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June/July 2002


Seating for Task Performance

By Karen M. Kangas, OTR/L

Seating has almost always been thought of as sitting in a particular, optimal position. In the past, developing seating systems for anyone involved planning for optimal support in a seated posture. Seating for task performance is not a single position, but rather an active repertoire of seated postures that the body requires in order for the mind to think, the eyes to read, and the arms to work.


Karen M. Kangas, OTR/L




PELVIC STABILITY

For any isolation, and adequate use of an extremity in a graded, controlled movement (for accurate and competent use in task completion), pelvic stability with pelvic weight-bearing must occur and be controlled by clients themselves. This stability of the pelvis is not a position of immobility but rather a position that allows a range of self-controlled (limited, graded) pelvic mobility.

Pelvic stability is a position of actively holding still rather than being passively restricted. Pelvic stability is not a position of relaxation or of stillness, but a position of co-activation or an active holding. In short, the body must allow muscle lengthening and controlled shortening simultaneously to allow controlled holding. This active holding will allow more active movement, which will then be controlled by an extremity.


Sandra, a 51-year-old woman with cerebral palsy, before receiving her seating system.

The final fitting of Sandra’s seating system for task performance.

In her new system, Sandra is now able to move forward, demonstrating more body control, and enabling her to use a communication device.


This pelvic posture is not simply a musculoskeletal posture, but rather a movement of the body, which includes an ongoing interaction of numerous systems including the musculoskeletal, neuromuscular, circulatory, respiratory, gastrointestinal, and endocrinological systems. These physiological systems become integrated and then can be used and developed with experience. This experience is specifically identified by the body for a particular task and its performance.

Pelvic girdle stability is required for shoulder girdle mobility. This relationship is critically related to weight-bearing and movement. It is pelvic girdle stability combined with shoulder girdle mobility that provides a foundation from which the head and hands can be controlled and moved.

THE SEATING POSITION

Generally, a seated position for task performance is one where clients' shoulders and head are aligned in front of their pelvis, rather than in line with the pelvis or behind the pelvis. Feet are on the floor, are weight -bearing, and are not symmetrically placed. A knee may be lower than a hip in alignment and one may be higher than the other (in line with the hip). The knees are used and held at a posture of less than 90° of flexion, placed below or under the body. Weight-bearing is not symmetrical, and lower extremity musculature (both hamstrings and quadriceps) is coactive. This posture is best described as the posture a person uses to get up out of a chair, but without getting up (imagine freezing just before rising from the chair, eg, leading with the head, the pelvis in an anterior tilt, the legs weight-bearing, and the shoulder girdle in front of the pelvis). This posture also assumes that the trunk is able to be slightly rotated or the pelvis or both, to obtain and manage power in this movement. (Power is strength combined with active range, through coactivation, or most often used by the body through movements controlled by rotation.) As experience with this posture is gained, movement is able to be initiated, maintained, and replicated as a task or routine would suggest. In contrast, individuals with increased tone are often seated in a reclined or overly tilted posture, with a wedged or antithrust seat (placing the knees higher than the hips, decreasing the hip angle to less than 90°, and forcing the pelvis into a static posterior tilt). Their feet are off the floor and raised onto footplates smaller than their feet. Their lower extremities are placed in symmetry, abducted, usually placing the knees at a position wider than the hips, rather than in line with the hips. The knees are located at a position of more than 90° of flexion, although their feet may be parallel to the floor. A pommel is added and straps are used at the chest, pelvis, ankles, and feet. This posture prevents weight-bearing on the thighs and the feet, and does not allow the pelvis to anteriorly tilt. In fact, the lower extremities are not allowed to be placed in line with the hips, and are certainly not permitted to touch each other.

Why does this matter? In human beings within earth's gravity, body control is interpreted and performed when the body understands its relationship to that gravity, primarily through the activation of the vestibular system. The ability to weight bear (interpreted through motor proprioceptors, kinesthetic receptors, and the vestibular system), and to stably, yet dynamically, react to gravity is critical to our every movement. In fact, the movement of the pelvic girdle (the shift of the pelvis into an anterior posture and the subsequent active weight-bearing in the lower extremities) is also an alerting reaction to the musculoskeletal system via the endocrinological system. If a person wants to perform a task, the pelvis must shift, sending messages to both the shoulder girdle and to the head that gravity is supporting the activity, and the body is ready to work. This shift of the pelvis forward also requires more power from the body and asks the body to kick-in tone in extension. This trunk extension lends additional power to the pelvis for increased stability, to the lower extremities for weight-bearing, and to the shoulder girdle and head for independent movement and control.

