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Hi all,

I am doing a research related to forces at the ankle joint, trying to get the quasi-stiffness of this joint. There are some cases that we cannot obtain ID just because we don't have external forces. So, the other option is to calculate forces through RRA tool. Using this tool We observe that the kinematics obtained by RRA matched with IK calculation provided by your software. Below, I show the behavior of kinematics obtained from Liu et al., 2008.
However, comparing ID with forces obtained by RRA tool the behavior does not match with ID, which is an expected result of quasi-stiffness of the ankle joint.
To conclude, I cannot trust in the optimization done by RRA and I have tried obtaining quasi-stiffness with other models and the result is the same. So, my question is, Am I doing something wrong? or The forces obtained by RRA are inaccurate by default?

Data taken from Liu, M. Q., Anderson, F. C., Schwartz, M. H., & Delp, S. L. (2008). Muscle contributions to support and progression over a range of walking speeds. Journal of Biomechanics, 41(15), 3243–3252. https://doi.org/10.1016/j.jbiomech.2008.07.031

You mentioned that you checked kinematics, but have you checked other outputs (e.g., residuals)? There might be some useful information in the "Best Practices and Troubleshooting" section on this page: http://simtk-confluence.stanford.edu:80 ... d+with+RRA.

Basically, checking the residuals, these are in the good range, so I think that something is not good because kinematics are well simulated by RRA but dynamics are not.

By the way, I'm reproducing the study the study from Liu, nothing was changed, so I think the process of simulation were doing well.