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The Science of Seating


the best chance in life

The role of the Occupational Therapist goes a long way further than merely helping to choose a mobility base & seat. The vision of the therapist is to give the wheelchair user & their carers the best opportunity to integrate & participate in everyday activities in the community (independence, work, learning, socialising etc.). Many studies relating to biomechanics of the human body & posture emphasis how the way we sit may affect many aspects of our lives. The Mobility Base & Seating are tools used to help achieve achieve the ultimate care & efficiency results for the user.










the importance of a comprehensive biomechanical seating assessment

A full biomechanical seating assessment methodically encompasses many aspects about the client to ensure the best mobility wheelchair & seat is chosen & set up in the best way for the user so their maximum potential is reached.


the 5 stages of a biomechanical seating assessment

1. Information gathering

• This is where information is gathered in depth about the client’s history, condition, previous surgery, previous mobility & seating experiences etc.

2. Supine Lying assessment

• The Supine Lying assessment is carried out on a firm surface such as a plinth. The therapist assesses & records information about the user to determine symmetry & alignment, the flexibility of muscles & joints & where to correct or accommodate the posture.

3. Sitting assessment

• The Therapist then assesses the sitting ability, balance, stability, symmetry, head control & the position of the limbs of the user in the seated position.

4. Simulate new posture & Centre Of Gravity

• From the information gathered in the lying & sitting assessments the new desired posture is simulated on a firm surface or assessment chair. Once this is achieved the user is gently & slowly rocked forwards & back to find the angle of the best centre of gravity for the user (the angle of the user’s back in relation to the floor). When the Centre of Gravity is found often the user will relax their arms & find it easier to lift their head. The angle is then recorded so it can be duplicated in the new seat.

5. Measurements taken

• Full body measurements are taken of the user whilst seated in the new simulated posture. Each side is measured individually.


 why postural management is important

Our posture, the way we hold ourselves, affects our overall wellbeing in many ways including the physiological function of our bodies, pain, pressure, stability & balance, vision, coordination & fine motor skills etc.

Posture may deteriorate with age, muscle weakness, muscle imbalance, inactivity, blocked or confused signals being sent from our brain to muscle groups & the development of compensatory postures.

When sitting in the optimum position, postural efficiency & sitting ability increases. As postural efficiency increases our subconscious effort on balance, stability & pain management reduces therefore much greater focus is able to be directed to the task at hand. This greatly increases the opportunity for improved coordination, fine motor skills & head control.

As our ability to control our own posture reduces we must increase our reliance on external supports.



Gravity is a powerful force on an unsupported body which over time, works to form a destructive posture. Body mass & lack of movement are targets for gravity. What begins as a muscle imbalance through asymmetry can end with a change in shape of individual bones & the whole skeletal structure making a reversal almost impossible.



Postural tone relates to the continuous partial contraction of muscles. When postural tone is normal, both the flexor (closing) muscles & extensor (opening) muscles have a constant balanced tone when at rest helping us to maintain a normal posture. Abnormal postural tone occurs when muscles or muscle groups become unbalanced. This may present itself in various forms including, poor coordination & integration, abnormal sensation, jerking movements, contractures, slumped or tense postures etc.

A client with an overall LOW TONE has a floppy or limp presentation & it is often difficult to maintain spinal extension & head control.

Clients with an overall HIGH TONE present with a rigid posture. Clients with HIGH TONE are at risk of  their tone increasing if they feel unstable or uncomfortable in their seating.



Pressure & Posture

Many pressure issues may be traced back to bad posture. Unequal weight distribution localises & intensifies pressure increasing the likelihood of pain & a pressure sore forming.







Image-63-neutral-Pelvic-Tilt-Diagram.pngwhat is the optimal sitting posture?

In sitting, the position where the least intra-discal pressure on the spine, the position least affected by gravity, requires the least effort to maintain & ensures better internal organ function is an upright, straight, symmetrical sitting position. Although the optimum sitting position is determined on an individual basis through the comprehensive seating assessment the principles remain the same. Where possible:

1. The position least affected by gravity.
2. Maintain symmetry & body alignment.
3. Neutral pelvic tilt (or slightly anterior tilt), pelvis level laterally, no pelvic rotation.
4. Upright back angle 95 -100°
5. Knees & feet at 90°.
6. Femurs slightly abducted, no femur rotation
7. Upper limbs in line with torso
8. Shoulders relaxed

In circumstances where it isn’t possible to achieve the optimal sitting posture, skill is required to determine where to correct or accommodate the posture in the seat.


Image-64-Seating-Sequence.pngSequence of seating

When fitting the client into a new wheelchair seat, the procedure is broken down into well-ordered sequential steps to ensure a successful result.


The pelvis is the largest & heaviest bone in the body. In seating the pelvis is the ‘cornerstone’. Always position & stabilise the pelvis 1st before attempting any other part of the seating.


take account of hip dislocation & subluxation, relieve tightened hamstrings by lowering the knee into the contours of the cushion, take account of wind-sweeping, internal or external femoral rotation as this directly affects the position of the pelvis. Failing to accommodate wind-sweeping can increase rotation & asymmetrical shaping in the torso region.


Work downwards positioning the knees & feet as this helps to stabilise the pelvis in the seat.


Once the pelvis is stabilised in the seat the torso is a lot easier to position. An upright backrest (95 - 100°) can help with spinal extension even with users who have low postural tone. Match the backrest to the user. Locating & accommodating even small asymmetries in the users spine & rib cage helps to minimise spinal rotation & the development of a compensatory posture forming. Gentle support under a kyphosis or in a lordosis cavity taking care not to overbuild can increase comfort & sitting tolerance.


The position of the head is critical as in seating the head through gravity leads the body. The Headrest position set up is unique to every user & may differ depending on the support requirements, the sitting ability of the user, whether it is used for function such as head controlled driving or merely as protection during transportation in a motor vehicle.


The position of the arms helps to support the torso & can affect the Centre of Gravity of the user. Upper arms should be relaxed & in line with the torso. When the forearms are too far forward on the armrests the shoulders become protracted & the user’s back will follow. If the forearms are too far back on the armrest shoulders may become retracted. Armrests positioned too high can cause shoulder painful elevation & armrests which are too low reduces head control, & may increase the risk of a kyphosis reducing spinal extension.


extension thrust

During our seating assessments of many "high needs" users we found that EXTENSION THRUST can apply both to clients with overall High tone or Low tone but mostly was caused by only two reasons: 

  1. Discomfort - Most importantly & commonly we found that tightened hamstrings combined with seating which incorporated a traditional style high pre-ischial shelf was the cause of great discomfort to the user. It seemed counter-intuitive, but enabling the leg or legs to be positioned lower to relieve the hamstring muscles combined with allowing the feet to move back on the footplate or using 90 degree legrest hangers, the clients sat for much longer periods without extending out of the seat or showing signs of discomfort. Sometimes repositioning the legs laterally to accommodate tightened abductor muscles or adductor muscles e.g. windswept postures, greatly aided  in the comfort to the user for long term sitting. Undiscovered hip subluxation or dislocation also causes a lot of pain & discomfort to the user & the client often will extend or twist in their seating to relieve the pain. Reducing any pressure in this area & reducing the tension on the hip belt can quickly help the client to relax.
  2. Mood - We all have bad days where we want to vent our feelings & frustrations. As these feelings go well beyond how they feel about their seat, this type of extension thrust will not be fixed by well configured & comfortable seating although good postural seating may reduce the frustrations making the client happier overall.