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ERGONOMIC SEATINGA TRUE CHALLENGE

BENGT ENGSTRÖM

Some pages from the book

Seating and Mobility for the Physically ChallengedRisks & Possibilities

When Using Wheelchairs

2 / ERGONOMIC SEATING - A True Challenge

"We are all individuals, searching for individual solutions.When used long-term a wheelchair should, be looked upon asa body orthosis on wheels."

I dedicate this book to each and every man and woman, boy and girlwho goes through life sitting in a wheelchair

ERGONOMIC SEATINGA True Challenge - When Using Wheelchairs

ISBN 91-972379-3-0

Book Design and Illustrations: B. EngströmCover Design and Cover Photo: B. Engström and A. Engström

The title of the first edition wasErgonomics, Seating and PositioningPublished in Sweden, 1993.

© Copyright 2002 Posturalis Books

All Rights Reserved.This book is protected by copyright. No part of the book may be reproduced inany form or by any means, including photocopying, or utilized in any informationstorage and retrieval system without written permission from the copyright owners.

Printed by Medio Druck & Logistik GmbH, Germany

The production of this book is generously sponsored by

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Contents

Foreword ........................................................................................... 5

1 Humans are physically intelligent ............................................. ... 9How should one sit? .................................................................. 10Influenced by the senses ............................................................. 12Factors that cause fatigue .......................................................... 13The sensitive human body ........................................................ 14The skeleton - fundamental principles ...................................... 15Normal sitting behavior ............................................................ 23

2 Fundamental seating principles .................................................. 37Tendency and reaction .............................................................. 38Firm or soft seating units .......................................................... 38Backrest recline ......................................................................... 40Tilt-in-space .............................................................................. 41The trunk, forwards ................................................................... 43From seat, to pelvis, to trunk ..................................................... 44Seat contours ............................................................................. 47Pressure and shear ..................................................................... 49Seat angle and contour .............................................................. 52Experience seating .................................................................... 54Posture and reactions ................................................................ 55Head- on- trunk sensitivity ........................................................ 57Correcting the trunk .................................................................. 58Interpret and understand ........................................................... 68

3 Physical intelligence and physical disability ............................. 71Fatigue and defence .................................................................. 72Stressed by pressure ................................................................. 73Equilibrium and stability ........................................................... 74Contractures .............................................................................. 76Posture and inner organs ........................................................... 79Posture and stimulation ............................................................. 82Pathological patterns ................................................................. 83Responsibility and financial aspects ......................................... 88

4 Wheelchair Design & Adjustments ............................................ 91Design and adjustments ............................................................ 92Seat cushions ............................................................................. 94Adjusting seat slope .................................................................. 98The chair’s back - a body orthosis ........................................... 100Stability for activity .................................................................. 110Adjusting step-by-step .............................................................. 111Positioning the feet ................................................................... 116Armrests and tray-table ............................................................ 118

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5 Wheelchair Mechanics .................................................................. 121Improving mobility .................................................................... 122Rolling resistance ....................................................................... 123Balancing the centre of gravity .................................................. 126Rotatory vestibular stimulation .................................................. 130Wheelbase .................................................................................. 131Wheels - camber & caster angle ................................................ 133

6 Wheelchair Mobility ...................................................................... 139Arm propulsion principles ......................................................... 140The pushrim - grip ..................................................................... 146Risking hand injuries ................................................................. 148Foot propulsion .......................................................................... 150Foot propulsion principles ......................................................... 151Compensation and adjustment ................................................... 156

7 Wheeling Skills ............................................................................... 163Basic propulsion ........................................................................ 164Prevent and correct falls ............................................................ 166Balancing on drive-wheels ........................................................ 167Climbing curbs and other obstacles ........................................... 168Slopes and hills .......................................................................... 170

8 Seating & Mobility Needs ............................................................ 173General aspects .......................................................................... 174Paraplegia .................................................................................. 180Quadriplegia / Tetraplegia ......................................................... 184Hemiplegia ................................................................................. 188Traumatic head injuries ............................................................. 192Cerebral palsy ............................................................................ 196Arm patterns .............................................................................. 198Leg patterns ............................................................................... 200Hip contractures ......................................................................... 202Knee contractures ...................................................................... 203Amputated legs .......................................................................... 204Elderly - women and men ......................................................... 206Thoracic kyphosis ...................................................................... 208

9 General Comments

Wheelchair Features / Guidelines .............................................. 213-231

Adjustments and Influence / Guidelines .................................. 233-239Standard wheelchair adaptation ................................................. 240

Literature / Internet Links ............................................................. 242

Index .................................................................................................... 243

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Humans ArHumans ArHumans ArHumans ArHumans Are Physically Intelligente Physically Intelligente Physically Intelligente Physically Intelligente Physically Intelligent

Regardless of what one sits on, an armchair, astool, an office chair, a car seat or a wheelchairthe body is always mechanically affected. Howseating is designed always has an impact.

This chapter deals with how the seating unitmay affect the body, in general, and more indetail. General influence means how the bodyreacts when general features like seat and backangles, and/or softness change. Broken down intomore detail the way the body reacts may beinfluenced by an area of a seat, the position of afootplate or the contour of a backrest etc..

When you understand how a body may beinfluenced into reacting by a specific backrestcontour, seat angle or seat contour it is easier todecide if a seating unit will suit a person’s needs.

To achieve successful and reasonable results,practical solutions and choices should be basedon how the seating unit affects the body, in theshort, as well as the long term. Long term in thisbook means many hours daily, for many years.

When wheelchairs are set up, the goal variesfrom one user to another, since every user isunique, physically and psychologically. Theenvironments users live in can be very different,as can the requirements as to how a wheelchairshould function. Each combination of user andwheelchair is new and, thus, always a challenge.

The result is always better when the personwho is responsible for choosing and setting upwheelchairs is open-minded and very receptiveto what the user wants and needs.

Adapying wheelchairs is sometimes a compli-cated puzzle. A user who has had a poor positionfor several years is not easily convinced that a“proper” and more upright position is better. Thetime the user needs to complete a transition isoften much longer than you expect.

This chapter is about important principles foruse when setting up seating units, and to a lesserextent for propulsion.

A person’s position is mainly dictated by howthe seat and backrest are designed, or adjusted,to suit the user. This also includes the armrestsand footplates.

The “pitfall” can be when other people dictatetoo much how the user should sit, based on whatthey know about seating. It is always worth whileto remember that very few of us can be seated inan active way for more than a very short while.If a user doesn´t accept a position, take itseriously and make the necessary changes.Experiment, don’t be afraid of making“mistakes,” they help us to learn. Just makesure corrections are made in time

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How should one sit?

How should one sit to be ergonomically seated?It is not easy to answer such a question.A list of what many people see as important

factors towards “how one should sit” can besimilar to the list below.

This points out some of the main factors thatare important for a decent sitting situation –allowing a natural balance between functionalstability and healthy activity.

Feeling safe is a crucial factor to enable anyoneto relax when seated. During long-term sitting,safety is more important than we realize. Whena person has difficulties in changing positions, asense of safety and security is of paramountimportance.

Feeling safe includes: comfortable pressure,reasonable stability with a possibility to move,and, last but not least, being able to decide when,and how, to move.

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A stable supporting surface is necessary to“anchor” the parts of the body that are active,and/or in motion.

When seated, the stability required is for thepelvis, thighs and feet. With good, but notexaggerated, stability provided for the correctappropriate parts of the body, people can be veryactive physically when seated. Seat stability isoften interpreted as meaning that the seat shouldbe firm. This interpretation is discussed below.

Besides providing the stability required, a seatthat gives such stability must not causeuncomfortable pressure. Pressure is what createsstability but is more efficient if distributed overa larger surface. If pressure is concentrated on asmall surface of the body it decreases stability.

Discomfort, due to pressure, results in muscularfatigue when the user tries to stabilize theinstability of the trunk.

Pressure distribution over a big, soft area givesmore comfort, but, if the area is too soft, stabilitydecreases.

One crucial detail in sitting is the capability ofbeing active forwards. It is obvious that forwardactivities are hindered if it is difficult to leanone’s trunk forwards.

When an office chair is adjusted with the seatsloping slightly forwards, forward activities areeasier to perform.

