material selection can impact wheelchair performance. Generally, reducing weight does have some impact on reducing rolling resistance.⁷ 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 wheel, F_rr = µ_RR x W.³

• In relation to components, clinicians must review the pros and cons of specific components with the client to determine which components are essential to meet individual needs. For example, Ott et al. conducted a study to evaluate rolling resistance of different tyres and castor wheels.³ To mitigate rolling resistance, the study found that rear wheels with pneumatic tyres were preferred over solid airless inserts, and 4-5 inch casters were preferred over 8-inch casters. In fact, compared to high pressure pneumatic tyres, 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 max tire pressure was an increase of 16 lbs, revealing that an airless or “flat-free” insert is less favorable than a tire that’s lost pressure.³
• In relation to materials, manufacturers design active user 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 the weight of materials used in modern wheelchair 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 all things were equal, meaning you were to compare wheelchairs with identical frame geometry, configuration, and component selection, how would you weigh the impact of weight on the client? 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 people who self-propel 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 client.

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 recognise 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 clients whether weight matters, and the vast majority respond with an unequivocal “Yes”. For those riders that have already optimised the configuration of their wheelchair that are still looking to improve the performance of their wheelchair, weight is a consideration. For the individual who is routinely folding and loading a wheelchair into a vehicle, weight may impact their independence.⁵ And for a person who is unable to achieve optimal weight distribution within the wheelchair due to physical impairment or functional limitations, 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.