FEM models equations using loadSoftBody ?

[evarcurto]

What are the equations/mathematical models behind the Neo Hookean FEM and the mass-spring FEM used in loadSoftBody? The research paper "Learning to Rearrange Deformable Cables, Fabrics, and Bags with Goal-Conditioned Transporter Networks" which is one of the first papers to use pybullet's new FEM method also has nothing detailed about it.

I needed to deeply understand the model used to use pybullet to get deformation data with ground truth.