In such a situation one can easily lean forwards,but if the trunk’s tendency of falling forwards istoo strong (due to the forward slope) the backmuscles become fatigued.

It should be as easy to lean back and relax as itshould be to lean forwards.

When both possibilities exist, position can becomfortably changed and dynamics are improved.Poor dynamics increases the static portion ofseating, which causes stress and problems.

How much support the back needs when seateddepends on what type of activity one wants to do.

Where should support be given?Again this depends on the desired activity,

static rest or dynamic movement.

One thing is clear though, the way the back issupported dictates the stability of the upper body.

Sometimes too much support, especially whenwrongly placed, makes seating more tiresomethan less support.

Another thing we very often overlook is thenecessity to move our legs when seated. Movingthe legs is one of the important ways of makingseating dynamic. We use our legs and feet asinstruments to influence the upper body whenseated. A more specific analysis of this is givenshortly.

As I mentioned earlier, it is important to feelsafely and securely seated.

A feeling of being safe can increase comfort,and thereby, the time that can be spent seated.

You become calmer, physically and mentally.This makes the seating situation more comfortable- and functional.

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Influenced by the senses

Our body is very sensitive to positions. Thereare many kinds of sense organs registeringchanges in the body. All of them are, of course,important, but in this book the focus is on theones that register the position of the body.

We cannot “shut off” our sense organs, they goon registering continuously.

If no external support is used, the body of aperson who is standing is continuously in motion,a balancing motion, known as postural sway. Wecannot “see” these movements of the body, butthey are present all the time.

Feet are designed for standing, walking andrunning. The sole, the foot joints and foot musclesas well as the leg joints and muscles registerpressure and pressure-directions when the foot isin contact with a surface of support.

The pressure on the sole influences the jointsdifferently, depending on the magnitude anddirection of the pressure.

To maintain balance pressure variations leaddirectly to muscular reactions throughout thebody.

When the muscles balance, and stabilize thebody, or its parts, in this way, we are seldomconscious that it is happening. The muscles areautomatically activated by the senses on asubconscious level. The reactions and correctionsare very precise and continuous.

Being seated on a chair makes the feet and legschange in relation to the trunk, radically andinstantaneously.

When standing, the centre of mass of the entirebody is located over the feet, which are thesupporting area. When seated, the centre of massof the trunk is only over the pelvis, with theischial bones as the supporting area.

This means that the smooth, balancing andcorrecting motion, which takes place when we

stand on our feet is not present when we areseated.

I am not saying that interaction between thelegs and the upper body disappears. But, whenthe feet are not loaded under the pelvis theirfunction of being sensitive “organs” formaintaining the balance of the body is eliminated.

Instead of relying on the feet for balance, we siton two tiny ischial bones. These two bones arethe support surface for the pelvis and, since thetrunk is on top of the pelvis, the entire upper bodybalances on the ischial bones.

Balanced seating requires a well-positionedpelvis. When the pelvis does not have a functionaland stable position the upper body cannot be keptin an upright position for more than a few, shortmoments.

The leg’s connection with the pelvis is throughligaments, joint capsules and muscles. Becauseof this connection, the position of the feet andlegs, as well as their muscle activity, influencethe position of the pelvis directly or indirectly.

In most seated positions the tendency of thepelvis is to tilt backwards. This tendency can beblocked or forced by the legs, something we docontinuously. It is a natural part of sitting.

Sit straight! Sit properly! Sit still!Most of us have said these to our children,

especially when they sit and eat.- But what is “proper” seating?If we tell a four-year-old to sit properly he will

probably become confused.If we do not know what it means to be seated

properly, how can we expect a child to know?To be seated “properly” is possibly a posture

that feels good - for the moment.Whenever our posture-sensing organs register

something negative in a position we re-positionthe body, over and over again.

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Factors that cause fatigue

I am sure you have experienced how awfullytiresome it can be to sit, especially when youdon’t want to sit.

Imagine sitting and listening to a lecture whichis not at all stimulating. It has been going on fornearly two hours. You are sitting in the middlepart of the lecture room, which is crowded withpeople, so you can’t “sneak out”...and it’s thirtyminutes to the next break!

Sitting makes the body’s physical stabilityincrease. If you also lean against a backrest youbecome highly stabilized. This is like putting thebody in a state of physical “rest”.

You can easily fall asleep when stabilityincreases and few things happen to keep youawake – things like a lively, positive lecturer!

Have you ever thought of the fact that an activeand inspiring lecturer makes us decrease ourphysical stability. We forget the backrest and sitmore upright, maybe we even lean forwardstowards the speaker - the person who is sointeresting. Physical activity is strongly linked tomental activity and vice versa.

Imagine you are sitting in a car and you are thedriver. You and your friend have been travellingfor many hours and the last hour you have bothbeen silent whilst you are driving on a highway.The engine is humming and the wind whispering.The weather is grey and it´s raining lightly.

Your eyelids begin to feel heavy. When youfeel you can hardly keep your eyes open, you pullover, park the car and take a short walk - to “wakeup the brain”.

Physical inactivity and mental under-stimulation, or monotony, when seated leads tofatigue and decreasing alertness. The brain “shutsthe systems down “.

Another situation can be when you are sittingon a chair designed for more active seating, suchas a chair in a restaurant.

It can be frustrating to be “forced” to stay at the

table after a heavy meal. If the company you arein bores you, and you don’t have anything incommon with them, I’m sure you would rather besomewhere else.

After a while it becomes torturous to sit uprighton “that hard chair”.

You begin to plan... What would be the bestway to get some rest... slide under the table orjust gently fold forwards and rest on the remainsof the dessert...

Other problems occur for those who travel onlong flights in economy class. It may be a ten-hour flight, which is a very long time to be seated.

You may have the middle seat, making it muchmore difficult to change position.

If you are even more unfortunate, you mayhave been allocated a middle seat between twopassengers with no interest in talking to you.

This psychologically under-stimulating andphysically locked-in position may come close towhat some people using wheelchairs experienceevery single day.

Below are some of the factors that may causefatigue when seated;

too much physical stability too much physical freedom uncomfortable pressure monotony / boring environment wrong chair for the activity sitting for too long!

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WHOLEBODY

THE LEGS

THE CHAIR

THE ARMS

How do we actually sit?

I always ask seminar participants this question.They are asked to spend a few minutes to findfive different ways to sit, using the chair they aresitting on for the moment. The most commonresults are presented here and form the basis foranalysis and discussion.

Participants experiment vigorously but comeup with more or less the same positions everytime. The different seating positions are givenbelow.

These different positions can be divided into afew categories - the whole body, the legs, thearms and the chair.

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Experience it!Sit upright, 90-90-90.Relax - sit upright again - relax - sit upagain - relax...By doing this you can feel that your trunkhas a tendency to flex when extendingmuscles relax.

MECHANICAL TENDENCYMechanical tendency is whenthe whole body, or a part of it, beginsto move when muscles relax.

MECHANICAL BALANCEis achieved when the centre of mass ofthe body (or body-part) has a tendencyto move away from its present positionbut you do not sense it.

Normal sitting behavior

On the left you find definitions of twoexpressions that are often used in the text, theyare - balance and tendency.

These definitions make it easier for you toanalyse the following exercises that you can doto experience seating more in depth.

The “90-90-90” position

To sit in a 90-90-90 position means to situpright with ninety degree angles of the hips,knees and ankles. This is accepted as a correctergonomic position in the workplaces seen inoffices and schools.

But, a 90-90-90 position is more a theoreticallygood position than a practical and functioningposition. In fact, most seated people use such aposition only for very short moments. This tellsus that there is a gap between ergonomicrecommendations and what is used in practice inreal-life situations.

What is the tendency of the pelvis and trunk ina 90-90-90 position?

Sitting without support for the back you noticethis tendency clearly. By relaxing the musclesthat keep the trunk erect, the tendency is for theupper body to collapes. It begins with flexion ofthe thoracic spine, which makes the pelvis tiltbackwards and the spine begins to collapse.

When this tendency is present and you want tostabilize the trunk, or adjust it to a more uprightposition, many muscles need to be activated.

Do this exercise slowly:Sit upright (90-90-90). Relax so your trunkcollapses. Adjust to upright again. You will verysoon notice how easily you collapse when yourelax your muscles, and that you really need towork to get back to, or maintain, an uprightsitting position.

