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I'm unable to import a SIMM musculoskeletal model in OpenSim using the Dynamics engine 'Simbody engine'. The error I get when I try to open the resulting model is: 'only Rot2Planar custom joints are supported'. What is meant by that?

I'm able to import my model using the Dynamics engine 'SIMM kinematics engine'. This engine, however, does not support dynamics.

Can somebody help me to solve this problem?

Hi Friedl,
Thank you for the post. Simbody has the capability of modeling biomechanically realistic joints that don't just consist of simple translations or rotations. A example of such a joint is the knee in which the tibia translates with respect to the femur as a function of knee angle. In OpenSim, we handle this joint in OpenSim with a Rot2Planar joint.

Unfortunately, the Rot2Planar joint is the only "fancy" joint we have in OpenSim at the moment. It takes a rotation and couples two translations to it, like a planar joint. If there are any joints in a SIMM model in which one degree of freedom is a function of another degree of freedom and this cannot be handled with the Rot2Planar joint, we are not able to handle that joint. This is what the message was indicating.

I should mention also that, currently, Simbody does not handle general user-defined constraints. So, this also means that some SIMM models are not going to tanslate directly to a Simbody-based model in OpenSim.

This coming March, Simbody 1.0 will be released, and it will be able to handle general user-defined constraints. In addition, it will have a much improved interface for handling custom joints. The plan is for the next release of OpenSim after the Simbody 1.0 release to expose all these new features of Simbody.

So, by Summer 2008, all SIMM models will be supported as Simbody-based models.

In the meantime, there are two options for getting your model converted to a dynamics-capbable model in OpenSim:

1. You can alter the original SIMM model so that it doesn't have any fancy joints, except perhaps those joints that can be modeled using the Rot2Planar joint. This mean that all joints (excpet the Rot2Planar candidates) are comprised of simple translations and rotations.

2. Use SDFast, as opposed to Simbody, as the basis of your OpenSim model. SDFast currently supports general user-defined constraints, and all SIMM models can be converted to an SDFast-based OpenSim model. The procedure for doing this is a bit lengthy. Next week we will be posting detailed instructions for how to build and SDFast-based OpenSim model.

I hope this helps.

Best regards,
Clay Anderson