Sitting is not the body's default
In the previous guides of this rung you met the wheelchair as a vehicle: the wheeled-mobility evaluation that decides who needs one, and the difference between a manual chair and a powered one. But a wheelchair is two machines stacked together. The lower one moves; the upper one holds. This guide is about the upper one — the seat — and the quiet truth that most prescriptions get wrong at first: a chair that rolls beautifully is useless if the person sitting in it cannot stay upright, comfortable, and functional for the hours they actually have to live in it.
Here is the central insight, and it is worth slowing down for. Sitting upright feels like a default — something the body simply does — only because, in a healthy person, an enormous amount of automatic muscular work is hidden underneath it. Dozens of trunk and hip muscles fire and relax thousandths of a second apart to keep your centre of mass over your base of support, the same balance idea you met in kinesiology. Take that automatic correction away — through a spinal cord injury, a stroke, advanced multiple sclerosis, cerebral palsy — and the body does not 'sit badly'. It does not sit at all. It collapses toward gravity. Good sitting, for these patients, is not something the body manages on its own; it is an engineered outcome that the seat has to supply.
The seating assessment: starting from the pelvis
A seating assessment is its own examination, distinct from the mobility evaluation, and a skilled therapist runs it in a deliberate order — almost always from the bottom up. The reason is mechanical: the pelvis is the foundation of the seated body the way footings are the foundation of a house. Tilt the pelvis and everything stacked above it — the spine, the shoulders, the head, the line of sight — tilts with it. So the first thing the clinician does is take the person out of the chair, often onto a mat, and answer one question by hand: which of this person's postural problems are still *flexible*, and which have become *fixed*?
That distinction governs everything that follows. A *flexible* deformity — a pelvis you can gently rotate back to neutral, a trunk that can be coaxed upright — can be *corrected*: the seat is built to hold it in a better place. A *fixed* deformity, where a joint has stiffened into a contracture (the immobility hazard you met earlier in this ladder), cannot be forced straight without pain or skin breakdown. It must instead be *accommodated*: the seat is shaped around the body as it is. Trying to correct a fixed deformity is one of the cruellest and most common seating errors — you end up loading a bony prominence that was never meant to bear weight.
- Off the chair, onto a mat: assess hips, pelvis, and trunk by hand, sorting each problem into flexible (correctable) or fixed (must be accommodated).
- Measure the body: hip-to-knee length, seat width, trunk height, and the bony landmarks that will bear weight.
- Simulate the target posture while still hands-on, before committing to any hardware.
- Translate that posture into a seat, cushion, and supports — then re-check it in the chair, under gravity, over time.
Building support from the seat up
Once the assessment is done, the seat is assembled as a postural support system — a deliberately layered set of surfaces, each with one job. The foundation is the cushion, which you will meet in full in the skin-protection guide; for now, know that it does two things at once: it distributes pressure away from the sitting bones, and it gives the pelvis a defined, level shelf to sit on. On top of that foundation sits the postural support system proper: a firm back support set at the right angle, lateral trunk supports if the spine tends to fall sideways, a pelvic positioning belt across the hips at roughly forty-five degrees, and — where head control is poor — a headrest. Each piece is added only because the assessment showed a specific body needs it.
Notice the pelvic belt detail, because it captures the engineering mindset of this whole field. A casual eye sees a 'safety strap', as if it were a car seatbelt for catching a crash. It is not. Its job is to stop the pelvis from sliding forward and rotating backward into a slumped C-shaped 'sacral sit', which collapses the trunk and drags weight onto the tailbone. Set it too high and it rides up onto the soft belly, doing nothing; set it at the right low angle into the crease of the hip and it anchors the foundation. A few degrees of strap angle decides whether the whole tower above stays standing.
All of this serves a posture clinicians describe with one tidy phrase: neutral pelvis, level shoulders, midline head. That is the alignment the assessment from the previous section was reaching toward — the same postural alignment principle from kinesiology, now hand-built into foam and frame rather than produced by living muscle. When it works, you can see it instantly: the person's eyes come up to the horizon, their hands come free of holding themselves up, and they look out at the room instead of down at their lap.
Tilt and recline: changing the angle without changing the posture
A static seat, however well built, has one problem: it asks the body to hold a single posture for the whole day, and no body tolerates that. The answer is tilt-in-space and recline — two movements that look similar and do very different things, a distinction worth getting exactly right. Recline opens the angle between the seat and the back, so the trunk leans back relative to the thighs, like a reclining armchair. Tilt-in-space keeps the hip and knee angles frozen and rotates the whole seat-and-back unit backward as one rigid piece, the way you might tip a chair onto its back legs without changing how the person sits in it.
Why does the difference matter so much? Recline opens the hip angle, which is wonderful for catheter care or a stretch — but it invites shear: as the trunk slides up and then back down the backrest, the skin gets dragged across the bone beneath it, and shear is even more destructive to skin than pressure alone. Tilt-in-space avoids that, because nothing slides relative to anything else; the carefully built posture from the last section is preserved exactly, just rotated. That is why tilt is the workhorse for someone with no protective sensation: it lets you shift load off the sitting bones many times a day without ever disturbing the alignment you worked so hard to set.
Stability and function: the two goals in tension
Every seating decision is pulled between two goals that quietly fight each other: stability and function. Stability is support — the more surfaces hold the body, the safer and more aligned it is. Function is freedom — every centimetre of movement a person keeps is a thing they can do for themselves. A blocky seat that locks someone in like a fixture is maximally stable and minimally functional. The skill is finding, for this person, the least restraint that gives enough stability — because a stable trunk is precisely what *frees* the hands and the gaze to do work.
Picture a teenager with cerebral palsy doing schoolwork. Without trunk support, both hands are busy propping himself upright and his head bobs, so he cannot read or write — all his effort goes into not falling over. Add a firm back, snug lateral supports, and a well-angled pelvic belt, and his trunk is suddenly held *for* him. The same hands that were holding his body up are now free to hold a pencil; his head steadies; his eyes find the page. The supports did not restrict him. By taking over the stability his nervous system could not supply, they handed his function back. That, in one image, is the entire point of seating.
And the seat is never finished. Bodies change — children grow, tone fluctuates, weight shifts, a flexible curve hardens. So seating is reviewed, not prescribed once and forgotten; the engineered posture is checked against a moving target. It also never stands alone: the alignment you build is what makes the day's pressure-relief weight shifts effective and protects the skin over the bony prominences — the thread that ties this guide to the skin-and-shoulders guide still ahead. Seating, in the end, is honest about what it is: it does not restore the muscles that once held the body up. It is a careful act of compensation — building, in foam and frame, the stability the body can no longer build for itself, so that everything above it can get back to living.