Let’s continue our discussion on seating and mobility considerations for individuals who are bariatric. (If you missed the first two articles in this mini-series, you can click here for Part 1 on Body Shape and click here for Part 2 on Assessment and Seating.) This month, we will look at the mobility base.
Let’s begin with looking at manual mobility. To think about the effect of size and body shape when propelling a manual wheelchair, consider the graphics below, where the red circle represents the individual’s approximate centre of gravity:
Graphics are copyright Sunrise Medical, Inc.
If these individuals are seated in a similarly configured wheelchair, the results will be vastly different. The wheelchair in the graphic represents either a standard wheelchair or a custom folding wheelchair with the rear wheels in the most rearward position. (See Manual Mobility: The Basics, for the differences between standard and custom folding wheelchairs.) In comparing the two graphics, it is easy to see that the person represented in the graphic to the right is more forward of the rear wheel than the person represented in the graphic to the left due to the excess posterior tissue on the individual to the right. This makes it more difficult to reach the rear wheels and causes excessive shoulder extension. It also affects the person’s ability to have a smooth push stroke for efficient propulsion. This is one of the reasons why it is more difficult for an individual with excess posterior tissue to propel a standard wheelchair.
In the above graphic to the left, if we draw an imaginary vertical line through the person’s centre of gravity, we would see that much of the mass of the user is over the rear wheel. In the graphic to the right, the same imaginary vertical line drawn through the person’s centre of gravity shows that much of the mass of the user is going through the front casters, which will load the front casters. We know that when the front casters of a wheelchair are loaded, the wheelchair becomes much more difficult to propel and to maneuver, whether the person is hand propelling, foot propelling or using a combination of the two. For an individual who foot propels, we may see instances of sliding forward in the wheelchair if the front casters are loaded as the effort required for heel strike and pull through pull the pelvis forward in the seat. The graphic explains why individuals who are bariatric require a wheelchair that has centre of gravity adjustability, such as found in the generic custom folding category of manual wheelchairs. For more on Centre of Gravity and Manual Wheelchairs, please click here.
Sometimes moving the rear wheel forward to alter the centre of gravity of the wheelchair is insufficient to have optimal weight distribution between the front casters and the rear wheels. Consideration also should be given to the position of the caster housing, which can be placed in either a leading or trailing position on some models of wheelchairs. Having the caster housing in a leading position will reduce the load through the casters. (For more information on front caster position and photos that illustrate the difference between leading and trailing caster housing positions, please see Front Caster Position in Manual Wheelchairs.) Some manual wheelchairs that are specifically designed for a bariatric population have the casters set in a more forward position to minimize the loading through the front casters, as the photo below illustrates:
Photo is copyright Sunrise Medical, Inc.
The effect of the load distribution between the front casters and the rear wheels is one factor that affects rolling resistance. Several other factors influencing rolling resistance are outlined in a previous post, Rolling Resistance in Manual Wheelchairs. For individuals who are bariatric, it is important to consider the choice between solid and pneumatic tires as solid tires demonstrate greater deformation as the load increases than pneumatic tires. This means that a heavier individual will experience greater rolling resistance with solid tires than someone who weighs less.
Another consideration when prescribing a wheelchair for an individual who is bariatric is lower extremity positioning. In the Part 1, we looked at the effect of excess tissue in promoting either hip abduction or adduction. Footrests and hangers need to be chosen to accommodate lower extremity positioning. The photograph below, to the left, shows that the distance between the hangers increases as the width of the wheelchair increases. This may be appropriate for someone with hip abduction. Someone who presents with hip adduction due to increased adipose tissue laterally may require an auto-folding platform, rather than individual footplates, on the hangers, as this would still allow the wheelchair to fold while having the feet positioned medially. The photo below, to the right, shows a centre-mount footrest option, which would be appropriate for someone who has hip adduction.
Photos are copyright Sunrise Medical, Inc.
With respect to power mobility, weight distribution between the drive wheels and the casters will affect performance of the wheelchair. Power wheelchairs have centre of gravity adjustability. Just as with a manual wheelchair, it’s important to ensure that the front casters of a power wheelchair are not taking too much of the individual’s weight.
Understanding a person’s unique shape and how this influences an individual’s needs with respect to seating and to mobility base set-up is the key to prescribing a seating and mobility system that will enhance function and mobility for an individual who is bariatric.