April 2001

In the Fast Lane

By Abu B. Yilla, PhD

In the Fast Lane

Seating and positioning for an active wheelchair user population.

Seating and positioning for the active user are dependent on the fundamental design of the modern, lightweight, wheelchair. Modern wheelchairs evolved from the box frame wheelchair developed by Joseph Jones and Bud Rumple in the late 1960s.¹

The box frame wheelchair freed designers from the restrictions inherent in the traditional folding chair design that dominated the wheelchair market until the late 1960s and introduced the now familiar concept of the lightweight. The box frame first became known as the “basketball chair” and then when adopted for general use.² The progression from basketball chair to lightweight has important ramifications for the modern health care professional.

Rumple and Jones designed the box frame to provide a better wheelchair for basketball. Conceptually, the inventors created a box and then attached main wheels to the rear of the chair and castors to the front. The box allowed for the use of lighter materials than the traditional, folding model and less material due to the absence of folding mechanisms and the inherent strength of a box design, which made the chairs lighter, sturdier, and more maneuverable. Additionally, the maneuverability of the wheelchairs could be manipulated by changing the relative position and angle of the rear wheels. Initially, high-functioning basketball players used these chairs and reveled in the improved performance that the design provided. Basketball players started using these chairs in everyday life. This was not without inconvenience as the rear wheels had to be wrenched on and off each time the chair was taken from a vehicle. The improved maneuverability more than offset this inconvenience and soon non-basketball players wanted to enjoy these benefits. In fact, for some individuals, notably those with quadriplegia, the improved handling characteristics of these chairs meant the difference between using a manually propelled wheelchair and using the less desirable powered chair.

Because of important health considerations (primarily postural), the medical community initially resisted the prescription of these chairs for everyday use. After an initial cultural clash, the nascent box frame wheelchair manufacturing industry made modifications in response to the resistance of the medical community and hence the modern lightweight evolved. While the folding concept has been integrated into the product models known as the lightweight, the inherent properties of the lightweight are revealed when examining the basic box frame. These important properties are the wheelchair frame, the rear wheel alignment, the camber angle (angle of the rear wheel relative to the frame), and the seat height.

The wheelchair frame primarily holds the other components of the wheelchair together. Because wheelchair propulsion is dependent on the transfer of energy from the user through the chair to translation across the ground, the stiffer the frame, the more energy is converted into forward movement. The box design of rigid chairs maximizes this energy conversion. However, many active wheelchair users need a chair with folding capabilities, reduced weight, and improved energy transfer. These should be weighed against the additional flexibility that a folding wheelchair provides. I believe that the medical community tends to overprescribe the traditional folding wheelchair while the experienced, active wheelchair user gravitates to the rigid chair when more in control of their prescription.

A critical first step in the prescription of the wheelchair is fitting the frame of the chair to the needs of the user. In fitting the frame, the two most important dimensions are the width of the chair and the relative positions of the seat and wheels. If the frame is too narrow, there will be insufficient clearance between the wheels and the body. This results in the wheel rubbing the person’s body, which not only produces frictional injury to the athlete but also slows the chair and reduces functionality. Conversely, if the frame is too wide, the handrim will be difficult to reach and even more difficult to push effectively. Additionally, a frame that is too wide will make it harder for the user to negotiate architectural barriers such as doorways.

Positioning in the modern chair is primarily dependent on the rear wheel and its alignment. The specific changes to the rear wheel can be identified as rear wheel camber angle, rear wheel alignment, and rear wheel position.

Main (rear) wheels are cambered to allow for superior turning and/or ease of pushing.³ A camber angle of as little as ⁴° can achieve this effect although the greater the degree of camber, the greater the benefit. However, when the main wheel is cambered, the overall width of the chair (the distance between the wheels at the ground level) is greater. Again, increased width may cause access problems with regard to doorways and other architectural barriers. Removing the plate that holds the rear wheel, adding more washers at the bottom of the plate than the top, and then replacing the plate can change the camber angle. With the increased sophistication of the lightweight wheelchair industry, however, camber is now adjusted in many different ways and each manufacturer’s product should be evaluated for the quality of their camber adjustment.

