Transferring load to flesh: part IX. Cushion stiffness effects.

Stresses developed within flesh in contact with a cushion are examined analytically. Compressive, tensile, and shear stresses within flesh in proximity to a bone are given as a function of cushion stiffness (major parameter) and the overall Poisson's ratio of flesh (minor parameter). It is shown that an individual already sitting on a soft cushion receives relatively minor benefit when a still softer cushion is substituted. This result follows from the fairly flat (saturated) trend of both shear and tensile stress with respect to cushion stiffness, once the soft cushion domain (less than 10 PSI) is entered. Only flesh normal stress (compression) responds significantly to incremental changes of stiffness within the soft cushion range; reducing the cushion stiffness 50 percent will typically reduce local compressive stress by roughly 20 percent. While such a gain is real, it is also modest.