Most of us who learned about seating and positioning in university were most likely taught rules for guiding appropriate wheelchair prescription – but how many of us have ever really taken a moment to consider why those rules exist, and whether or not they actually make sense or hold any value? We’d like to take this opportunity to deconstruct some common rules that were taught in the 80’s, many of which are still being taught in graduate programs at top universities today.
1. 90°-90°-90°
The Rule: Person should sit at 90°/90° angles (hips/knees); the cushion should be level so that thighs are horizontal to the ground and even with hips.
Think about this 90°-90°-90° concept.. just for a minute. Do any of us sit in this way consistently? Can we function when we try to sit this way? The answer is in all likelihood “NO”.
Perhaps 90°-90°-90° would be ideal for SpongeBob SquarePants. But not for the humans we serve on a daily basis in clinical practice.
A full hands on assessment of the person in sitting and supine is essential to determine their unique body tolerances which in turn will give us the information needed to provide alignment strategies that are purposeful from a postural, skin preservation and functional point of view.
On top of this, from a technology perspective - where are these angles meant to be measured? Are we meant to measure the wheelchair frame, or are we supposed to also factor in the seat cushion (loaded or unloaded), back support and foot support?
Once the human body functions and structures have been analysed during the hands on assessment and the seated simulation has been completed, we will now have this information:
How much immersion is happening and necessary in the seat cushion ( without bottoming out) - this will impact pelvic thigh angle as well as potentially the pelvic trunk angle
Where the persons feet need to be placed on the foot supports relative to their knee (thigh to lower leg angle as measured during the assessment) - there’s even a chance that the right foot will be placed somewhere different than the left! Symmetry is not always optimal!
What the desired pelvic thigh angle is (this is different to the thigh to trunk angle) - clients with significant lumbar lordosis and limited hip extension will benefit from a much different setup than clients with limited hip flexion and tight hamstrings!
What the optimal pelvic trunk angle is ( as identified during the evaluation) - lets really think about what we’re measuring here and how it can affect function!
How much gravity assist (orientation in space) is needed for optimal function - your client with non-reducible thoracic kyphosis might need tilt to function and to improve their visual field, while a client with limited hip flexion might need tilt because their limitation is accommodated through opening the back support in order to address their body structure!
With the answers to these critical questions, we now can speak about angles: body angles and technology angles. Nowhere in this list is there a preconceived notion about 90-90-90.
Check out the Guide to Standardized Wheelchair Seating Measures for terminology and a more in-depth explanation of measurements.
2. Seat Depth
The Rule: Subtract 2-3 inches from the measurement of popliteal crease to back of buttocks; the seat cushion should end 1.5-2” from popliteal fossa
For many years, I actually believed this without questioning. Then one day, I decided to question everything I was doing as I worked on creating for myself a clinical pathway for my assessments. During this experience, seat depth was no exception.
WHY are we told that we need to subtract from the measurement we have taken from the back of the buttocks to the back of the knee (buttock/thigh depth)? WHY is it so easy for us to believe this without question?
I have asked so many clinicians around the globe exactly that. The majority of answers are related to pressure on the popliteal fossa. If we think about this - really think about it … pressure on the popliteal fossa is way more likely due to the foot supports being too low than the seat depth being brought to the back of the knee.
I relate seat depth to the inferior sitting footprint/load bearing area of the seat support surface. We know that pressure is force/area. We know that by increasing the area in a well fitted cushion, we can theoretically reduce pressure.
Once I know the assessment findings related to body structures and functions and have simulated the concept, I measure buttock/thigh depth from the posterior buttocks to the back of the knee in the desired seated posture, paying attention to the medial hamstring tendon.
If there are tight hamstrings OR if there is a large calf mass that needs to be respected in combination with the clinical need for immersion and skin protection, then I have choices related to the front of the cushion and desired seat depth. For example, we could shorten the seat depth to respect these findings at the cost of decreasing the loading surface; we could scallop the front of the cushion or bevel it back so as to maintain the loading surface while allowing for the shortened hamstrings or larger lower leg mass.
None of this decision making included the thoughts of a standardised approach of subtracting 2-3” or 2-3 fingers from the human measurement taken.
Another example might be related to the person who functions from a wheelchair who self transfers. Perhaps they need to put their hand under their distal thigh to lift their legs sideways so as to do a side transfer from the chair to another surface. This person may need full seat depth for the purposes of skin protection/loading area as well as for lower extremity positioning, but if they don’t have the endurance to do a two step transfer ( forward first to get their hand under their distal thigh and then transfer laterally), it may be necessary for function to select a shorter seat depth - and that is how I would document the decision.
