Ground Reaction Forces

[Jonas Frischknecht]

Hy there.
First I'm very sorry for this question. I saw it was already discussed a lot. But with every sentence I read, I got more and more confused.
So I'm a newbie and want shortly explain what I've already did. Maybe someone find some more mistakes.
My goal: I want on a theoretical way find out, how much muscle force is necesseray to move from this...

stand.JPG (23.52 KiB) Viewed 729 times

into this...

stoop.JPG (26.32 KiB) Viewed 729 times

Position.
Theoretical means, that I don't have any measured data. I've calculated the position of the marker in Matlab and generated a .trc file.

First I placed my markers and scaled the model. After that I could calculate the IK. I also generated the externalLoads.mot file with a constant ground force. So far so good. After that it was possible to run the SO Tool.

Now this is the first point I'm not sure. Is the SO the correct tool for this task or do I have to take another way?
My next problem are the results. They don't look like I expect it. And that's because I've chosen a constant ground reaction force. And during this motion it isn't realy constant. The problem is, that I don't have this _grf.mot file with all the data. Can OpenSim calculate this value or is my task just with measured data possible? Do I realy need it or is there a way to reach my goal without the grf data?

Would be nice if someone got a soloution for my questions..
Thank you..

[Dimitar Stanev]

Theoretical means, that I don't have any measured data. I've calculated the position of the marker in Matlab and generated a .trc file.

It is not necessary to perform IK. You can interpolate the values of the coordinates from the reference pose to the final pose and bypass the IK step because this is a very simple movement.

Is the SO the correct tool for this task or do I have to take another way?

Yes, if you knew the GRF.

They don't look like I expect it. And that's because I've chosen a constant ground reaction force. And during this motion it isn't realy constant. The problem is, that I don't have this _grf.mot file with all the data. Can OpenSim calculate this value or is my task just with measured data possible? Do I realy need it or is there a way to reach my goal without the grf data?

As you correctly pointed out your only problem is the GRF estimation. If the model is not moving its legs, you can try to add a WeldConstraint between the feet and the ground. The WeldConstraint will find the necessary constraint forces that will keep the model from falling, thus simulating the effect of the GRF. Unfortunately, the constraints are not accounted during Inverse Dynamics (used in SO). You will have to solve the problem in a Forward Dynamic manner. Fortunately, you can perform the same analysis by using the CMC tool, which solves for the muscle activations in a Forward Dynamics manner.

https://simtk-confluence.stanford.edu:8 ... %27s+Guide

Best

[Jonas Frischknecht]

Thanks for your response.

It is not necessary to perform IK. You can interpolate the values of the coordinates from the reference pose to the final pose and bypass the IK step because this is a very simple movement.

I'm sure this should be a very simple movement. Unfortunately, I don't get it working. How can I simulate that with interpolation?

As you correctly pointed out your only problem is the GRF estimation. If the model is not moving its legs, you can try to add a WeldConstraint between the feet and the ground. The WeldConstraint will find the necessary constraint forces that will keep the model from falling, thus simulating the effect of the GRF. Unfortunately, the constraints are not accounted during Inverse Dynamics (used in SO). You will have to solve the problem in a Forward Dynamic manner. Fortunately, you can perform the same analysis by using the CMC tool, which solves for the muscle activations in a Forward Dynamics manner.

So I also tried this solution. And I successfully implemented the constraints. For the mot file I run the IK tool. But when I associate the motion data after running the IK they don't fit anymore to my scaled model. That looks like:

offset.JPG (14.39 KiB) Viewed 698 times

Any idea why that happens?

In the CMC tool I use the IK.mot. But I'm confused what kind of information have to be in the xml file for the tracking task? The coordinates and weights from the IK? I tried it with the IKSettings.xml but my model was going crazy with that.

CMC.JPG (77.93 KiB) Viewed 698 times

Thanks for your help

[Dimitar Stanev]

Hi,

'm sure this should be a very simple movement. Unfortunately, I don't get it working. How can I simulate that with interpolation?

If it will not be a bending ok, but if only one coordinate is varying then you can interpolate this coordinate and generate the .mot file yourself. It is simpler because you only define one variables, instead of defining the marker positions. But this is not so important.

So I also tried this solution. And I successfully implemented the constraints. For the mot file I run the IK tool. But when I associate the motion data after running the IK they don't fit anymore to my scaled model. That looks like:

Yes because your model is grounded by the constraints. If your motion file requires that your model is translated then this is impossible. You will have either to transform your motion or to translate the initial position of the model and the point of application of the constraints. Note that constraints would be tricky to get working, so you have to understand their modeling implications. I was thinking of another solution to your problem that is more "relaxed"; that is instead of using constraints, you can add a BuchingForce at each foot and try to find a good set of parameters for the stiffness and damping that work for your problem. These forces will allow for fluctuations of the foot position, making the problem easier to satisfy during the FD face. Nevertheless, this may not work for sure and you will have to give it a try or find something that works for you best.

https://simtk.org/api_docs/opensim/api_ ... Force.html
https://simtk-confluence.stanford.edu:8 ... Sim+Models

Go through the user guide, it will save you a lot of trouble:

https://simtk-confluence.stanford.edu:8 ... d+with+CMC

PS: It may be that SO will also account for the constraints. I am not sure, I will have to look at the implementation. You can give it a try.

Please let us know if you solve your problem.

Best