strain energy function

So today what i learn is strain energy function
what a headache problem.
http://www.comsol.com/community/forums/general/thread/5999/

http://comsol-users.blogspot.com/2006/11/crash-course-on-comsolfemlab-for.html

http://www.comsol.se/community/forums/general/thread/16322/

http://www.comsol.com/community/forums/general/thread/24219/

a good explanation although i still not understand is
http://solidmechanics.org/text/Chapter3_5/Chapter3_5.htm

i should search for cardiac muscle properties