The work is focused on the elastic properties of soft tissues, as the brain. Agar gels are tested through a rheometer: in particular cylindrical samples of different diameters undergo a torsion test, obtaining torque and normal force required to twist the samples at constant rotation velocity. These constitutive parameters are useful in order to determine the constitutive model for better fitting experimental data. The gels are modelled with Mooney-Rivlin equations, highlighting the presence of a positive Poynting effect, i.e. they expands in the direction perpendicular to the plane of twisting. Once Mooney-Rivlin parameters (shear modulus µ and C2) are figured out, they are used to implement a Finite Element simulation in Abaqus and then experimental, theoretical and numerical data are compared.