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Fundamental Seating PrinciplesFundamental Seating PrinciplesFundamental Seating PrinciplesFundamental Seating PrinciplesFundamental Seating Principles

A principle is like a rule that stays the same.The seating principles in this book are also likebasic rules that do not change.

A very simple example:A man sits on a chair. On the chair's seat is a

paper, shaped like a little ball. The paper is underone of the man's buttocks.

At first he does not pay attention to the factthat he is sitting on the paper, but after a whilehe begins to feel it, it becomes uncomfortable.

- What do you think the man will do?Take the paper away of course! But what if it

is not possible to take it away? It does not haveto be a paper, it can be something in the designof chair seat.

Now we can look at a principle, the principleof pressure. Should pressure against tissue (herea buttock) become uncomfortable, the pressureneeds to be changed to decrease discomfort orto improve comfort.

The principle of pressure is very powerful!I do not think anybody with normal tissuesensitivity ignores uncomfortable pressure. Thereaction to go away from pressure that stressesis deeply integrated in biological life, it is a sub-conscious reaction. This principle is directlyrelated to the principle of pain.

We do not want pain, of course not. If the manin the example above does not move away fromthe uncomfortable paper-pressure, his buttockwill, sooner or later, develop pain. Pressure leadsto pain when there is too much pressure-time andthe pressure is long-term. This fact is simple tounderstand. Most people do understand the

necessity in changing their sitting positionfrequently to vary the pressure on the body.

The Principles of Pressure and Pain areimportant since they guide us when we adaptseating systems for long-term sitting.

The second set of principles is the principle ofstability and the principle of balance, closelyrelated to the pressure-principle.

Pressure is necessary for stability. No pressuremeans no contact, which means no stability.Functional stability on a seat means that it ispossible to reach/lean sideways without falling.The seat surface is supportive, stabilizing thebuttocks, hips and thighs. Should the seat surfacenot be supportive, sideways stability is lost.

Adapting wheelchairs is always a compromisebetween comfort and function. Comfort meanspressure-friendliness and safety, but also positiveactivity (an activity comfortable to perform).Function is what activities you can perform,based on your body's present position.

Well understood and used, the principles ofpressure, stability and balance lead to situationsthat are comfortable and functional.

The most important principle of all is theprinciple of time. Time is what often changescomfort and function to become the opposite.

The principles mentioned are logical and can,to some extent, tolerate modification. There arealso principles of softness, angles, contours...

What we cannot modify so easily, is the personwe are adapting for, the user. Humans have, inprinciple, limitations on how much change thatcan be tolerated, and it is always time-related.

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SOFT SURFACES

The Pelvis -> Backward Tilt

The Trunk Collapses Forwards

Stabilizing the spine

Ex.: Arms crossed on chest

Ex.: Crossed lower legs

Muscular Fatigue

Collapsing Sliding Forwards

TRUNK FALLS FORWARDS TRUNK COLLAPSES Compensation: Slouching Compensation: Slouching

FIRM SURFACES

The Pelvis -> Stable

The Trunk Falls Forwards

Stabilizing the trunk

Ex.: An arm put behind backrest

Ex.: Crossed thighs

Muscular Fatigue

Falling Forwards

TendencyFIRM / 90-degree angle

TendencySOFT / 90 degree angle

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From seat to pelvis, to trunk

A seat’s quality and features directly affect thepelvis. The features are angle, softness andcontours.

When you sit on a horizontal, planar seat withyour feet on the floor just in front of the seat, thepelvis has a tendency to tilt backwards. It is easyto feel the muscles extending the spine when onerelaxes.

By changing the seat’s angle, backwards orforwards, this tendency increases or decreases.Simultaneously, the trunk - the spine - isinfluenced to flex or extend.

This co-operation between the seat, pelvis andspine is easy to observe and feel.

FORWARD SLOPINGPlace someone on an ordinary chair, such as a

kitchen chair. The person sits uprightapproximately five centimetres from the backrest,with her feet on the floor under her knees. Tell theperson to maintain an upright and well-balancedsitting position.

Now tilt the chair forwards a little.When this is done, the seat slopes forwards, so

the pelvis is moved up and forwards. The person’sreaction is to extend the trunk to prevent herselffrom falling forwards.

Rock the chair slowly. The seat surface changesbetween horizontal and sloping forwards. It iseasy to see the trunk’s movement, extensionwhen rocking forwards and flexion when rockingback again.

As long as the person’s feet are on the floor thetrunk extension is well controlled.

Try this when the person’s feet are placedunder the seat and her soles face upwards.

The legs´ stabilizing effect on the pelvic girdleis lost. Pelvic stability (blocking it from rotatingforwards) is greatly needed to prevent the trunkfrom falling forwards when the seat is rockedforwards.

It is mainly the buttock muscles, the glutei thatprevent the pelvis from tilting forwards. Theykeep the pelvis in position when the feet pressagainst the floor.

By having both feet under the seat, thepossibility to press is gone and the response isdifferent.

The extension of the spine is larger and moveshigher. The reason is that the pelvis, “anchors”the trunk-extending muscles. The position isn’tstable, which leads to compensation. To preventa fall forwards one moves weight to a higherlevel in the trunk.

The forward-tilted seat increases the trunk’stendency to fall forwards and stimulates trunkextension, so it is very important where the feetare positioned.

To give you a direct and unquestionableexperience of how correct this statement is, trythe following:

Sit upright on the front part of a chair’s seat.Lean your trunk forwards without letting yourarms have support against your legs. Stay in thisposition for a few seconds.

Now, slowly lift up both feet. Can you do itwithout falling forwards? No!

To avoid fatigue when sitting on a forward-sloping seat (planar surface) requires normalmuscle functions and endurance in the trunk andlegs. Even with functional, fully functioningmuscles it can be very tiresome to sit on a seatthat is sloped forward.

When it becomes tiring, a normal compensationis to slide forwards for stability.

I do not mean that sitting on a forward- slopingseat is unhealthy. What I mean is that it can be, ifthe contours are incorrect.

It is always better if forward sloping is combinedwith anatomical contouring since this improvesstability. The key is pelvic stability. If the seatstabilizes the pelvis, it decreases its tendency torotate forwards. This allows the pelvis to be thetrunk’s “anchor” and extension will be morenormal.

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A backward-sloped seat tiltsthe pelvis backwards, whichmakes the spine collapse.

A forward-sloped seat tiltsthe pelvis forwards , makingthe spine extend.

The reason I analyse forward-sloping seats inthis book is their fantastic influence on users.Using this type of seat I have often had theopportunity to stimulate natural trunk extension(even for severely impaired persons) in a muchbetter way.

Many more persons than we believe benefitfrom sitting on a seat that is slightly slopedforwards, and which is well contoured.

BACKWARD SLOPINGLet the person stay on the same kitchen chair.

Tell her to try to maintain her trunk in an uprightposition when you tilt the chair backwards.

Tilt the chair back in space, slowly.This time the seat tilts the pelvis backwards.

For the trunk to stay upright, the pelvis needs tokeep its position, which requires many musclesto work.

As soon as the person relaxes her trunk-extending muscles, her thoracic spine flexes.This forces the pelvis to tilt backwards.

A backward-sloped seat stabilizes the pelvis ina backward-rotated position, which makes naturaltrunk extension more difficult.

Backward sloping stimulates trunk flexion,while forward sloping stimulates extension.

The leg response (opposite to forward sloping)is that when leaning forwards the legs extendeasily, making the feet come off the floor (orfootplates).

This isn’t at all strange. When a person wantsto tilt the pelvis forwards, from a backwardposition, the hip flexors and knee extensors areactivated to assist the pelvis as it is tilted forwards,and the legs lift.

A backward-sloped seat also makes the trunkmore stable backwards.For the pelvis to have a functional, more uprightposition on this seat, the backrest needs to supportthe pelvis efficiently.

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Soft, planar seat surfaces easily makethe pelvis tilt sideways, whichdevelops scoliosis in the long term.Stable surfaces can prevent this, if thepressure does not cause discomfort.

Horizontal, planar seat surfaces oftenincrease pressure against the front ofthe thighs and on the ischials.When there is a backrest thisstimulates sliding on the seat

SEAT CONTOURSContours should be looked upon as many

“angles” which, together, form a shape.What is true for angles is true for contours, the

final result is important since it decides theposition of the legs and pelvis. A seat is morefunctional and comfortable when it isanatomically contoured.

