Joint Reaction Analysis - Muscles with Wrapping

[Dan Mulla]

Hi everyone,

I've been trying to figure out how OpenSim performs its Joint Reaction Analysis when muscles are wrapped around objects. Specifically, given a muscle force, how does OpenSim determine the XYZ contributions of that muscle force to a joint when using Joint Reaction Analysis tool? This seems pretty straightforward for a muscle represented by a straight line, as the XYZ components of that vector can be easily computed, but I'm interested in what happens when the muscle is wrapped around an object.

I've seen the following plugin to determine the muscle line of action, but from what I can gather, you manually choose the effective origin/insertion points to determine the line of action and thus, the XYZ components, and this is separate from the Joint Reaction Analysis.
https://simtk.org/projects/force_direction

Does the Joint Reaction Analysis tool identify the effective origin and insertion of the muscles when they are wrapped and then calculate its XYZ components? And if so, how does OpenSim do this? I've tried going through the source code on GitHub but can't seem to find how OpenSim goes about through this step, so if anyone could guide me the right way that would be really appreciated.

Thank you!

Dan

[Christopher Dembia]

Here is where OpenSim computes the forces applied by muscles to bodies: https://github.com/opensim-org/opensim- ... h.cpp#L291

These forces are passed to Simbody and Simbody uses these body forces when computing joint reactions.

[Dan Mulla]

Thanks Christopher!

Is there a way through Matlab and the API to output the vector components (Fx, Fy, Fz) of a muscle? Or alternatively, its direction cosines?

Thank you!

Dan