simtk.org

Hello,

I would like to get the force and moments acting on a body due to muscles. I know you can get muscle moments acting at generalized coordinates and muscle forces using an analysis, but I'm not sure how to break down this force into x,y,and z components. Also, I'm not sure how to read the moments. If I have a kinematically constrained knee that is dependent on knee flexion angle, I can get knee flexion moment due to muscles. What about moments in the other 2 directions?

Thanks,

Anne

Ok, so here is a proposed solution I have found.

1. Use BodyKinematics analysis to determine the location and orientation of the body COM (in global coordinates).

2. From this, you can determine where the body origin is in global.

3. You know where the muscle points are in the reference frame so you now know where they are in the global (transformation).

4. Knowing the muscle points in space, you can calculate the muscle line of action.

Hi Anne,

I think I sat behind you in the recent OpenSim workshop (not sure as I am bad with names). But I was trying to get this same information for use with FE models. Your process flow is pretty much what I came up. The problem was that if you wanted all the forces acting on a segment, other forces (besides muscles and joints) may be included in that set, and those forces were not as readily available. I submitted a request to have a function written that given a body segment, would return all the forces acting on that segment.

In conversations with Ayman and Matt Demers, we all agreed that this type of function would be beneficial to many users (because then you could write an Analysis plugin that calls this at every time step and exports the data in any format you want). Matt said he would need a function like this for some of his upcoming research. So depending on if you can wait (hopefully for the next release of OpenSim), this functionality might be pre-packaged.

Maybe Matt or Ayman can comment on if this is going to make it into the next release.

Best,
David Wagner

Hi David,

Yup. That was me in front of you. I agree it would be nice to have an analysis plugin that calculate the resultant moments and forces (due to muscles) acting on each body center of mass expressed in the global reference frame.

What do you think Ayman? or any other OpenSim member?

Thanks,

Anne

Hi,

Have you been able to get any response for the plug in?

Thanks,

Daniel

No response for a plugin yet. I have some self written Matlab code that I may try to implement this semester.

The other two moments are joint reaction forces. You can use the joint reaction analysis to get them.

You can use a PointKinematics analysis to get the muscle point locations in the ground frame to compute the direction of force application.

Is the joint reaction analysis available with the current version of OpenSIM (1.8.1)... I could not find the option in the Analyze Tool....

Hey Ananth,

You have to download version 1.9 and look for it in the Analyze tool

Yup, got it.... Thanks Dan!!