I will briefly describe what may happen on aseat surface that is planar.

A horizontal seat surface often makes the thighslower near the knees, due to a compression of softtissue. A mechanical pivotal point is created atthe front edge of the seat (top figure on this page).

The part of the leg in front of the seat movedownwards and the upper part of the leg moveupwards. This lifts up the pelvis, which is thentilted back by the weight of the trunk, oftenmaking the pelvis slide forwards.

This result is decreased, or eliminated, by abackward sloping seat surface.

A horizontal, anatomically contoured seat isthicker under the thighs, preventing them frombeing lowered. This makes them have a morehorizontal position, which improves the stabilityof the pelvis.

To distribute the body’s pressure against theseat surface evenly, the contact should be ascongruent as possible. For many people, a planarseat surface is uncomfortable to sit on long-termdue to high, local pressure on the ischial bonesand other bony parts.

On the other hand - for most people (but not all)- an anatomically contoured surface is muchmore comfortable for the long-term.

A planar, firm seat is said to improve uprightsitting. This is perfectly correct.

The reason a firm, planar and horizontal seatstimulates trunk extension better than anequivalent but soft surface is the position of thepelvis in space.

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An anatomically contoured seat is adaptedto the skeleton. The shape distributes pressureefficiently against thighs and pelvis.The “Ischial Support” stabilizes the pelvis,which is beneficial for the entire spine.

Contouring distributes pressure and improvesstability. The supporting surface under theischial bones should be horizontal (line) toavoid pelvic obliqueness..

Supporting surfaces against the thighs fromeach side also stabilizes the pelvis.

A soft seat allows the ischial bones and sacrumto “sink in” more than a firm seat, which makesthe pelvis tilt more.

It must be added that the degree of pelvic tiltdepends on the interaction between the seat andthe backrest.

A firm seat surface makes it easier to achieveupright sitting. Why is this so? How long does itlast? Is there a time limit?

“Sling” seats are not very healthy to sit on in thelong-term. They often make you sitasymmetrically. A planar and firm seat is betterbut, after a while, may be too hard.

A soft seat is comfortable in the beginning, butpoor stability makes the trunk collapse, whichcauses fatigue in the long term. What is a “correct”and proper seat?

The pelvis and legs need stability and an evendistribution of pressure on the seat.

An anatomically contoured seat is efficient. Itprovides pressure distribution as well as stability.Should it be necessary to decrease pressure, thecontour can be changed, or the seat made softer,or both.

When experimenting with seat softness andcontours, the pressure experienced by soft tissueand the stability experienced by the skeleton andmuscles are altered.

Setting up a seat is a balance between pressuredistribution and fair stability.

Even if a planar seat can be uncomfortable(long term) some persons need such a seat.

People having a pronounced/severe lordosisand a forward-tilted pelvis often shift weight tothe thighs if the part under the pelvis is too low.For them, an anatomically contoured, horizontalseat, or a planar, backward-sloped seat can causestress. If, on the other hand, the seat is contouredand slightly forward-sloped it often functionsbetter. Such a seat can be very appropriate forpeople with Polio, Rheumatism or Spina Bifida.

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Seat shear develops when the pressureon the backrest increases.The shear distorts the soft tissue layerswhen the person slides forwards.

When standing, shear on the feet fromthe floor develops when the person leansagainst the wall.

PRESSURE and SHEARPressure per unit area is lowered when

distributed over a larger contact surface. Thus,the goal is to create a large contact surfacebetween the body and the seat.

For low pressure on the seat, body contoursdictate seat contours.

The pressure we normally talk about isperpendicular to the seat surface. This pressurecompresses soft tissue.

Besides the perpendicular pressure there areforces that deform tissue from the side. These areknown as shear forces and are difficult to measure.

Shear forces “push” parallel, soft tissue layersaway from one another, which tears and stretchesdifferent connections and blood vessels,penetrating many layers.

It is uncomfortable to stand if the pressureagainst the soles is static. We want pressure tochange, in other words, we want pressure to bedynamic.

When you lean gently against a wall it is moredifficult to change the pressure on the feet,pressure is more static.

The pressure on the wall increases when yourfeet move away from the wall, and the shearforces on your soles increase.

The same thing happens on your pelvis whenyou sit. When your trunk leans against a backrest(the “wall”), pressure from the backrest pushesyour body forwards. Due to seat friction, the softtissue stays in place whilst the skeleton, in thiscase the pelvis, moves - slides - forwards withinthe soft tissue.. This pelvic sliding of the ischialbones deforms the soft tissue between the seatand the ischial bones.

Seat cushions may protect soft tissue fromperpendicular pressure. A decrease of seat shearis achieved by contouring the backrest. Trunkbalance and extension decrease pressure againstthe low and middle part of the backrest. Thisdecreases seat shear.

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Interpret and understand

There are several goals in sitting. So we sit verydifferently. An important condition whenanalysing and interpreting other people’s sittingbehaviour starts with discovering these goals.

Of course, most of us can “see” that people sitdifferently but the ability to see what a positionreveals, physically and psychologically, issomething you learn.

It cannot be emphasized too often that the mosteffective way to learn how seating “works” is touse your own body as an “experimentalworkshop” and to thoroughly observe peoplesitting in many different environments.

Every human who sits is an individual with hisor her own conditions and desires. Major demandsare placed on anyone who sets up andindividualizes a seating unit for somebody else.If you want to become a skilled “wheelchair-fitter” you have many reasons to observe andpractise seating.

Sometimes you encounter complicatedproblems to solve to enable the user to sit withfair-to-good functionability. From time to timeyou may feel that the problems you are responsiblefor solving are “unsolvable”, especially if you donot know where to begin.

For the many sitting positions in which the goalis some sort of activity it is important thatfunctional conditions are created for the uppertrunk, head and hands.

It isn’t easy to create a functional position if theperson who is seated doesn’t have the capabilityof sitting the way “I want to”. Most of us have an“inner picture” of how people “should” sit, apicture “guiding” us when we want to make goodpositions for others.

If you meet a person who, due to long-termpoor seating, has physical difficulties such asstiff joints or tensed muscles, it is good to realizethat the change you want to create for this sittingperson requires some time. It may be a long-termprocess!

By looking at how a person is sitting youreceive many invaluable clues as to — why?

A sitting position can tell you what a person istrying to achieve or create with the help of histrunk, head, legs or arms.

Users sitting with their knees pressed togetherdo so for several reasons. If you change theposition of a user’s legs and feet to a more“correct” one you may influence the user’s trunkin a negative way. It may seriously influence thisuser’s ability to work with his hands.

If, instead, you divide a sitting position intofunctional body segments — feet, legs, pelvis,lower trunk, upper trunk, head arms, and hands— you may very soon discover how the legsinfluence other parts of the body. Starting byadjusting for some other part first, usually thetrunk, the user’s legs may correct themselves.

Interpreting a sitting position is like putting a“puzzle” together. Sooner or later you learn,understanding what the pieces mean and how toplace them in relation to each other. Practice andexperience develop your skill in putting the pieces“correctly” together, using less and less time.

Every person you meet is a new “puzzle” butsometimes this new person does not match yourexperience. This is a challenge, and opportunity,to learn more!

A sitting position is a position of the body thatfunctions well, for the moment... This momentcan last for a few seconds or many hours. Howlong it lasts depends on if the person who isseated is capable of changing position or not.Those of us who work directly with setting upand designing chairs, with or without wheels,must realize that no matter what we do — peoplestill sit the way they need in order to achievecomfort and functionability.

I maintain that this is true even when theseating unit doesn’t function well, the person issitting very poorly and the sitting-time isextensive.

The big difference between persons who canchange position and those who can do it only

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with great effort is that the person with difficultiesuses fewer positions during the same amount oftime. These persons’ “right now” positions justlast longer.

What is it that makes us continuously changeposition when we sit?

Life! Standing as well as seated...Physical forces always influence your body.

How your body is influenced is, to a great extent,dictated by how your body is built and what youdo when you sit.

The chair you use has the greatest influence onyour body. This means that the interpretation ofhow a person places her body in the chair tells usabout this particular seating unit, its seat andbackrest and, perhaps, also armrests.