Again, there is no mention or thought here about the methodology frequently taught.
3. Seat Width
The Rule: Add ½ to 1 inch (1.75-2.5 cms) on both sides of greater trochanter/hip width measurement
I see this language in clinical references all over the globe still today. All I have to say about this is that I don’t choose shoes for myself or my family that are a 1/2” or an inch bigger than our feet.
A wheelchair is an extension of the human body.
It must fit intimately for function, skin protection, postural protection and shoulder/wrist protection.I have not added centimetres to this measurement for years and I am thankful for the wheelchair users I have met for validating why this is so important.
Sometimes we are told to add width for children to facilitate growth. Be very careful with this. If the child you are working with is propelling a manual wheelchair, it is NOT in their best interest to have the chair wider than their hips. A wider wheelchair will impacts the child’s ability to access the push rims on the wheels, putting them at risk for shoulder and neck problems down the road. If it is necessary to prescribe a wheelchair that is wider than the person sitting in it, adding camber may be an option to decrease the strain on the shoulders and wrists by bringing the wheels closer to the wheelchair user. From a postural protection standpoint, in all likelihood the child will grow into the chair that is starting out too big for them, but this will likely happen with deviated postures in the absence of optimal dimensions and support.
4. Seat to Floor Height
The Rule: Measure lower leg length and add ~ 2-4 inches for clearance of foot plates
Seat to floor height is a critical measurement that impacts
Transfers involving stand and pivot
Foot propulsion where the hamstrings have the range making this technique possible
Access to the environment ( from rolling up to tables to traversing curbs)
Access to wheels
Stability
Seat to floor height really cannot be determined without knowing
Lower leg length measurement bilaterally from popliteal fossa to heel (with shoes +/- AFOs factored in )
Cushion thickness at the front under load
Any asymmetries that are being accommodated related to hip limitations. For example right thigh needs to be supported 1” (2.5 cms) lower than the left because of a right side hip flexion limitation - this means that the right foot will be 1” closer to the floor when supported
Curb/environmental obstacles that need to be traversed regularly
Desired floor to top of head measurement considerations
Any specific wheel size considerations/requirements
All of this information is gleaned during the thorough hands on assessment.
5. Arm Support Height
The Rule: Measured from the seating surface to the elbow with shoulder relaxed and elbows flexed to 90°
This measurement is probably reasonable as long as we allow for non reducible asymmetries such as scoliosis, where one arm support may need to be lower than the other.
The language used for the human measurement is elbow height which translates to where the arm support should be located. We have so many choices for arm supports now from manufacturers that we can get what is necessary relatively easily in many parts of the world.
A challenge I see related to arm supports is often more about overall location than actual height.
If the width of the seating system or chair is much wider than the persons shoulder width, it can be challenging to get the arm support optimised. Also in the case of a hemiplegic presentation, getting the arm and shoulder supported in a way to not induce shoulder dislocation can be challenging due to location.
6. Back Support Height
The Rule: Measure from the seat surface to the inferior angle of the scapula and subtract an inch OR measure from the seat surface to the shoulder (which I have noticed was/is used a lot with paediatrics)
In todays world we now use the language Back Support Length and Back Support Height, which are two different and equally important measurements.
Back support length is the actual dimension of the back-support, whereas Back support height represents the vertical location or placement of the back support relative to the seat. Adjustment of the back support up or down will change the back-support height but not the back-support length.
I like to think about the level of support needed by the individual being assessed for optimal function. This includes not only where the top of the back support should end, but also where the bottom of the back support should begin. Putting it all together with the cushion and wheelchair configuration is when I need to consider back support height.
For all populations, including children, we need to know where we want the back support to begin and end on the body. We need to know the human measurement.
For example; perhaps it is desirable to have the back support start at the PSIS level and end at T10/11 for an active person with an incomplete thoracic SCI. The length of the back support will be determined by measuring the distance between these two points on the persons back while they are supported in their optimal alignment. Once the seat cushion and optimal wheelchair configuration is determined, the back height can then be adjusted through the hardware/attaching mechanism.
Back support fitting is such a critical consideration for optimal seat cushion performance related to function, skin and posture protection. Getting the length and height sorted is one of the essential steps of wheelchair prescription.
7. Seat Dump/Squeeze
The Rule: Seat to floor height should be lower in the back than in the front for persons with SCI and poor trunk control to improve balance and stability.
