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Many manual wheelchair users and industry professionals claim that a rigid orthopedic backrest automatically prevents a wheelchair user from slumping into posterior pelvic tilt. But my observations have been that even if a wheelchair has an orthopedic style backrest, slumping by the user may still occur.
When a wheelchair user with limited trunk control sits up vertically, they are at risk of their upper body falling forward due to lack of stabilizing muscles.
Falling forward could happen due to:
The forward tilt that is created when descending a slope.
The result of being jostled around while traversing on bumpy terrain.
A push on the person’s upper body from the rear.
Holding an object/arms out in front of the person's torso.
Leaning forward when rapidly propelling the wheelchair.
Leaning forward to pick up an object.
Leaning forward when ascending a slope.
An orthopedic backrest, which provides the support for the WC user to sit vertically while on a level surface, will not alleviate these above situations of falling forward. A traditional solution for high level SCI is to use a strap to hold the user's trunk in a vertical position. While this strap will indeed provide more stability, it will typically hinder the person's overall mobility by preventing a forward lean when pushing. This forced vertical position decreases their power to propel their WC up slopes or on rough terrain. This situation is an example of the tradeoff between static stability and wheelchair mobility.
Note. A well-positioned elastic abdominal strap may provide an increase in dynamic stability which leads to an increase in mobility for some manual wheelchair users. In this case, they are still able to lean forward, but they are prevented from falling forward.
Another traditional method for increasing static upper body stability is to add armrests for the user to grab onto for support. High armrests typically hinder the user's ability to access their rear wheels. Thus, limiting their wheelchair propelling power and upper body range of motion.
Wheelchair setup factors that contribute to upper body instability:
Negative dump or no seating dump.
Hanging (unsupported) legs and feet.
Not having a stable lap to use as a brace for the user’s hands when needed.
Having an unstable footrest platform.
Having an unstable cushion.
Considerable weight on front casters leading to forward jolts when moving.
The user is unable to wheelie and create a dynamic reclining position when needed.
A high backrest which prevents the user from getting their shoulders positioned back.
Note: There are also bodily factors that contribute to upper bodily instability such as sudden spasms, tight muscles, and more. Consult a medical professional to understand these issues.
The WC user will naturally seek to stabilize their upper body. The easiest way to stabilize is by slightly reclining by scooting their hips forward on the seat, regardless of whether or not they have an orthopedic backrest. This forward shift may not even be noticed by the WC user. They just feel more “comfortable” in this position, so they keep doing it. Then it becomes a habitual position. In doing so, the following happens:
The person's torso rests in a slightly reclined position which is now less likely to fall forward.
Moving their hips forward, moves their COG forward, relative to the rear axle. This action has the effect of making the WC less likely to flip over backwards. Thus, seemingly creating wheelchair stability.
Their shoulders move rearward relative to the rear wheel axle which makes the wheels feel easier to access IF the wheels have been positioned too far rearward in the first place (for stability).
The WC user dramatically decreases their ability to ergonomically push their wheelchair by incorporating the powerful muscles of their back. Their pushing technique is restricted to the much less efficient back and forth arc, rather than the more powerful elliptical motion.
This habitual wheelchair slumping has the effect of creating positional upper body stability at the negative cost of poor posture and the development of posterior pelvic tilt with its many associated health problems. In this case, the (expensive and specialized) orthopedic backrest may provide static stability on flat surfaces, such as a rehabilitation hospital. But does not provide stability in the real world environment of uneven/bumpy terrain, and for acts of daily living.
The bottom line is that it is not enough to assume that a wheelchair equipped with an orthopedic backrest will automatically create proper posture. The backrest must be sized and positioned properly for use in a range of environments. It is also necessary to examine the other factors involved in the user/wheelchair mobility system to create the desired postural result.