A wheelchair’s seating unit may also beequipped with footplates, leg rests, calf supports,trunk supports and a headrest.

Seats and backrests have features such as beingsoft or hard, horizontal or angled and planar orcontoured.

The human body is a live mechanical systemmade of pieces of bone and muscles. Jointpositions and muscle tension are registered andcontrolled by the nervous system every moment.

Every change, like altered pressure on a body-part or an altered position of the entire body orbody-part is registered by sense organs, directlyreporting to the brain which acts and influencesnerves, if necessary.

We, the observers and interpreters, need to seeand understand how each seating unit influencesand creates reactions in the body of the user whosits in it.

A person who is capable of continuously shiftingposition avoids the risk of harmful, static seating.

Persons with physical dysfunctions may not beable to defend themselves against the dangerousinfluences of poor seating units.

Their sitting positions and behaviour are whatwe need to interpret and understand, based onindividual functions and how the seating unit’sfeatures influence the user in several ways.

- Why does the person slide on the seat?

- Why does the person always lean towards theright side - why not the left?

- Why does this man slide off the seat of hiswheelchair even though he has a belt around hischest? And always at lunch-time!

- Why does this woman place her left foot on theseat so often?

- Why does this girl keep crossing her legs allthe time?

- What is the cause of this woman not beingable to keep her head up?

- Why does this boy always extend his legs androtate them inwards?

- Why is it more difficult for him to keep histrunk up when his legs are corrected?

We can keep on asking many, many questionsas to why people using wheelchairs sit the waythey do. We can also keep on forever trying toanswer all the questions if we do not steer awayfrom focusing on symptoms (what we see) and,instead, start searching for what is the cause ofwhat we see.

When we begin to know what actually createsthe positions we see in people using wheelchairswe can begin to change things more appropriately.

Knowing how to interpret sitting positionsincreases our knowledge of how seating unitsinfluence their users.When we begin to grasp “what influences what”we will better understand how to design andadapt wheelchairs, not just simply for sitting butfor much more functional sitting!

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Physiological Sensitivity InPhysiological Sensitivity InPhysiological Sensitivity InPhysiological Sensitivity InPhysiological Sensitivity InPhysically Challenged HumansPhysically Challenged HumansPhysically Challenged HumansPhysically Challenged HumansPhysically Challenged Humans

The wheelchair, an assistive device, alwaysinfluences the user in some way.

It can create seating of reasonable comfort,which functions well, or lead to additionalproblems. Physical treatment influences the bodybeing treated. Effects can be positive, neutral ornegative.

Wheelchairs influence. For a user who usesone every day, with few opportunities to standand walk, a wheelchair is a physical treatment.The goal of this treatment is to improve, developor maintain functions and to prevent furtherdeterioration and/or injury.

We must never overlook poor sitting posturesin wheelchairs, postures that may lead to seriousimpairments and injuries.

If a user is sitting poorly you will always findthe cause when you examine the wheelchair.Faulty seating is not the fault of the user! Userssit the way they are influenced to sit - by thewheelchair.

What then, is a poor sitting posture?Any posture, which is, or may become, static

and long-term may develop into a harmful sittingposture.

Some people assume that when a wheelchairuser sits poorly it is because the user has impairedfunctions, is handicapped, and “this is the reasonhe/she cannot sit properly”.

Being deeply involved with seating andwheelchairs you soon learn not to blame poorpositions on a user’s impaired function. If that isthe case you should change your profession! It isusually possible to achieve sitting positions thatprevent serious sitting problems.

We have only two choices – to do things wellor not to do them so well. There are NO short-cuts to creating good seating.

Common problems that develop because ofpoor seating are pressure injuries, joint and musclecontractures (shortness-stiffness), muscleweakness, high or low muscle tension andnegative effects on inner organs.

Other common and very negative functionalproblems are difficulties in staying awake,decreased appetite and impairment of vital dailyactivities (primary ADL) such as eating anddressing.

When reasoning about problems it is also naturalto wonder how much seating problems “cost”society. Pressure sores are concrete and the costto heal a sore can be “calculated”. It is, however,not so easy to put a price-tag on a stiff hip, ascoliotic spine, decreased lung capacity,obstipation, swollen legs or neck pain. Poorseating is probably extremely costly! If we wantto know how costly, more research is needed.

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When trunk-extending muscles fatigue, you collapse.- How do you protect your body from collapsing?

Fatigue and defence

What happens if the chair you sit on is notsufficiently comfortable?

It creates discomfort and makes you tired.The many different “techniques” we use (like

crossing our legs or arms) can be effective waysto temporarily increase comfort.

There can, however, be a big difference betweena person with optimal functions and a personwith impaired functions when comparing howarms and legs are utilised.

The non-disabled person does not sit still,frequently changes position. When sittingbecomes tiring, it is natural to stand up and dosomething else.

A wheelchair user may have no other alternativethan to sit in the wheelchair the entire day. Someusers are able to change their sitting positions,others are unable to do so.

If a user’s trunk has a strong tendency tocollapse, crossing his/her thighs, or his/her armson the chest, prevents this from happening.

But, if this is not possible, there are otherfunctional alternatives to help the trunk. Active

extension of the spine or sliding forwards on theseat are two other ways of helping.

Crossing arms and/or legs and extension of thespine are activities which may cause muscles tofatigue after some time.

The alternative - sliding forwards on the seat -is convenient and the most common way peoplestabilize their trunks, to avoid excessive load onmuscles. This is not a problem as long as it isshort term. It will, however, become harmful ifmaintained long-term, leading to muscularinactivity and back problems.

Every person compensates a poor posture andso does a wheelchair user. We do what we needto do to avoid discomfort, to prevent the bodyfrom being harmed.

All human beings are very intelligentmechanically and physiologically. If we are sittingunpleasantly our bodies’ senses send informationrequesting protection against physical stress. Weare “defensive” because we do not want to losecomfort and functions and try to stay away fromsituations that cause pain!

A wheelchair user’s protection against physicalstress, a perfectly natural reaction, often causesother types of serious problems.

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Stressed by pressure

The first thing one experiences when sittingdown is how hard, or soft the seat, backrest andarmrests are. It may feel fine at first, but after afew hours a chair, which was comfortable,becomes uncomfortable. This is the normal casein real life!

Time is always the deciding factor when“comfortable” changes to “not comfortable”.We all have a tolerance level that decides whenpressure is becoming uncomfortable. Somepeople are thin and others fat, some are youngand others old, some people have big, well-builtmuscles whilst other´s muscles have never donehard work.

Soft tissue can adapt to, and thus tolerate, aseating unit’s long-term pressure. But even if thisis so, pressure against the body will, sooner orlater, become a source of stress for a person whosits for a long time.

- How much is a chair pressing against you andwhat can be dangerous pressure?

Do the following exercise:Sit on a chair and make sure your pelvis and

back have good contact with the backrest. Putyour feet flat on the floor.

Feel how the seat presses against your thighsand ischial tuberosities (sit-bones).

Now, lift your heels up (let your toes staydown). This increases the pressure against yourischial tuberosities. This is the situation whenwheelchair users have their footplates too high.

Move both feet forwards on the floor. Thisinstantly increases the pressure on your thighsand decreases the pressure on your sit-bones!This is a common and effective way of decreasingunpleasant pressure on ischial tuberosities.

But another thing is also happening.Press your thighs a little harder against the

front of the chair seat. The ischial tuberositiespressure decreases more and the upper part ofyour trunk presses against the backrest (the midto top part).

The pressure on the backrest and seat-front ofthe chair makes your pelvis lift. When you stoppressing with your legs you may slide forwardsa bit. If you relax you can feel that the pressure onyour upper (thoracic) spine has increased.

If this is uncomfortable and you want to changethe pressure by changing position it can only bedone if you move your trunk away from thebackrest. Feel where, and how, the backrestpresses against your back, and the seat againstyour pelvis and legs.

Now re-position yourself. Sit on one buttock,with your pelvis in an oblique position. Thismakes pressure move from your ischialtuberosities and sacrum to your buttock’s softtissue.

How does the backrest press?Your trunk rotates when you sit obliquely and

presses against your rib-cage on the same side asthe buttock you are sitting on. After a while thiscan cause discomfort.