Changing camber angle also changes the alignment of the main wheels. It is essential that the main wheels point straight ahead in order to reduce rolling resistance. If the main wheels point slightly outward (toe-out) or slightly inward (toe-in), it significantly slows down the wheelchair. O’Reagan et al4 demonstrated that toe-in or toe-out of as little as 3° increased rolling resistance tenfold. It is not common practice to check for this alignment, and rehabilitation professionals are encouraged to do so or require that it be done by the wheelchair manufacturer representative.

The rear wheel position is perhaps the most critical seating consideration for the active wheelchair user. By moving the main wheel forward or backward, and up or down, relative to the user’s center of gravity, the wheeling characteristics of the chair will change. This will fundamentally affect how the wheelchair user should be seated.

The change in horizontal position of the rear wheel from a neutral position A to a more forward position B has the benefit of moving the wheelchair user’s center of gravity to the back of the rear wheel, thus making the chair considerably lighter in the front. This reduces the resistance encountered by the smaller front castor wheels and eases the wheeling of the chair over small disturbances in the wheeling surface. As with many of these positioning considerations, there is a trade-off. In this instance, moving the rear wheel forward leads to an increased propensity of the chair to rear up and tilt backwards, ultimately causing the user to fall out backwards. Controlling this propensity depends on the user’s ability to force the front end of the wheelchair down when necessary. This in turn depends on the stomach and leg function of the user. On discharge after the acute phase of spinal cord injury, the wheelchair user may not yet have fully recovered, which complicates the appropriate wheelchair prescription. If the prescribed wheelchair has adjustments for both horizontal and vertical position of the main wheel, then a certain amount of fine-tuning is possible to maximize mobility without the necessity of purchasing a new wheelchair.

The rear wheel can also be adjusted for vertical position. From the neutral position A, the chair can be raised or lowered relative to the rear wheel. Normally, the desire for everyday wheelchair use will be to raise the chair. Sitting taller facilitates activities of daily living by permitting the user to better reach objects, light switches, and other amenities designed for a world where people are at a standing height. Some experienced active wheelchair users exaggerate this benefit with artificially tall wheelchairs that allow for more normal societal interaction by raising the user closer to the eye level of the ambulatory population. These types of self-esteem benefits should not be minimized when rehabilitating individuals who, preinjury, were used to interacting with people at an equal eye level. There are, however, very real negative consequences when seating a wheelchair user in a tall position. This position elevates the user’s center of gravity, thereby creating increased challenges to balance that can be offset only if the user has good leg and trunk function. The position will also increase the rolling resistance of the front castors and may make it difficult to negotiate architectural barriers such as getting underneath tables and other work surfaces.

Making these adjustments to the rear wheel of the chair has serious postural implications. Maximizing maneuverability includes reducing the rolling resistance of the front castors. When achieving this, there will be a tendency for the user to have to lean their body weight forward even if they are in a bucket-type configuration. This invariably exaggerates the long-term postural complications inherent in chronic wheelchair users, who tend to utilize a small range of motion at the shoulder and the elbow for propulsion.⁵ Because the forces of extension during propulsion are much greater than the forces used in recovery, the asymmetry can lead to an imbalance in the joints particularly around the shoulder joint. This imbalance can lead to inappropriate posture and subsequent chronic health problems.5 This can be offset somewhat by encouraging active wheelchair users to incorporate a strength training program that emphasizes the recovery muscles and the development of these posterior muscles not normally targeted by users. When identifying the appropriate seating configuration, postural considerations should be given a high priority. Adjusting sitting posture will change the location of pressure on the user’s sitting surface.

Abu B. Yilla, PhD, is assistant professor in the Department of Kinesiology at the University of Texas at Arlington.

References

1. La Mere TJ, Labanowich S. The history of sport wheelchairs–Part I: The development of the basketball wheelchair. Sports ’n Spokes. 1984;9(6):6-8, 10-11.
2. Yilla AB. Getting functional. Sports ’n Spokes. 1998;24(2):23.
3. Yilla AB. Enhancing wheelchair sports performance. In: Winnick J, ed. Adapted Physical Education and Sport. 3rd ed. Champaign, Ill: Human Kinetics; 2000.
4. O’Reagan JR, Thacker JG, KauzIarich JJ, Mochel E, Carmine D, Bryant M. Wheelchair dynamics. In: Rehabilitation Engineering Center. Wheelchair Mobility 1976-1981. Charlottesville, Va: University of Virginia; 1981.
5. Curtis KA. Wheelchair sportsmedicine: part 2—training. Sports ’n Spokes. 1981;7(2):16-19.

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