  Finite element analysis of soft tissues is a well-developed method that
allows estimation of mechanical quantities (e.g. stresses, strains). A
constitutive law has to be used to characterise the individual tissues. This is
complex as biological tissues are generally visco-hyperelastic, anisotropic,
and heterogenous. A specific characteristic, their nearly incompressibility,
was well reported in the literature, but very little effort has been made to
compare volume variations computed by the simulations with in vivo
measurements. In the present study, volume changes of the fat pad during
controlled indentations of the human heel region were estimated from segmented
medical images using digital volume correlation. Indentations were repeated
with high and mild intensity normal and shear loads. The experiment was
reproduced using finite element modelling with several values of Poisson's
ratio for the fat pad, extracted from literature values (from 0.4500 to
0.4999). Estimated fat pad volume changes were compared to the measured ones to
assess the best value of Poisson's ratio in each indentation case. The impact
of the Poisson's ratio on the Jacobian of the deformation gradient and the
volumetric strains was also computed. A single value of Poisson's ratio could
not fit all the indentation cases. Estimated volume changes were between 0.9 %
- 11.7 % with a Poisson's ratio from 0.4500 to 0.4999. The best fit was
obtained with a 0.4900 Poisson's ratio except for the high normal load where a
value of 0.4999 resulted in less error. In conclusion, special care should be
taken when setting the Poisson's ratio as the resulting estimated deformations
may become unrealistic when the value is far from incompressible materials.
