The mobility performance of a person using a wheelchair is the result of the output of a complex adaptive system made up of the wheelchair, the wheelchair user, the immediate environment, and the cause and effect interactions between the entities of the system.
These four variables can be described as:
(W) – The physical characteristics and configuration of the wheelchair
(U) – The physical, cognitive, and emotional characteristics of the wheelchair user.
(E) – The physical factors of the environment in terms of terrain, weather, wind, lighting, temperature, man-made and natural obstacles, presence/absence of other people, clothes, etc.
(I) – The cause and effect relationships between the variables (W,P,E) which include volatility, uncertainty, and unknowables.
A person’s wheelchair skills are examples of the cause and effect relationships between the User/Device, User/Environment that adapts over time. As with any acquired human skill, this ability is subject to dramatic change making it volatile. It cannot be accurately measured in many situations which creates uncertainty, and its qualities cannot be fully predicted in the future making certain aspects unknowable.
Due to these factors, the wheelchair system is described an open complex adaptive system. It is an open system because it can be influenced by factors outside the system. It is an adaptive system because it can modify itself and evolve. It is complex because of the existence of interactions between the elements of the system.
Therefore, we have the impossible to solve function equation for the system where P is the variable for the performance output of the wheelchair system.
P = f(W,U,E,I)
If you understand the complexity of the wheelchair system, then you also recognize that no person, no doctor, no physical therapist/occupational therapist, no Assistive Technology Professional or seating specialist, no wheelchair sales representative, no researcher, no wheelchair user, no family member, etc. has the ability to accurately predict the mobility behavior produced by a person using a wheelchair by examining only part of the system. This is the case regardless of their personal level of experience or pedigree. There are simply too many variables involved.
The complexity of the wheelchair system means that it is not possible to measure the physical characteristics of a person in need of a wheelchair and then come to an accurate conclusion as to the exact wheelchair configuration that will produce the greatest mobility performance due to the existence of the other variables in the system (because some the other variables will change, have uncertainty, or are unknown).
The behaviors of Complex Adaptive Systems are highly unpredictable. Their behaviors cannot be determined by simple linear equations. For example, you can accurately measure the length of a person’s bottom-of-foot to bottom-of-bended-knee. But that doesn't mean you know how long the wheelchair’s leg support should be fabricated due to the other variables of varying shoe heel height, presence/absence of footwear/cushion thickness, and individual preference (which may adapt/change over time).
You cannot accurately determine the placement of the rear axle of a manual wheelchair’s rear wheels since there is no way to determine the person’s future wheelie balancing skills and personal emotional comfort level with stability/instability due to the future adaptation of the wheelchair system.
Effective problem solving for complex adaptive systems requires a constant strategy of probing, testing, experimenting, and responding to feedback over time. This requires the ability to modify responses (the configuration of the wheelchair) in order to discover the most effective configuration at a given point in time (since the system keeps evolving).
On the other hand, a wheelchair that has been made-to-measure/customized with a fixed geometry is a singular and final response to the prediction of wheelchair system behavior. This decision has long term consequences for the future mobility, comfort and life of the wheelchair user (since it cannot be changed). And due to the inherent uncertainty of the complex wheelchair system, particularly for a first time wheelchair user, this prediction is highly likely to be wrong in the long term.
When someone makes an irreversible decision regarding wheelchair configuration they are in effect saying that they have solved the P = f(W,U,E,I) equation in that particular case. Maybe they have, but it is more likely that their lack of understanding of the overall complexities of the wheelchair system has led to a mistaken conclusion.
In summary, creating effective wheelchair seating decisions requires having an in-depth understanding the behavior of the entire wheelchair system, not just a piece of it (regardless of how well that particular piece may be understood).