Slide forwards a bit more and the pressurechanges. Every time you slide, pressure againstthe backrest increases. Should you choose todeviate by leaning your trunk sideways, thearmrest-plate may stop you and press againstyour ribs (it hurts!).

Pressure against the body often develops asdescribed here. So, it is not surprising thatwheelchair users may risk sliding off their seatsand ending up on the floor. When that happens,it is not unusual to stabilize the user in the chairby using specific stabilizing devices. Differenttypes of belts, vests or even tables can prevent theuser from sliding off the seat.

People sit the way they are influenced to sit!Uncomfortable pressure - from the seat, backrest,headrest, armrests, leg rests and footplates createsseating problems. The cause why many users sitpoorly is to be find in the seating unit. Stress iscompensated - it is physiologically intelligent!Poor seating should never be blamed on the user.

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Collapse of the thoracicspine makes the head dropforwards. Over time thismakes the neck region stiffand painful.

To be able to lookstraight ahead aperson with astrongly kyphoticspine, leans the headto one side incombination withneck rotation.Long-term, thisleads to neckcontractures.

If the thoracic spine flexesin a backward-tiltedposition, the head fallsbackwards causing over-extension of the neck.

Equilibrium and stability

As a factor causing discomfort, pressure israther well understood. Two other factors, whichare greatly underestimated as causes of harm insitting, are poor equilibrium and lack of stability.

Most people can feel that poor equilibrium andstability in standing makes it difficult to beingfunctional when standing and walking.

A person who has consumed alcohol, impairinghis balance, often needs a wider surface of support.One way to create more stability is to move thefeet apart.

If you are sitting with poor equilibrium andhave a tendency to fall (in any direction) you willimprove stability, sooner or later, by decreasingthe “fall-tendency”. At the risk of saying this toomany times, I repeat - the most natural thing to dois to slide forwards on the seat, to a greater orlesser extent. This is, in fact, the most commonposition in a wheelchair. Being in this situation isnot the problem, it is staying in such a position,long-term.

When the spine collapses the head falls forwards- or backwards.

A user slides forwards on a horizontal seat. Theunit’s backrest is upright. This make the head fallforwards and downwards.

If the seating unit is tilted back, when thethoracic spine flexes the head has a tendency tofall backwards, so a correctly adjusted headrestis needed.

Many users who are positioned like this, andwho drop their heads backwards or forwards, areelderly, or have had traumatic brain injuries.

The spine must be prevented from collapsing.Improving natural, spinal alignment allows amore functional and balanced position of thehead. This is achieved by adjusting the seat andbackrest correctly.

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Time and compensations

It is to some extent possible to "classify" sittingpatterns based on physical impairment and qualityand features of seating units. Many positions wesee in long-term sitting are logical and, thuspossible to explain.

Here are some explanations, based on changesmade to a seating unit and the user's response.The bottom-line is to change how physical forcesare influencing a user.

SLIDE FORWARD AND RELAXThe most common compensation among

wheelchair users, and every other human being,is to slide forward on the seat.

The backrest is too upright or high, "pushing"on the trunk from behind. This causes musclefatigue and trunk instability. The smart thing todo for more trunk stability is to move forward onthe seat and relax. This makes the spine flex --downwards, forwards and sometimes sideways-- and you risk the development of a majorthoracic kyphosis and, sometimes, a scoliosis.

The measure to take is to improve the trunk'sstability and upright posture against the backrest.If it is easy (low muscle activity) to have thetrunk against the backrest the need to slidedecreases.

PRESSURE - FOR STABILITYA way to "assist" the trunk to stay upright

(prevent flexion) is pressing the feet against thefoot plates and the hands against the armrests

This is a very direct and effective way tostabilize the trunk, preventing it from fallingforward. If though, "pressing" becomes a habit itmay result in a user "trapped" in an extensorpattern. This may create serious functionalproblems such as difficulties to lean the trunkforward for - eating - standing up - transfering -walking, etc..

If this "pattern" has been used for a long time(several years) it could be difficult to solve, but

Time and influence may lead to...?

Sliding forwardson the seat is shortterm positively stable.

But...sliding andslouching alsocreate inactivityand fatigue,sometimes pain.

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it can also change rapidly to the better. Themeasure is to use (once again) to adjust thebackrest, making the trunk stable and balancedagainst the backrest. These users are often verysensitive to where the trunk is placed in space.Leaning too much backward or forward maytrigger the extension. Be gentle and patient!

LEG AND TRUNK EXTENSIONUsers who have physical impairments from

birth like in Cerebral Palsy often have seatingunits that make their sitting tilted back in space inrelation to gravity.

The trunk is moved to behind the position itshould have had if it was upright. Suppose thatthis person wants to move the trunk forward to sitmore upright.

To make this mechanically possible the Centreof Gravity (CoG) of the trunk needs to moveforwards. To be sitting upright the CoG needs tohave a position over the hips.

Step 1 The chest moves forwards / trunk flexion

Step 2 The pelvis rotates forwards

Step 3 The trunk extends

After step 3 the trunk should be upright...The beginning of the movement, step 1 is a

normal "sit-up" activity. The problem is that it isdone "uphill" - the seat is backward-sloped.Doing a sit-up uphill requires more muscle force.An attempt will trigger the legs to extend and, ifthe person tries to sit up as "a habit", the extensionof the legs is an important part of this habit.

If this person's abdominal muscles are weakand he/she tries to lean forwards from a tiltedposition and the legs are triggered to extend, theresponse is trunk extension. Extended legs helpthe pelvis to pivot forwards. If the abdominalmuscles cannot keep the ribcage's position theforward rotation of the pelvis, and the thighs'pressure against the seat, extends the spine.

By pressing against oneor both armrests and/orone or both footplates thetrunk is assisted - orpushed - to an extendedposition.

A pattern often seen in childrenwith Cerebral Palsy. The patternis so common that it is lookedupon as a "CP-pattern".

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An often encounered arm and leg pattern(sitting as well as in bed) among braininjured adult persons after head trauma.

The extension of the spine presses the trunkagainst the backrest. This blocks the trunk'smovement forward and the movement becomesmore blocked when the thighs press against theseat front.

The solution is to make it easier for the personto move the trunk forward (if this is what he/shewants!). A fast way to test a person's response isto place a small pillow behind the pelvis, put ona hip belt and place wooden plates under thedrive-wheels -- tilts the seating unit forward. Letit stay like this five-ten minutes.

Sometimes it is surprising to see how fast aperson's arms and legs relax and are lowered in amore upright sitting position.

Success when doing a permanent set up inaccordance with these guidelines is to give thelegs and lower trunk functional stability and theupper trunk a possibility to move. It is a delicatebalance, demanding observation and follow up.

One thing that needs to be observed is when theperson has a mild or moderate scoliosis. A moreupright posture makes the scoliosis increase, ifthe backrest does not have functional sidesupports. Remember, if the seating system isdynamicit is an advantage to alternate the person'strunk position during the day.

ARMS AND LEGS FLEXA brain-injured person often has a wheelchair

with a high back and a headrest and is seatedtilted back in space.

A brain-injured person who has been afunctional walker used to use normalcompensations like crossed legs and crossedarms. If the tendency of the trunk is to collapseand/or fall forward it is normal to stabilize thetrunk by using the legs and arms in this way.

If coordination of arms and legs is a problem,crossing them can be difficult to do.

If the sense organs register and "tell" the brainthat the trunk is losing its position it may triggerspecific muscles to begin bending the armstowards the chest and bending and pulling up one

Changing position now and then, beingdynamic, is a good way to influence forchanging undesirable, negative sittingpatterns to become more natural.

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leg. These movements may be an attempt tocross the arms and legs. It is not done consciouslybut subconsciously because this functioned beforethe injury.

We can also see this as primitive reflexes! Butif the flexed arms relax when the seating situationis improved and the hands stay on the legs, oneimportant cause was the poor position.

Create balance and stability. Make sure that theupper part of the spine, the shoulders and thehead can move smoothly.

PELVIS FORWARDSTilting the pelvis forward is an effective way to

create an extended and stable spine. The spinebecomes stable in all directions and can be usedto compensate impaired trunk muscle functionand balance.

Some users are highly dependent on suchcompensation, to be able to sit upright.

