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Hi!

I have a question about the implementation of the Thelen2003 muscle model. I encountered three documents about this topic.

The first is the respective article from Thelen called Adjustment of Muscle Mechanics Model Parameters to Simulate Dynamic Contractions in Older Adults

The second is the article from Millard from 2013 called Flexing Computational Muscle: Modeling and Simulation of Musculotendon Dynamics

The third is a document from Chand T. John called Complete Description of the Thelen2003Muscle Model


I feel that these three documents given different informations, especially due to the material properties of the muscle - tendon unit.

Is the Thelen2003 muscle an Equilibrium Musculotendon Model or a Rigid-Tendon Musculotendon Model?

Are the expressions presented in the document from Chand T. John accurate?

Which one should I follow to describe such implementation?

Thank you in advance,

Rodrigo Mateus

Is the Thelen2003 muscle an Equilibrium Musculotendon Model or a Rigid-Tendon Musculotendon Model?

The tendon is compliant and in equilibrium with the fiber. Please see the description here: https://simtk.org/api_docs/opensim/api_ ... ml#details. Also see the Properties associated with the Thelen2003Muscle class, one of which is tendon strain at peak isometric force.

Yes this goes along with what I was thinking.

And what about the expressions on the document of Chand T. John, are they accurate and really the expressions used in the model? Or should I use the expressions given in Thelen 2003, with the corrections provided in Millard 2013?

Thank you again!

Rodrigo Mateus

what about the expressions on the document of Chand T. John, are they accurate and really the expressions used in the model?

The document to which you're referring is outdated (though it may still describe the Thelen2003Muscle_Deprecated model). The Thelen2003Muscle model was updated in OpenSim version 3.0 (see the release notes here: https://simtk-confluence.stanford.edu/d ... scleModels). I recommend referring to the Millard 2013 paper, the doxygen and Confluence documentation, and the code itself. Here are some links:
- paper: http://nmbl.stanford.edu/publications/p ... rd2013.pdf
- doxygen: https://simtk.org/api_docs/opensim/api_ ... ml#details
- Confluence documentation: https://simtk-confluence.stanford.edu/d ... scle+Model
- Thelen2003Muscle.cpp: https://github.com/opensim-org/opensim- ... Muscle.cpp

Thank you so much this will be very helpful for my dissertation!!