muscle force orientation?

[Patricia Kramer]

Dear OpenSim gurus,

I want to use joint reaction and muscle forces of the thigh/femur (calculated through the normal OpenSim procedure using the 2392 model) as boundary conditions (i.e., applied loads) in a finite element model of the femur. In order to do that, I need both magnitude and direction of the force vectors. The joint reaction analysis output is set up to report in particular coordinate systems (e.g. ground) so that works perfectly for my purposes, but I have not found a way to report muscle forces with an orientation (other than along their lengths). I can, of course, calculate the orientation of the muscle using output information about its path/origin/insertion, but it would be more convenient to use an existing routine if OpenSim has that capability built in. So:

Does OpenSim provide a method for reporting both the magnitude and the direction of a muscle force?

Or, can OpenSim report muscle forces in the ground coordinate system?

Thank you!

[Dimitar Stanev]

Your approach seems correct. Please note that if you use the results from joint reaction analysis, the muscle forces are implicitly accounted for, so there is not need for you to apply then on the FE model. Furthermore, you can re-express the reaction forces into different frames, e.g. read the convention used:

https://simtk-confluence.stanford.edu/d ... s+Analysis

You can also take a look at the following project, related to the extraction of muscle forces and lines of action:

https://simtk.org/projects/force_direction/