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Hi all,
I apologize if this is an obvious question, I have been primarily using OpenSim to analyze prescribed movements and I am now testing the waters of predictive simulations.

I am trying to convert my workflow from OpenSim using the GUI to using OpenSim MoCo. I wanted to test the model I was using in the GUI to see if it works prior to trying my hand at MoCo. The model I am using is from the following paper by Miller and Russell Esposito (2021): https://peerj.com/articles/11960/. This model is based on the model published by Rajogopal et al, with two major differences to ensure that it is compatible with MoCo: changing the wrapping objects to conditional path points and replacing the "Millard2012" muscles with "DeGrooteFregly" muscles.

I have run both models through my GUI workflow, with the Rajogopal model completing CMC without issue. The Miller model immediately crashes CMC without even an error message. I have looked at my CMC task file and my reserve actuators to ensure that they have names that match the appropriate coordinates and whatnot, but that does not seem to be the issue. My hypothesis is that CMC does not play nice with this muscle type. I was wondering if anyone can confirm that's the issue. I was wondering if doing what I am doing is even possible with this type of model. Thanks in advance!

Dan

There's been some discussion about this on the Moco forum (see here) and CMC sometimes works with DGF muscles, but not always. It depends on the muscles and the motion in question.

When it crashes, I'm pretty sure the crash is caused by a failure of the muscle to equilibrate at the beginning of the simulation. Whether or not the muscles successfully equilibrate I think depends on the initial length of the muscle relative to its optimal fiber length. I've been meaning to put together a testbed model with just one muscle pulling a block to see if I can tease apart what initial conditions cause CMC with DGF muscles to fail, but I haven't gotten around to it yet.

Thank so much for your reply and pointing me in the direction of the Moco forum. After looking over that thread, it appears to be the similar issues that I am facing. Thanks so much for also taking the time and effort looking into this!

Dan