coordinate actuators in static optimization toolbox

[Tom Augenstein]

In the documentation for the for the static optimization toolbox, it is recommended that if your model is too weak, that coordinate actuators should be appended to get an idea of "how much 'reserve' actuation is required at a given joint" over the course of some motion and to then strengthen the muscles in your model accordingly.

I'm having a hard time understanding how this is helpful for interpreting the toolbox output. If you increase muscle strength on a physiologically-accurate model to match the reserve actuation, aren't you nullifying the static optimization output because the muscle strength is no longer accurate?

[Thomas Uchida]

If you increase muscle strength on a physiologically-accurate model to match the reserve actuation, aren't you nullifying the static optimization output because the muscle strength is no longer accurate?

The model may not be physiologically accurate; it may be necessary to adjust a muscle's path, maximum isometric force, tendon slack length, etc., depending on the subject. There's some relevant discussion in Section 3 of this paper: Hicks, J.L., Uchida, T.K., Seth, A., Rajagopal, A., Delp, S.L. Is my model good enough? Best practices for verification and validation of musculoskeletal models and simulations of movement. ASME Journal of Biomechanical Engineering 137(2):020905, 2015. http://nmbl.stanford.edu/publications/pdf/Hicks2015.pdf

[Tom Augenstein]

Ok, thank you for the link.

However, the model I'm using I downloaded from the opensim website (https://simtk.org/projects/upexdyn). Should I question whether or not this model is accurate even though it is a very popular model (nearly 3000 downloads) that I retrieved from the Opensim website?

[Thomas Uchida]

Should I question whether or not this model is accurate even though it is a very popular model (nearly 3000 downloads) that I retrieved from the Opensim website?

The authors would have built the model according to the data they obtained from one or more individuals and it may be very accurate for those data. The subject for whom you are generating a simulation will certainly be different in some respects, the movement may go beyond the range of motion for which the model was validated, etc. Whether the model is appropriate for your research and whether any modifications are necessary will depend on many factors. You should be familiar with how the model was built and validated, as well as the model's limitations. You may want to perform your own validation as well.

[Tom Augenstein]

Ok, that makes sense. Thank you for the help