Lixin-
The properties of any model are defined (generally) from experimental data and used to build the model. If you read the published papers associated with the models, it will give you information on how properties (like mass, etc) got defined.
The different names (thelen, Millard) are the different muscle models that can be implemented in OpenSim.
For all of these questions you should have a read of the confluence documentation on 'Theory and Publication', which also includes a description of muscle models;
http://simtk-confluence.stanford.edu:80 ... blications
Gluck,
-james