Key: COG (center of gravity) COM (center of mass)

This discussion is specific to weight-bearing on the buttocks and thighs and assume that sitting posture is with the pelvis in a neutral axis orientation

• Our sitting footprint indicates how and where pressure is distributed between our buttocks and thighs and the corresponding support surface.
  
  
• From the time we are infants and throughout all of our growing years, our sitting footprint expands in length, width and total area.
  
  
• During our growing years our sitting footprint expands at a slower rate than our body's increase in mass.
  
  
• As our body mass increases, the contact pressure within the slower expanding footprint, increases as well.
  
  
• Even though we reach a point in life where our body is no longer growing our sitting footprint continues to expand due to continued weight gain and changes in weight distribution.
  
  
• Given the same weight and mass, a person with a larger footprint will exhibit lower contact pressure.
  
  
• Pressure readings within the sitting footprint may represent only a moment in time, will vary from location to location and may not be symmetrical, left to right.
  
  
• Areas associated with bony prominences will exhibit higher pressure readings than the adjacent anatomy.
  
  
• Adult women are likely to exhibit a larger footprint than adult males by the fact that, given the same height and weight, the intra-ischial distance and hip width for females is typically greater.
  
  
• The sitting footprint continues to expand until approximately the early 40s for males and early 50s for females.
  
  
• From about 40 years of age and older, even though a male's weight may continue to increase, his concentration of body mass begins to rise above the waist. At the same time his sitting footprint begins to decrease. The result is increased pressure at all regions of the footprint.
  
  
• On average, a women's weight and sitting footprint continue to increase through age 55, with most weight gain at the pelvic region and below.
  
  
• Beyond these general time frames, the sitting footprint for both sexes steadily decreases until, as part of the aging process, weight-loss and muscle atrophy can progress no further.
  
  
• At the point that the size of the sitting footprint begins to diminish (no matter the reason), so does the quality of the footprint. That is; young and healthy muscle tissue can better support body weight than aged or compromised muscle tissue. This diminished quality of pressure distribution can also set the stage for increased risk of chronic discomfort, ischemia and skin trauma.
  
  
• Emaciation due to aging can combine with disuse atrophy, impaired sensory feedback and a compromised flight response to further increase the risk for skin trauma. Such that, as the quality of the footprint diminishes, your client looses the ability to reposition their body away from the cause of discomfort, pain or ischemia.
  
  
• In total, as the quality of the sitting footprint diminishes, the distribution of weight becomes more concentrated into an ever-smaller area of support and the potential for repetitive stress or ischemia increases with time. Secondarily, the potential for permanent skin trauma increases as well.

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The same thinking regarding the diminished size and quality of the sitting footprint can be applied to any individual experiencing disability related disuse atrophy. The most significant difference is that while there is a diminution in the size of the footprint, the amount of superincumbent mass (the weight above the pelvis) may not diminish proportionally.

In fact, superincumbent mass may continue to increase as part of the maturing process. Consequently, as the pounds-per-square-inch concentration of body mass increases so does ischemia-producing pressure and the sensation of discomfort. Ultimately, to reduce pressure under the ischii, the shape of the seat cushion must include more pronounced convolutions.

In the case of birth related abnormalities such as Spina Bifida, weight gain above the waist is typically disproportionate to the lower body at an earlier stage in the growth and physical maturity cycle. Additionally, there is considerable likelihood that combined with a structural deformity, weight distribution will never be either symmetrical or balanced. The most common occurrence of these deformities is anterior pelvic tilt. Unfortunately, anterolateral postural collapse is very difficult to prevent, delay or manage.

As for asymmetry and imbalance, a similar picture will arise with early onset SCI (tetraplegia), though it will likely progress at a slower pace. However, in pediatrics, once a spinal fusion is introduced, the disparity between natural growth and suppressed growth regions will impact postural imbalance in a pronounced manner.

Sitting Footprint Pressure Variables

• Weight shifts and posture changes affect pressure distribution and can even produce short periods of localized ischemia.
  
  
• The feet and lower legs play a roll in pressure distribution regardless whether they are weight-bearing or not.
  
  
• Unilateral limb amputations (upper or lower) produce asymmetrical pressure distribution.
  
  
• No matter the duration, an extended, unsupported lower limb creates a torque moment from the hip to the toes. This position increases pressure at the distal thigh and decreases pressure at the ipsi-lateral pelvis. The consequence is obliquity and anterior migration. This is a common occurrence with hyper-kinetic individuals.
  
  
• The location of highest pressures within the footprint will remain the same no matter the density or conformity of the support surface.
  
  
• We can manipulate the size of a sitting footprint by varying the shape and density of the support surface. The more pliable the support surface, the larger the footprint.
  
  
• The more that a support surface conforms to, or mimics the concave profile of the human body, so does the size of the footprint increase and contact pressure decrease.
  
  
• With the most reliable consistency, air-cell cushions provide the greatest amount of immersion and lowest overall pressure readings, at the expense of increased buoyancy and instability.
  
  
• For short periods of time, gel-pack cushions can produce very low pressure readings. However, those readings can oscillate to ischemia producing highs, through the course of any given day and any given posture.
  
  
• A built-in pommel can expand the sitting footprint and thus decrease contact pressure, whereas a flip down pommel cannot.
  
  
• A contour-shape foam cushion can maximize the sitting footprint, increase pressure distribution, reduce pressure at bony prominences (to zero if desired) and provide optimum stability in a reliable and reproducible manner.

Richard Xavier Cushmaster
CUSHMAKER.com

©Copyright March, 2008

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