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Our aim in this series of discussions is to create a baseline understanding of balanced seated posture to which specific sitting disabilities and postural anomalies can be introduced and compensated for. This in fact is a discussion of the fundamental principles and theories of adult seated equilibrium but viewed through the interplay of geometry, physics, disability and other factors. Any conclusions or deductions extracted from this or any subsequent discussions are not meant to be scribed in stone. Instead, they are intended to form the theoretical bases for future scientific investigations. A problem with any discussion of sitting dependent posture is that most authors fail to fully identify the target population and in many ways imply that their guidelines have universal implication or application. The baseline that we seek here is not intended to describe an accommodating platform for all fixed or flexible orthopedic deformities, especially those present (or initiated) at birth. Our focus instead will be directed to stabilizing the proximal body architecture through the axial alignment of body segments, relative to each other, the line of gravity and other physical forces. So, clear the table and set aside all your training induced biases about the principles of custom wheelchair seating because this is a discussion about a specific type of sedentary postural balance which is undisturbed by vestibular dysfunction and depicted with full sensation intact. For the purposes of discussion, our theoretical subject sits on a contoured/molded seating system constructed of foam. Also, our subject's body composition is: medium height and build, has no structural abnormalities except the presence of adult onset, little or no tone. Let's further clarify that within the context of these discussions, a wheelchair sitting posture is recognized when; in the sagittal plane, the relative angle between the hips and pelvis are 55º or greater and the posterior pelvic support of the backrest is positioned at 55º or greater above horizontal. Take note that this definition differs significantly form the ANSI/RESNA definition of seated posture as being "not more than 45º to the vertical when viewed in any direction". (1) Why little or no tone? Because: a.) with this profile the influencing torque variables of arm function and other upper body movements do not distraction us from focusing purely on the position and orientation of the body segments in question, b.) with little or no tone it is less likely that a test subject will use muscle contraction to compensate for diminished postural control, c.) this profile eliminates the question of trunk muscle strength as a predictor of sitting stability, d.) there should be little argument that the test subject will be consequently deprived of arm function activities resulting from any concluding proposals of this review, e.) in the present scenario, mobility is accomplished through manual assistance, and d.) this test model allows us to record and analyze length, angle and torque measurements to determine the postural stability probability of the subject without actually conducting a stability test. The assumption with this theoretical exercise is that with a flaccid and pliable body structure, each body segment can be placed in an optimal alignment that is conducive to long term stability and equilibrium. If and when a formula is determined it can then be tested using a live subject to determine how effectively it translates from theory to application. We can also then determine whether additional components or adjustment must be applied when dealing with dynamic living structures. The ultimate goal, though beyond this discussion, will be to create a computer model based on the center of mass/center of gravity profile of a person which will illustrate an alignment predictor for each body segment. Eventually, through this alignment predictor, various anomalies can be introduced and compensated for. The results will then be used to compliment the use of seating simulators as well as assist with seating construction and assembly. It is also feasible that the product composition specifications can be used as base elements to determine the mesostructure for Solid Freeform Fabrication (SFF) techniques such as Laser Sintering, additive manufacturing, 3D Printing or other Rapid Manufacturing processes. This yet to be developed group of seating design programs will also contain a list of commercial product specifications which can be matches to the established support criteria for the person being assessed. Most quoted material for this discussion is readily available from public sources and I expect that reviewers of this material will seek out additional documentation on their own. I will not identify the authors within the text, partly because some quotes are either anonymous or difficult to authenticate. However, I will provide references or links to all quoted text at the conclusion of each chapter. Also, though we have set a sequence of chapters for discussing the various aspects of seating, there completion and this outline will not follow the actual sequence for conducting assessment and funding decisions. Using currently acceptable seating and positioning principles we will start with the placement and stabilization of the pelvis and work our way distally to the ankles and feet. Once completed, we will return to the pelvis and this time work superiorly through the trunk, upper extremities and head. We'll also introduce several common postural anomalies as we proceed, and then discuss specific design criteria for the structure and composition of the seat and backrest. Well, that's a tall order, so lets get started. ©Copyright August 15, 2005 - CUSHMAKER.com
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