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Hi everyone,

I am trying to evaluate joint reaction forces from muscle forces (not estimated in Opensim). In order to do that, I want to access the direction of the muscle force at each state.
I found the function addInEquivalentForces, but I am not sure if it is the way to go, nor how to implement it.
Additionally, in which frame will the forces be expressed?

Thank you in advance for your help !

Najoua

The "OpenSim plugin to extract the muscle lines of action" SimTK project may be helpful: https://simtk.org/projects/force_direction/.

Hi,

Thank you for your reply.
I have already checked this plugin, but it's only accurate when there are no wrapping objects, however, I have some on my model, so I can't use it.
That's why I checked the functions Opensim uses under JointReaction analysis, and found the addInEquivalentForces, but I am not sure about it.

At this point, I realize the state up to the positions level, I equilibrate the muscles. For each muscle, I call the path and apply "addInEquivalentForces(state, Tension, Body_forces, gene_forces). Where Tension includes the muscle forces I have obtained from the previous step. Body forces is a vector of spatial vectors (set to 0), and gene_forces is a vector set to zero.

It seems redundant to me to feed the generalized forces from the state, since they were already included in the estimation of the muscle forces, but I am not sure. I appreciate if you could give some clarifications and comments.

Regards,

Najoua

You should be able to access each Muscle's PathPointSet via GeometryPath (https://simtk.org/api_docs/opensim/api_ ... yPath.html).

Hi,

Thank you for your reply, but I am afraid I don't see why I would need the PathPointSet.

I don't see why I would need the PathPointSet

I believe the PathPointSet is where you can find the coordinates of the points that define the path of the muscle. The first two points and last two points could be used to compute the direction in which the muscle force is applied to each body.

Hi,

Sorry, I am afraid I haven't been clear enough. Since my last mail, I have already succeeded in implementing the AddInEquivalentForces. I was just wondering if the forces are expressed in the local body frame or in the ground frame.

Thanks for your help.

Najoua