Optimization Potential of the Method - Preventive Biomechanics - page 318

Fig. 7.37 a-f Different stages of buttock loading: a no deformation, e at 40 % body weight
loading and f maximum deformation at 100 % body weight loading

7.2.2.2 Stress and Strain Analysis

The resulting tissue and support material deformation process at different stages
during the process of sitting down in section cut view at the ischium is depicted in
Fig. 7.37 , starting from left to right at zero deformation, Fig. 7.37 a, to the state of
static equilibrium, Fig. 7.37 f.
As already presented in Chap. 6 'Mechanical Interactions', in the seated
posture, maximum tissue direct stress occurs underneath the ischial tuberosity in
the muscle tissue, Fig. 7.38 , and maximum shear stress occurs laterally to the
pelvic bone, Fig. 7.39 in the muscle tissue. These two body sites were used to
establish stress over cushion thickness relations. (Note: max. direct and shear
stress differ by one order of magnitude!).
In the contour plots shown in Figs. 7.38 and 7.39 , element corner output was
averaged across component boundaries at the element nodes using a simple
averaging

method. Averaged stress was used since absolute values are not

necessary to demonstrate the approach.
The above loading scenario was simulated altering the cushion thickness from 0
to 300 mm in steps of Ds = 20 mm, Fig. 7.40 . In Fig. 7.40 tissue stress represents
mean values collected from the domain comprising 8 continuum elements in the
particular tissue region. In general, it can be observed from simulation that the
tissue location where stress accumulates changed with cushion thickness. With
increasing cushion thickness the location displaced laterally. At zero cushion
thickness major

contact loads were induced underneath the ischium and as