I have to admit that over the years I have had a lot of confusion between terms Seat dump: Squeeze: Seat slope etc. What do these terms even mean? Is there a consistent language?
Seat Dump and Squeeze: Both of these terms tend to be referring to a seat slope, where the seat to floor height is higher at the front than the back. These terms tend to be interchangeable some places in the world, yet they tend to mean two very different things in other places in the world. For example, when I hear the term ‘dump’, I have a vision of a wheelchair with an open seat to back angle (>/= 90 degrees), and when I hear ‘squeeze’, I have a vision of a wheelchair with a closed seat to back angle (<90 degrees). When Jenn was learning about wheelchair prescription, and throughout her time working on a spinal cord inpatient rehab unit, she learned that ‘dump’ was with a closed seat to back angle, and had never heard the term ‘squeeze’ before coming to Europe! The definition of ‘dump’ and ‘squeeze’ might depend on the manufactures you are working with, but truthfully these terms are outdated and we should instead be referring to seat slope and seat to back angle when describing this position.
Seat Slope/Incline: This is referring to the slope created by the difference between front seat to floor height and rear seat to floor height, which is visible in the frame of the wheelchair. An inclined seat can be used in combination with an open seat to back angle, perhaps for someone with a hip flexion limitation, or with a closed seat to back angle, such as for someone who prefers to have their knees higher than their hips for stability. Seat slope/incline on it’s own does not inherently have anything to do with the seat to back angle. Many terms have been used to describe this, such as seat tilt, seat incline or seat plane angle. The terms seat sagittal angle and inclined seat are recommended to maintain consistency in terms following the principles of the international standard.
Language is important. We need to do our best as professionals to stay up to date with current best practices, including terminology. Language is so important that you’ll see a future blog post dedicated to just that! Now that we are on the same page with regards to terminology, let’s get back to discussing why adding ‘seat dump’ or ‘squeeze’ shouldn’t be a go-to rule.
What I have learned to appreciate is that I need to know a few important things to be able to get what is needed for the person I am assessing:
The influence of hip flexion on pelvis trunk alignment and stability.
This is very important to know this before the seat to back angle is determined. If we create a significant difference in seat slope for someone with limited hip flexion with the front being higher than the rear in combination with a closed seat to back angle, we are likely contributing to a compensatory kyphoscoliotic posture, uneven pressure distribution, and respiratory compromise.
The desired front seat to floor height, as we discussed above.
The desired rear seat to floor height which may be related to wheel access, postural stability and overall function.
The cushion that will be used.
I need to know about immersion which means I need to check to ensure that the person is in the cushion rather than on the cushion, and that we have the required 1.75-2 cms of protective material beneath the ischial tuberosities. With many cushions on the market, by design there is “seat slope happening” through immersion. Therefore we should factor this in before making a decision about the wheelchair seat slope configuration.
The adjustability of the posterior/back support.
Knowing this will provide us with options for the appropriate seat to back angle once we have determined the appropriate front and rear seat to floor height.
So, in summary, I use the language ‘front seat to floor height’ and ‘rear seat to floor height’ to describe an inclined seat, and the desired ‘seat to back angle’ to describe whether it is an open or closed angle rather than using the language seat ‘dump’, ‘squeeze’ and ‘slope’. I strongly recommend referencing the Glossary of Wheelchair Terms and Definitions for consistency and use of standardised language.
8. Lateral supports
The Rule: Use lateral supports to correct lateral trunk lean
Lets just start by saying we will be VERY disappointed with laterals if we believe that they correct lateral trunk leans….
I use the Sitting Footprint in daily practice to describe the inferior and posterior loading surfaces (Primary Support Surfaces), and the lateral and anterior loading surfaces (Secondary Support Surfaces, or assistants to the primary support surfaces).
As secondary support surfaces, laterals should only be used after the primary support surfaces are optimised and we have identified how much and in what orientation gravity can be most helpful for the individual. In all likelihood, the lateral is merely a band-aid on a symptom (the symptom being a lateral lean) rather than a solution for the cause (which we determine from our hands-on assessment).
It all goes back to the hands on evaluation as this is where we gain the NECESSARY information about the human body and the impact of one segment upon the other relative to sitting. In the absence of this we are guessing and this seldom yields positive long term outcomes.
Rather than thinking about Rules that dictate seating choices, I like to think about Guidelines to facilitate appropriate equipment selection.
What kinds of rules have you heard in regards to wheelchair prescription?
What would you add to our list of considerations for the rules we have listed above?
Are there any resources or guidelines you find helpful in practice?
Leave a comment below!