Sitting Outside of the Box

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November 2001

By Jean L. Minkel, PT

Jean L. Minkel, PT

Sitting Outside of the Box

Clinicians need to let go of the 90°/90°/90° seating rule to explore more efficacious alternatives.

Jonathan Marks wrote, "The vast majority of ideas that most scientists have ever had have been wrong. They have been refuted; they have been disposed of."1 In the area of seating and positioning, we need to let go of an old idea-the 90°/90°/90° seating position for all wheelchair riders. This two-dimensional approach to providing postural support for a wheelchair rider is not only simplistic, in many cases, it does not work.

When it does not work, we, the professionals providing recommendations for postural support systems, just add straps-as many straps as needed to get the person to stay in the proper 90°/90°/90° seating position. These 90°/90°/90° seating systems are even less effective when mounted in a wheelchair with the back support in a perfectly vertical position. The result of this relentless quest for the ideal seating posture is what I refer to as therapist-induced immobility. A person who may have sufficient motor control to achieve dynamic seating has now been (s)trapped into the proper seated position.

As an able-bodied person, consider just one functional activity you routinely do in the seated position like driving a car. Do you share the driver's seat with another driver? Other than the absolutely functional requirement of moving the seat forward or backward to access the gas pedal, do you need to readjust the seat after the other person was driving? Do you need to change the backrest angle? Do you have a precise angle you need the backrest moved to, before you can even back out of the driveway? Look around-at your office chair, the conference room chairs-very few seats/chairs are made with a completely vertical backrest, or even a 90° seat to back angle.

Sling Back
Often, when first studying the need for adaptive seating, the lack of support provided by the sling seat and back of a wheelchair is noted. The older, stretched-out sling upholstery especially contributes to slouched postures and sliding out of the chair. However, before we throw the baby out with the bath water, we should look at the pros and cons of the sling back in particular. To understand the advantages of the sling back, and to comment on the improved upholstery now available, we need to step back and look at postural control in the seated position.

Those of us with an intact neuromuscular system can achieve and even maintain (at least briefly) a 90°/90°/90° seating position, which is sitting straight up. We don't choose to stay in this position too long, however, because it takes too much work; our stomach and back muscles have to be working together to allow us to hold this upright position against gravity. In a relatively short period of time, we are looking for some kind of external support-leaning against a backrest or leaning forward on a table or desk. We may even slide our hips forward and slightly posteriorly tilt our pelvis to achieve a comfortable (though slightly slouched) seated position.

Dynamic Seating
Our sitting is dynamic, however. Our intact neuromuscular system also provides us with a sensory feedback system. Eventually, that feedback reminds us that we need to shift our position, get off our sacrum, and sit up to allow the blood to flow. We shift, we move, and we do not stay in the same position for long periods of time.

Biomechanically, when we sit in the car seat and adjust the backrest angle, we are positioning the center of gravity of our trunk in a comfortable position relative to our base of support (our pelvis and legs). Often, we feel most stable (able to use both arms for driving and changing the radio station) when our center of gravity is slightly behind the base of support. Open the door of your car and look at the position of the driver's seat-not only the seat to back angle but also the position of the whole seat, relative to vertical.

Persons with poor postural control in the seated position often achieve this same stability (able to use their upper extremities for functional activities like wheeling, eating, or keyboarding), by assuming a kyphotic sitting position. In a sling-back chair, it is relatively easy to get the upper back behind the base of support, due to the slinging of the upholstery. The problem is that there is no support for the pelvis in the sling upholstery and the resulting posture is a sacral sitting and a kyphotic trunk.

The goal of adaptive seating, however, needs to be more than positioning the pelvis in neutral and straightening the trunk to achieve 90°/90°/90° at the hips, knees, and ankles. Rather, the goal should be to provide the external support, at the angles needed by an individual, to achieve an upright, stable, and functional seated position.

Consider the wheelchair rider, without fixed deformity, who has low trunk tone and needs the support of his or her hands to sit over the edge of a treatment mat. If asked to lift up both hands, this person will sink into a posterior pelvic tilt and kyphotic seated position, in an attempt to maintain sitting balance. If, as a therapist, I sit behind the person, reposition the pelvis into a neutral tilt, and support the trunk in an upright position directly over the pelvis, I will most likely need to provide both anterior and posterior support. Without the anterior support, the person feels he will fall forward.

In many cases, clients will fall forward because they do not have the intrinsic muscle control to hold this upright position. However, rather than just allowing the person to slump back into the kyphotic position, I can alter my supports by continuing to keep the pelvis in a neutral position, but allowing the upper trunk to move behind this base of support. Keeping the trunk extended, and allowing the upper thoracic region of the trunk to move behind the base of support, allows for increased stability, without sinking into a kyphotic position. From a side view, this supported seated position is curved (slight kyphosis in the thoracic region and a slight lordosis in the lumbar region)-not flat, as in the side view of a 90°/90°/90° seated position.

The biggest difference with this supported upright position, compared to the slumped kyphotic position, is that the pelvis is not in the sacral sitting position, but rather weight bearing on the ischial tuberosities. The upper trunk is reclined slightly, relative to the pelvis (3°, 5°, maybe 7°; not a lot, certainly not reclined back 15° or 20°).

Achieving Stability
There are several products now available that can assist a user in achieving a comfortable, stable seating position, without the need to slump. An effective intervention is redesigned back upholstery, referred to as tension adjustable back upholstery. The tension of the lower straps can be tightened to provide aggressive sacral and lumbar support, while the upper straps can be loosened to allow the thoracic region to rest, a bit further back, behind the base of support. Once properly adjusted, the straps are covered with a piece of upholstery, which provides an aesthetic cover over the straps.

Back supports, which replace the upholstery, may also provide similar support. Take the cover of the back support and look at the contours of the foam or the adjustments available through hardware. A couple of back supports are now available with segmented supports, which allow for individual fitting of each segment. The sacral supports can be adjusted separately from the upper portion of the backrest, which provides thoracic support. Once the pelvis is positioned, very small adjustments of the thoracic sections can make a tremendous difference in the person's postural stability. As noted earlier, small changes of 3°, 5°, or 7° may result in large changes in the person's sense of postural stability. Once stable, riders can often move their upper trunk freely-slightly forward and then resting back-eliminating the need for restrictive straps.

Even back supports that do not have separate sacral and thoracic components may be effective in providing a stable seating support. Those supports with angle adjustable mounting hardware can be adjusted from the vertical position to a slightly reclined position, which may improve a rider's stability.

Chairs with angle adjustable back posts-rigid frame chairs or chairs designed for postural support-can also be adjusted to improve the rider's sense of stability when sitting with an extended trunk.

Once a desired sitting posture and support have been identified, there are multiple methods for implementation. The combinations of adjustable mobility bases and/or postural supports can be overwhelming. A collaborative effort, actively involving the wheelchair rider, a supplier, and a clinician, can facilitate the identification of the combination of products that is most likely to succeed. The identification of a rider's personal, stable, functional position while using a wheelchair is a far more interactive process than just applying a formula like 90°/90°/90°.

Jean L. Minkel, PT, directs the consulting firm, Minkel Consulting, in New Windsor, NY. The firm provides consultation services to all team members in the field of assistive technology. She can be reached at jminkel@aol.com.

Reference
1. Marks J. Human Biodiversity: Genes, Race and History. Hawthorne, NY: Aldine de Gruyter; 1995.

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November 2001