TRUNK DEVIATION AND ROTATIONSide-deviation and rotation of the trunk is

normal when we sit. It is also very common inhemiplegia.

This user moves the pelvis asymmetri-callyforward on the seat. The pelvic position makesthe spine flex forwards and sideways and it alsorotates. This makes the trunk tend to fall sideways.

The cause can be impaired trunk musclefunction in combination with poor bodyawareness. The cause can also be the user's foottrying to reach the floor or the hand trying toreach the pushrim.

The most common cause, however, is whenwheelchairs are not set up correctly forcomfortable sitting and effective mobility.

A user who has had a stroke often slidesand deviates the trunk to one side whichcan be the only effective way to staymobile.

Forward pelvic tiltextends the spine.It mechanically createsfunctional stabilitywhen trunk muscles areweak. When it works itis also used by some ofthe strong users.

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A home-made seat solution

In many countries the possibility to have afunctional seat cushion is very limited. This canbe because there are no such cushions availableor society will not pay for it. After all, high-techgood cushions cost a lot of money. However, themoney spent on a cushion is worth every cent ifit prevents pressure sores and posture problems.Of course! But real life may be that the money (orthe will to pay) may not be there!

Even, if you do not have a functionalmanufactured seat cushion it is still possible tomake the seat surface function rather well if youfollow some basic principles shown in the illus-trations on the following pages.

For better pressure distribution, the surfaceshould smoothly follow the body contour -- thecontour of the skeleton. In the example here onhow to build a functional seat surface it is assumedthat a backrest is used.

1 The seat surface under the pelvis should bea little lower (approx. 2-3 cm) than under thelegs, to make the pressure against the ischialtubesosities and nearby soft tissue decrease.

2 An important reason for making the surfacechange from low under the pelvis to higher underthe legs is that the area connecting low and highcreates an anti-thrust function. It makes the pel-vis more stable by preventing it from slidingforwards. It is very important that this area issmoothly rounded, to avoid high pressure locally.

3 The slightly higher surface under the legsimproves leg support and make the legs morestable sideways. This also, makes it easier for thetrunk to stay against the backrest, which is theimportant reason for doing this. Functional thigh-against-seat pressure improves trunk-against-backrest pressure. The result is less muscle forcebeing needed to maintain the upper body againstthe backrest.

1 Slightly lower under the pelvis.

2 In front of the pelvisit should be slightly"uphill" to preventforward sliding.

3 The seat needs tobe a little higherunder the thighs foreffective leg support.

1 2

3

When factors 1, 2 and 3 are functional theycreate;

Pressure Relief Pressure Distribution (also on surfaces) Pelvic Stability / Slide Prevention Leg Support and Trunk Stability

Such a seat surface also makes the legs, thepelvis and the trunk more stable.

If you have a planar foam cushion youcan make it function much better bychanging its shape a little.Add a pelvic stabiliser (opposite page),or a foam-layer with higher densitythan the seat cushion, under the cushionwhere the legs are (dark grey). Makesure that it is placed just in front of thethe ischial tuberosities (sit bones).The function of this more anatomicallycontoured cushion improves seating.

1 2 3

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BASIC SEAT CONTOUR

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Many wheelchairs have sling seat upholstery.The problem with this, especially when the slingis hanging down a lot, is that the seat cushionfollows the sling (see below, A) making the legsmove closer to the to midline of the seat. Onesolution to this problem is to put a wooden boardon the seat to improve leg position. But a pressureproblem may develop if the firm board under thecushion begins to create local pressure.

Instead of a board it is possible to better utilizethe sling shaped seat by placing a cone-shapedwedge on the seat front in midline under thecushion (inside the cushion cover) (see B). Thewedge should be 2-3 cm high forwards and,closer to the pelvis, it should be very thin.

Such a piece used on sling upholstery makesthe front of the seat contoured to the legs and thearea under the buttocks stays rounded (C).

Seat cushion informationThere are plenty of cushions to choose from

and the only way to really know or have a chanceto know is to try them out, not only on users. Wewho prescribe them need to try them outthoroughly.

Many seat cushions are said to be specificallydesigned for long-term sitting. Some are clinicallytested and some are not. Be critical, read theproduct information and ask questions.

Our main responsibility is to protect the userfrom developing a pressure injury and othertypes of seating problems. It is not uncommonthat people who adapt wheelchairs compromisebetween protection against high pressure andprotection against physical deformaties (long-term poor posture) and choose pressure protectionbefore postural function.

It is fully possible to fulfill both objectives -pressure and posture protection if we know what(product) influences what. It is important to learnhow the different products you see and try reallyinfluence the user. Seat cushion representativesinform us but it is our responibility to evaluatethe user-benefit of every product we prescribe.

A cone-shaped wedge under the front of thecushion improves the sling seat by making itmore anatomically contoured (see text).

Pelvic Stabilizer (VICAIR) function areas.

3 32

1

A

B

C

Legs before

Rear

Legs with wedge.

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The backrest is a personalorthosis for the trunk

Numerous reports have been written aboutfactors leading to the development of pressureinjuries during prolonged sitting. In order toprevent serious harm from pressure, many typesof pressure-decreasing seat cushions are in usetoday.

Some manufacturers have developed adjustableseating systems for wheelchairs. Such systemsinclude both seats and backs. Some manufacturerssometimes integrate contourability as a standardfeature.

A seat needs to be individually contoured foruser protection and trunk stability. The seat worksas the base for the trunk, but, for the seat to befully functional, it needs to be in harmony withthe backrest.

The seat's role is to distribute pressure andcreate necessary stability. The backrest assiststhe trunk to reach fair stability and the balancerequired for various types of activities.

Let's suppose that a seating unit's seat andfootplates are well adjusted. This gives us aneffective basis, and about half the unit is set up.The footplates and seat surface create a stableplatform, focusing on the pelvis and legs. Thisgives half the body a good start.

Now, the other half of the seating unit is adjustedfor the rest of the body, the trunk. This includessetting up the backrest and, in most of cases, thearmrests.

When considering the contouring of the seat itis relatively simple to find a shape and softnessthat suits many people.

Individual factors influencing a person'spossibility to sit on a seat surface are soft tissuequality, blood circulation and, for some users,impaired skin sensibility and- of course - duration.There are different needs, but many humans havea relatively "standardized" shape against a seatsurface, based on the shape of the skeleton.

This "standard" means that, despite minor

differences, many wheelchair users can becomfortable on standard seat cushions.

The backrest on the other hand needs to bestrictly individualized since every person needsto be individually balanced and positioned inrelation to gravity.

When standing upright we balance our bodyusing our feet as support. When sitting uprightwe need to balance the trunk with the pelvis(ischial tuberosities) as support.

It is much more difficult to maintain upper-body balance when sitting. So, the backrest needsto be adjusted optimally for each individual. Thegoal is to assist the upper body to achieve itsoptimal position, a position that makes it possibleto change between balanced activity and stablerelaxation. The backrest's function is to stimulatenatural upper-body function.

For any user who is in long-term contact witha backrest, and who may not easily changeposition, the backrest should be considered anindividual trunk orthosis.

Not even for the many, many people who donot have any physical impairment and who canshift and change, a functionally contouredbackrest may not function well (car seat, officechair, airplane seat) if it is insufficiently contouredby design. Salvation in such situations is thepossibility of standing up and doing somethingelse...

For many physically challenged people whoare sitting in wheelchairs this may not be anoption. Their real life situation may be to sit intheir wheelchair or to be in bed. Because of this,the wheelchair's backrest is one of the factors thattotally dictates the seating unit's influence on theupper body, and thus, the whole body.

Spinal Cord Injury, Hemiplegia, RheumatoidArthritis, MS and Cerebral Palsy are all differentdiagnoses and conditions requiring differentbackrest solutions.

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Backrest Contour

A backrest's contour should be such that thesurfaces in contact with the trunk and spinefollow their contours.

The objectives are pressure distribution,stability and functional balance.

Begin with the position and stability of thepelvis, then balance and support the upper bodyby contouring for extension of the thoracic spine.Lastly, the area of the backrest against the lowerback should be in gentle contact with the lowback lordosis.

First the pelvis, then the thoracic spine andfinally the lower back. A functional position ofthe head, shoulders and upper trunk is the goal.

