I would like to the model the viscoelastic behavior of active fibers, as you done in the paper: "Modelling heart tissue using a composite muscle model with blood perfusion" or as here: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.488.5795&rep=rep1&type=pdf and I was wondering if is easy to implemented in sfepy and/or if there is some predefined term in Sfepy, since for example the term "dw_tl_fib_a" doesn't include the "exp{k min(0;de/dt}}" that appears in the paper.

Thank you very much for your help :)