Thank you for your answer, where can I find the code of the functions getLength(s) and getPreScaleLength(s) ?

That answers my question, thank you !
However, now I would like to know how upd_GeometryPath() works (mostly where it is) and how the path.getLength(s) and path.getPreScaleLength(s) calculate the muscle length ?

Hello, In the OpenSim documentation in "How Scaling Works" https://simtk-confluence.stanford.edu:8443/display/OpenSim/How+Scaling+Works , it is mentioned that "...muscle's new optimal fiber length and tendon slack length are computed during the scaling process. A scale factor is computed as the rati...