The origin of stiffening in cross-linked semiflexible networks
  Strain stiffening of protein networks is explored by means of a finite strain analysis of a two-dimensional network model of cross-linked semiflexible filaments. The results show that stiffening is caused by non-affine network rearrangements that govern a transition from a bending dominated response at small strains to a stretching dominated response at large strains. Thermally-induced filament undulations only have a minor effect; they merely postpone the transition. 
P.R. Onck, T.Koeman, T. Van Dillen, E. Van der Giessen