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I aimed to enhance the degree of freedom in the knee joint of Rajagopal Model, which currently has only one degree of freedom.
If I were to modify your model to increase the degree of freedom, would that be acceptable, or would it be considered inappropriate without ligaments?

Hi, it depends on your research question. You can find a model with knee ligaments and more knee DoF here: https://simtk.org/projects/opensim-jam

But generally, it might not be a good idea. Because the other knee DoFs cannot be measured accurately using skin mounted markers [1] and it leads to inaccurate and unreliable muscles kinematics, abnormal muscles activity/force, and hence, knee joint contact force. Please see this:
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viewtopicPhpbb.php?f=91&t=12919&p=0&sta ... eec78cd468

[1] Benoit et al. (2006). Effect of skin movement artifact on knee kinematics during gait and cutting motions measured in vivo. https://doi.org/10.1016/j.gaitpost.2005.04.012

Dear Mohammadreza Rezaie,
Thanks
Prof. Bogert's response was convincing. What if I include knee ligaments in the Rajagopal model?
Example : https://simtk.org/projects/kneemodel/

Kind Regards,
Rohan