Freestyle Seating

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January/February 2004

By Mark Schmeler, MS, OTR/L, ATP; Michael L. Boninger, MD; Rory A. Cooper, PhD; and Rosemarie Cooper, MPT, ATP

Mark R. Schmeler, MS, OTR/L, ATP, (right) discusses wheelchair caster suspension options with a client at the Center for Assistive Technology at the University of Pittsburgh Medical Center.

Considering the human body in a seated position seems like a simple concept, yet it is complex. A reason for this is that the human body is not designed to sit, or at least not for prolonged periods of time in the same position. For people who sit in and propel manual wheelchairs, identifying the most effective seating system can be a challenge. The purpose of this article is to briefly review principles of seating and the components of a seating system for the active manual wheelchair user.

The human body is a dynamic structure that is designed to perform and engage in a variety of tasks in a variety of positions. From a purely biomechanical perspective, the seated body is unstable because the pelvis becomes the base of support and is able to tilt in all directions. Stacked above the pelvis are several vertebrae and a large cranium, making the structure dynamic but very unstable. The human body maintains balance through the neuromuscular system as well as through bones, joints, and ligaments. The human body also has an inherent need to move and change position in order to find comfort (relieve pressure or stretch) or find a better point of stability to function in a particular task. A simple way to grasp the concepts of seating and positioning is to pay attention to various postures you assume throughout the day or the postural behaviors you engage in when forced to sit for prolonged periods such as sitting in a theater or on a long flight.

Long-term sitting—especially in a poorly fitted seating system—can be pathological, as sitting has been associated with secondary complications such as pressure sores, back pain, joint contractures, postural deformities, and edema of the lower extremities. For people who have impairments to their neuromuscular system that require long-term sitting and the use of a wheelchair, seating and positioning becomes an even more important consideration. The wheelchair is the user’s primary means of mobility, therefore the person uses the same seating system for prolonged periods across an entire realm of daily activities. Power wheelchairs can offer features such as power tilt in space, reclining backs, and elevating legrests to give the seated person greater options for postural changes, as compared to people who self-propel manual wheelchairs.

Propulsion efficiency and mobility are important factors for manual wheelchair users. To achieve them, the wheelchair frame should be constructed of lightweight composite materials such as aluminum or titanium and configured in a manner that reduces rolling resistance, provides good access to the push-rims, and accommodates a seating system that meets the user’s needs. Many of the components on modern wheelchairs are adjustable, including the seat and back angles to allow for individual preferences and setup. However, active users eventually tend to prefer rigid or fixed components to reduce weight, increase durability, and further increase propulsion efficiency. The dilemma then becomes determining an ideal seating system to address a person’s needs across a multitude of activities.

When identifying an optimal manual wheelchair frame and seating system for a person, it is important to assess the user’s lifestyle, understand their physical motor and range of motion limitations, identify their goals and preferences, and, most important, allow them to try various configurations of manual wheelchairs and seating systems prior to making a final equipment choice.

When trying equipment, the user should perform a sequence of typical functional activities such as transfers, reaching activities, and stowing the device in a vehicle. To assist in identifying an optimal seating, several seating system components will be discussed.

Seat Cushions
There are a multitude of wheelchair seat cushions on the market, and choosing the right cushion is a factor of understanding the user’s goals and being familiar with the performance characteristics of the material’s properties. To stay focused on the concept of the seated person seeking function, stability, and comfort, it is important to understand that many cushions attempt to address all these needs, but there is no cushion that perfectly addresses all of them. Therefore, the benefit of one feature may need to be traded off for another more important one.

Air floatation cushions are lightweight and tend to provide the greatest pressure distribution when adjusted properly, but may give some users a sense of postural instability. They may also be more difficult to move around on for position changes or to slide across for transfers. Likewise, a flat foam cushion with a solid insert may give a better sense of stability, be easier to move around on, and require less maintenance, but may not provide adequate pressure distribution and may wear out or bottom out sooner than other materials. Other materials used for the construction of cushions include foams, polymer gels, viscous fluids (gels), plastics, and combinations, and help achieve pressure distribution, pelvic stabilization, and moisture control, as well as allow the person to move around on the surface.

