||In the context
of support surfaces, pressure is considered to be the force per unit area exerted on the
body by a mattress, seat cushion, or other body support.
In engineering terms, this quantity would be referred to as normal stress. The component
of the force exerted on the skin acting along the skin surface can produce what is
commonly referred to as shear and will be discussed subsequently. The distribution of
pressure on a seat cushion or mattress depends on the relative fit between the body and
the support surface, the mechanical characteristics of the body tissues and the cushion or
mattress, and the distribution of weight in the body.
The ideal pressure distribution
would be one where the soft tissue was not deformed relative to its unloaded condition
(Chow, 1978; Levine, 1990; Brienza, 1993). Such a condition would minimize many of the
effects believed to lead to the development of pressure ulcers. These effects are
capillary blood flow occlusion (Kosiak, 1961; Reswick, 1976; Daniel, 1981), impairment of
lymph flow, and excessive interstitial fluid flow (Reddy, 1981a; Mak, 1994). The precise
underlying mechanism for cell damage has not been determined. However, since pressure
distributions with low peak pressures and low pressure gradients likely minimize each of
the presumptive harmful mechanisms leading to pressure ulcers, it is reasonable to strive
for loading conditions with these characteristics.