With 300,000 paraplegic persons only in France, ischial pressure ulcers represent a major public health issue. They result from the buttocks' soft tissues compression by the bony prominences. Unfortunately, the current clinical techniques, with - in the best case - embedded pressure sensor mats, are insufficient to prevent them because most are due to high internal strains which can occur even with low pressures at the skin surface. Therefore, improving prevention requires using a biomechanical model to estimate internal strains from skin surface pressures. However, the buttocks' soft tissues' stiffness is still unknown. This paper provides a stiffness sensitivity analysis using a finite element model. Different layers with distinct Neo Hookean materials simulate the skin, fat and muscles. With Young moduli in the range [100 - 500 kPa], [25 - 35 kPa], and [80 - 140 kPa] for the skin, fat, and muscles respectively, maximum internal strains reach realistic 50 to 60 % values. The fat and muscle stiffnesses have an important influence on the strain variations, while skin stiffness is less influent. Simulating different sitting postures and changing the muscle thickness also result in a variation in the internal strains.