I have more questions than answers, unfortunately. There are a LOT of muscle models out there in the literature that are based on Hill's formulation. What have you implemented? Also, since you have implemented the model, it seems natural to ask: Shouldn't you know which history variable you are using to save muscle forces in? Also, is it a continuum mechanics formulation or a 0D approximation? If it is the latter, then how is it any different from what has already been available in Dyna for a while? If it is the former, MAT_295 has an active module in it that you might find interesting. However, please know that MAT_295 is very new and its features are not available in the graphical user interface and the documentation may not be up to date either :)