Avril et al from Ecole National Superieure des Mines in France have published work on fitting constants in a Neo-Hookean strain energy function to soft tissues in the lower leg, on the macro scale, in 2D based on unloaded and loaded MRI sections.
Journal of Biomechanical Engineering 2010 vol 132, 'Mixed Experimental and Numerical Approach for Characterizing the Biomechanical Response of the Human Leg under Elastic Compression'.