Muscles have natural stiffness (spring-like behavior) and damping (resistance to movement) properties. To help researchers use muscle models effectively and analyze the influence of these muscle properties on movement, I developed custom muscle models with certain combinations of properties removed. I also created the first comprehensive, mathematically accurate description of the exact implementation of the Thelen 2003 muscle model in OpenSim. This description is available in the appendix of the article listed below.
See custom muscle models, compatible with OpenSim 1.5.5, here. For the rest of the software framework, see the tagged version 1.5.5 of the OpenSim source code. If you are not a member of the OpenSim project, please request access to the source code from one of the project leads listed on the OpenSim project page.