Seat Angle and Orientation
The angle as well as the shape of the seat contribute to pelvic stabilization. Without stabilization, the pelvis tends to rotate in a posterior orientation, causing the buttocks to slide forward in the seat. This can cause shearing forces in the buttocks and lead the spine to collapse into a slouched posture. Providing a posterior slope to the seat angle, wedging the seat, or designing an ischial shelf in the shape of the cushion can provide greater pelvic stabilization and a better sense of balance, but may not always be ideal. It could make it more difficult to slide forward out of the seat for transfers, increase pressure in the ischial tuberosity regions, or promote more of a posterior pelvic tilt if the back support is not adequate. A posterior slope may also not be ideal for people who self-propel with their feet. It is also important to understand the fact that people tend to want to slide forward in their seats and slouch to relax and change position at various times throughout the day, as sitting upright continuously is difficult and fatiguing.

Back Supports
Wheelchair users are vulnerable to collapsing deformities of the spine, which can lead to breathing and other compromised vital organ functions. Back supports can be categorized by their shape, height, and stiffness. Ideally, a person is looking for a backrest that provides posterior and lateral support but does not inhibit freedom of movement in the trunk and upper extremities, which is needed for propulsion and other activities. A contoured or curved back should accommodate the width of the user and follow the natural curves of the spine to provide enough trunk support without compromising movement or function. Flat backs allow for more movement but might be less comfortable and stable as they provide no lateral stabilization, nor do they conform to the curves of the spine. A high back will provide greater support through the spine but may interfere with trunk rotation and shoulder movements. Back supports below the scapulae and thoracic spine allow for greater freedom of movement but may result in long-term spinal deformities. Fabric backs with adjustable tension straps are now more common in manual wheelchairs and tend to be lightweight as compared to rigid shell back supports and can be adjusted for varying needs and shapes. However, they do stretch and wear out over time and do not provide a sense of rigid stabilization.

Current trends in backrest design are focusing on taller style backs with lateral curves around the thoracolumbar regions with cut-outs in the scapular region to provide spinal support, yet still allow for movements at the shoulders. Attention is also being focused in the area of using adjustable yet durable carbon-fiber materials in the construction of back supports to address previous design shortcomings.

Arm Supports
Arm supports are necessary to rest the arms and provide a greater sense of lateral stability at the trunk, to hold onto with one hand while reaching with the other, or are used to push from for weight shifts or transfers in and out of the wheelchair. Armrest assemblies can also serve to contain the seated portion of the seat to keep the thighs in tight alignment as well as to protect clothing from rubbing on the tires. They do, however, add weight to the wheelchair and can get in the way of accessing the push-rims for effective propulsion. Therefore, many active users choose to forgo the use of arm supports. If arm supports are to be used, it is ideal for them to be removable or capable of swinging out of the way. They should also be set at an ideal height for the user or be height adjustable.

Foot and Leg Support
Foot and leg supports can be classified by the angle to which they position the knee and, in turn, position the location of the feet. Traditional swing-away footrests are designed to keep the user’s feet out in front with about 60 to 70 degrees of knee flexion to avoid interference with caster swivel and rotation. This is not necessarily a natural seated position as it promotes a slouched posture through pulling the hamstrings and the pelvis into a posterior pelvic tilt.

Elevating legrests pose an even greater problem. A perceived purpose of elevating legrests is to assist with edema management. However, being seated in an upright position with the knees extended will do little to assist with edema unless the feet and legs can get above heart level, which can be accomplished only by tilting the seat back and reclining the backrest. Elevating legrests without these features will also likely pull further on the hamstrings, promote more of a slouched posture, and cause serious problems for people with limited knee extension.

Most people prefer to sit with their knees at a more natural 90-degree position or flexed with their feet tucked under the seat to provide additional postural stability and shorten the length of the wheelchair for maneuverability. This position also warrants tapering the footrests inward to keep the feet from interfering with caster swivel. Some active users also prefer to have the foot supports fixed in a one-piece structure to add more rigidity to the frame. However, removable or swing-away foot supports should not be neglected for people who stand to transfer out of the wheelchair or propel with their feet.

Conclusion
It is clear that there now exist many options with modern wheelchair designs, and there does not appear to be one ideal positioning solution for everyone’s needs across all activities. The challenge lies with the practitioner working closely with consumers and qualified Rehabilitation Technology Suppliers to identify needs and preferences, as well as to try various options prior to making a final decision.

At the University of Pittsburgh Medical Center, Mark R. Schmeler, MS, OTR/L, ATP, is director of the Center for Assistive Technology; Michael L. Boninger, MD, is medical director of the Human Engineering Research Laboratories, associate professor, and research director, Department of Physical Medicine and Rehabilitation; Rory A. Cooper, PhD, is chairman, Department of Rehabilitation Science and Technology; and Rosemarie Cooper, MPT, ATP, is an evaluation specialist for the Center for Assistive Technology and an instructor for the Department of Rehabilitation Science and Technology.

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January/February 2004