#1 "ROOM" FOR THE PELVIS

By adjustment of the lowest part of the backrestthe basis is created for making the backrest abovethe pelvis optimally contoured and functional.

One very common cause of seating problems isinsufficient contouring of the area of the backrestthat is in contact with the pelvis.

One common mistake in backrest design is thatthis pelvis-area is too flat, or sometimes evenforward convex! Such a contour decreases theroom needed by the pelvis for an upright andfunctional position.

If, due to a flat surface, the pelvis presses hard

Contouring of the backrest - Levels and Principles

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against this area it will rotate backwards veryeasily if/when the thoracic spine begins to flex(remember that thoracic flexion makes the pelvisrotate backwards).

When the pelvis' position is upright or forward-rotated it is more stable. This influences theposition of the trunk's centre of gravity (CoG). Amore upright pelvis mechanically influences thethoracic spine to extend and the CoG movesupwards and backwards. This shortens the CoG'smomentum and decreases the muscle forcerequired to keep the trunk's CoG-position.

To make the backrest, as a whole, functionalfor the trunk its lowest part must allow the pelvisto have an optimal position.

By "positioning the pelvis" several differentstructures are influenced. The pelvis' position inspace dictates factors like the placement of thesacrum's upper surface (the base of the spine),hip joints (position) and leg and trunk muscles(muscle length). All these factors influence thepelvis and spine differently.

When the pelvis has a "final" position, thisposition decides the spine's mechanicalconditions, its balance and its possibility to move.For most people it is better to make the backrest'slowest part, which is in contact with soft tissueand the lowest part of the pelvis, too "roomy"than too tight. If more contact against the backrestis needed, or wanted, it can always be created...

You may wonder why I focus on this low-pelvis-area of the backrest so deeply?

The reason is very simple. If this area of thebackrest is insufficiently addressed, the result isthat seating problems will develop very soon.

Give the pelvis the room it needs to be in aposition that makes it muscularly easy for a userto keep his/her trunk extended!

#2 PELVIC STABILITY

After the pelvis has been placed, its positionneeds to be stabilized by making the backrest

support the pelvis' upper sacral area, from side toside. It is important that the pressure against thepelvis is not too great. Should it becomeuncomfortable, the user decreases the pressureby moving the pelvis -- by sliding forwards onthe seat.

For many people, supporting the back meansthe same as supporting the lower back. The goalis primarily to stabilize the pelvis, not to supportthe lower back.

When the pelvis and sacrum, which are thelowest part of the spine, are sufficiently stabilized,the backrest above the pelvis is easier to adjust,in order to balance and stabilize the rest of thespine.

A roomy contour for the pelvis in the backrest'slower area, creates the conditions for the pelvicposition. With the backrest supporting the pelvisfrom behind the condition needed for a stableposition is maintained.

Contouring for room and sacral supportingcomplement each other to give the pelvis afunctional and stable position in relation togravity.

#3 THORACIC EXTENSION

When the backrest's lowest part, the area incontact with the pelvis, is well adjusted the nextstep is normally not to adjust for the lower back.

For a better, faster and often more accurateresult, instead of focusing on the lower back,adjust for the thoracic spine first, then for thelower back.

The reason is simple. When adjusting thebackrest for upright sitting, the goal is to assistthe lower back to maintain its forward convexcurve, its lordosis. The problem is that you veryseldom know what kind of curvature you areadjusting for. Is it big or small? Perhaps it's stiff!

Can the person you have in front of you situpright with a lower back lordosis?

To know for certain, he or she should be seated

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#4 THE LOW BACK CONTOUR

Good contact and individual contour on bothsides. Observe that the curve of the lower back,the lordosis, is created and regulated byinteraction between the pelvis and thoracic spine,their positions respectively, in relation to oneanother.

Avoid excessive lower back contouring. It over-extends the spine and, if the user finds itunpleasant, (extension and/or pressure)stimulates sliding on the seat.

A lower-back support is only seldom func-tional for maintaining a lumbar lordosis. The keyto keep a lordosis is a functional and stable pelvicposition in interaction with the upright extendeddynamically moving thoracic spine.

#5 NECK AND HEAD BALANCE

The purpose of the design (or adjustment) ofthe backrest is (mostly) to make it easy for thehead to have an upright and functional position.

The balance of the neck (cervical spine) anddynamic position of the head is based on how thepelvis and thoracic spine cooperate and interact.The best condition for the head is when the pelvishas a functional and stable (not fixed) positionand the thoracic spine extends and can changenaturally between extension and flexion.

When a backrest has been set up, the user's headwill show you if it is functional. If the user moveshis/her head to a position other than upright,relaxed and controlled, you should re-set thebackrest.

stably and relaxed, with the thoracic spine upright,naturally extended.

After the pelvis has a stable position, the nextstep is to contour the backrest's upper area. Theobjective is individual and comfortable extensionof the thoracic spine, giving the trunk a functionalbalance, and minimizing its flexion tendencywhen extensor muscles relax. Now the backrestshould be contoured to be in good contact withthe lower back.

If the thoracic spine's extension is maintained(with very small or no muscle force) the result isthat the spine stabilizes the pelvis and its backwardrotation is blocked. Thoracic extension restrictsthe pelvis' possibility to rotate backwards. Aslong the thoracic spine is extended the pelvis ismechanically hindered from doing so.

Thoracic extension is positive for the trunk andfor pelvic stability. The pelvis and thoracic spinealways interact and influence one another. Whenthe pelvis rotates backwards, the thoracic spineflexes and when the pelvis rotates forwards thethoracic spine extends. If you flex your thoracicspine it makes your pelvis rotate backwards andwhen you extend your trunk your pelvis rotatesforwards.

If you want to block your pelvis or thoracicspine from moving when the counterpart moves,you need to use many muscles. Thus, for athoracic spine, the backrest contouring should befor balanced extension.

The backrest's contact area against the ribcageneeds to be large and very comfortable. If it isn'tcomfortable it isn't working! Functional supportfor the trunk is created by contouring the backrest'smiddle-to-upper area. This stabilizes the trunksideways and stabilizes one of the spine's "pivotpoints", the lumbo-thoracic junction, whichstimulates and makes extending the spine moreeffective.

Let the upper part (4-5 cm) of the backrest havesome slack, or be a little softer, to allow extensionwhen needed, which is very often!

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Backward seat slope. A high, reclinedbackrest makes the pelvis and most ofthe trunk stable. It allows activity in theupper part of the trunk.

Backward seat slope. A low backrest,angled forwards, makes the pelvis andlower trunk stable. It allows moreactivity above the area of stability, whichmeans most of the trunk.

ACTIVITY

STABILITY

ACTIVITY

STABILITY

Stability for activity

The trunk's pressure against the backrest createsupper body stability.

How much upper body stability is created andin what direction(s) depends on where the backrestis in contact, the magnitude of the pressure andthe contact area. Together, the backrest and seatangle decide how great the pressure against thebackrest will be.

If the whole seating unit is tilted back in spacethe body-backrest pressure changes - the greaterthe tilt, the greater the pressure against thebackrest.

On a seating unit with a ninety-degree seat-to-back angle and a horizontal seat, seat pressure ismaximized. When this unit is tilted back ninetydegrees it maximizes the pressure against thebackrest. Tilting backwards decreases seatpressure and increases backrest pressure, whichmeans that trunk stability is increased.

Stability through backrest pressure alwaysmakes it more difficult to move the trunk awayfrom the backrest. This is the principle to usewhen users have impaired upper body musclefunction and stability.

A backward-sloped seat makes the pelvis havea strong tendency of backwards rotation. Aforward-angled backrest, and more hip flexion(seat slope) increases stability by making thepelvis be pressed harder against the backrest. Payattention to this pressure for sensitive users!

For users who need a lot of pelvic and trunkstability, slope the seat backward and use a high,reclined backrest (the upper figure on this page).

For users who need pelvic and lower trunkstability for freedom of movement and activityof the upper trunk, slope the seat backward anduse a shorter, well contoured and forward-angledbackrest (see lower figure on this page.)

Published 2002, hardcover, 250 pages, 4-colour illustrations

Posturalis BooksJägarvägen 14, SE-139 40 Värmdö, Sweden

Phone/Fax: +46 8 650 51 71 / E-mail: engstrom.posturalis@swipnet.se