In short, the pelvis and its movement signal the entire body to become ready to perform a task by encouraging an increase in tone (weight-bearing and anterior pelvic tilt), in power (rotation of the pelvis and/or trunk), and in alertness (adrenaline added to motor initiation).

CASE STUDY

For example, consider a child with athetoid cerebral palsy with lower tone exhibited in the trunk and higher tone exhibited in her extremities. In her current seating system, her trunk and lower body were supported with a tall back, a wedged (antithrust) seat, foot straps at both heels and toes, a separate chest butterfly strap, and a large head support. As she attempted to reach a switch, her pelvis would thrust posteriorly, she would attempt to push on her legs and feet, and her arms would fly up. She was diagnosed as having too much thrust, too much tone, and too inconsistent with switch access. Why? As she needed power from her body, and attempted to move her pelvis, the only place it could go was posteriorly due to her seated posture. This power or increased tone, rather than being used to help her head and hands, banged against the headpiece and footplates, further trapping her within her seating system. The more she worked, the more tone she required, the more banging and extensor tone occurred, and success at reaching a switch was impossible.

However, when her seating was changed to allow her to perform a task, her tone was able to actually provide adequate power for real task use. The seating was changed by first placing her into a carved, firm seat, with deep thigh channels (this provided her legs with tactile/kinesthetic input). The seat itself was placed in a very subtle anterior tilt. The footplates were extended under the seat, allowing the feet to be placed there. The back was replaced with one of shoulder height. Deeper trunk positioners close to the trunk were added not curved, but rather simply available to cue her trunk, and deep enough to be present when she would bring her trunk forward. The chest strap was removed, as well as the headpiece. Velfoam straps were added in a simple posture, around the knees, and in front of them.

This seating allowed some containment of the pelvis, support at the knees, and prevented her from pushing off of the footplates when back extension was initiated for head control. Trunk support existed but was not constricting. She was now able to move and see. She had more control of her body, and was ready for the task. She did not use a head support. In fact, head control was evident, whereas previously, it appeared to be questionable.

BYPASSING THE 90/90/90

The 90/90/90 position is a totally symmetrical posture. This posture may and often can passively and temporarily reduce tone. As a resting position or posture not allowing tone, it subsequently prevents power for any control or performance for tasks. When task performance is needed, movement and tone are needed.

This position (90/90/90) is too restrictive. To maintain clients in this posture, especially those who have increased or varying tone, they must have a multitude of straps holding them in place. Using straps does not permit the body to learn how to move. Restriction assists only in preventing movement, or paradoxically, due to the strapping, the body learns to fight the system, adding more power directly at the point of the strapping.

TASK PERFORMANCE POSITION

First of all, seating for task performance is not a seated posture to be maintained all day, or for long periods doing nothing or resting. A task performance position is one that must be able to be assumed, maintained, and then moved from to perform a task of interest and intent. In short, the seating has to allow a change in postures. This can best be developed with the use of a tilt-in-space function as well as less restrictive seating while the individual performs a task.

The individual must be able to move and control the movement. This control is learned, through practice and through repeating an activity that is enjoyed Most often with young children or with inexperienced adolescents or adults, I find that controlling powered mobility with head access can provide an excellent opportunity for providing seating for task performance. With a powered system, the seating can be changed without changing the seat with which the individual is already extremely familiar (in her manual wheelchair). The seating for powered mobility can be adjusted for task performance, the chair can be programmed for head access, and while observing the individual involved in the activity, what seating changes (while performing this task) need to be made, and where, can be determined. For example, if the seat is too anteriorly tilted, the individual will lose trunk control. If the lower extremities kick out, or show increased tone, or move into adduction, the pelvis is not adequately stable. If the trunk collapses, the anterior support is not enough.

All individuals already carry the knowledge of the task performance posture with them. We actually do not learn this posture; it is already a part of us, and reveals itself over time increasingly as tasks are performed. However, those of us who use a wheelchair have great difficulty, without experience of weight-bearing, finding these postures. That is why, those of us, here to help, must better understand how a body without adequate weight-bearing works, and provide systems that will allow weight-bearing, pelvic stability, and mobility to occur. With that, tasks can be performed and independence attained.

Karen M. Kangas, OTR/L, is a seating and positioning specialist and clinical educator in private practice in Shamokin, Pa. She can be reached via email: kmkangas@ptd.net.

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