One extra external load

[Zhen Yang]

Hi,

I want to simulate CP children (with GMFCS V, gross motor function classification system, level five) standing in a standing shell. So I would have external loads at the ground as usual, but then I will have 2 additional forces at the inside of the thigns, since the CP children could not stand by themselves.

So my question is, is it possible to add two external loads at the inside of the thigns? If so, how should I do it?

Thanks,
Zhen

[Stephen Riutta]

Hi Zhen,

Depending on the force data you have, you can apply either a point or body force to the model. The body force will be applied to the COM of the body you specify. There is more information on both of these approaches on the following page:
http://simtk-confluence.stanford.edu:80 ... amics+Tool
Scroll down to the "External forces" section.

Best,

Stephen

[Stephen Riutta]

Hi Zhen,

Regarding your question on adding the force to the femur. You can modify your ground reaction force .mot file to include other forces. The important thing is to follow the correct format. Forces columns in the x, y, and z directions should be in the following format: name_vx, name_vy, and name_vz, where "name" anything you choose.

Go to "External Loads Specification" on the http://simtk-confluence.stanford.edu:80 ... amics+Tool page to see an example of how to apply those forces to the femur.

Hope that helps,

Stephen

[Zhen Yang]

Hi Stephen,

thanks for your reply!
I have tried to modify force file, and I have difficulty defining the point position which the extra force will apply, I am thinking using COM of the femur. But I don't know how to compute it.

Can you help me?

Thanks,
Zhen

[Stephen Riutta]

Hi Zhen,

If you want to apply a force to the COM of the femur, there are two options that I'm aware of. If anyone else has a better idea on how to do this, feel free to pass that on.

1. Use the Analyze/Body Kinematics tool to compute the COM positions of all the segments. Add the COM positions of the femur to your GRF force file. Then choose the "point force" option in the external forces box.

2. If the origin of the body frame is located at the COM of the femur, you can apply a body force to the femur. (The body forces are applied to the origin of the body frame). If the origin of the body frame is not located at the COM of the body, you can add a small, massless body and use a weld joint to position it at the COM of the femur. Apply a body force to this body.

Best,

Stephen

[Zhen Yang]

Hi Stephen!
Thanks for quick reply! I really appreciate your answer!

I have one question: I use Analyze/PointKinematics to compute the COM of the femur, as in the PointKinematicsExample online. Was I wrong?

I suppose now that in the example, the pendulum is considered as a point?
And in my case, femurs are body segment, and I should use BodyKinematics instead as you suggested?

Best regards,
Zhen

[Zhen Yang]

Hi Stephen!

Another question regarding BodyKinematics: What do I need for files to run this analyze?

In PointKinematics example, I use only result from inverse kinematics, .mot file, without any actuators or external loads, is this correct even for BodyKinematics?

Thanks,
Zhen

[Stephen Riutta]

Hi Zhen,

In the output file from BodyKinematics, it states:

This file contains the kinematics (positions and orientations, velocities and angular velocities, or accelerations and angular accelerations) of the centers of mass of the body segments in model workerModel.

You could place a point at the COM of the femur with PointKinematics, but I think BodyKinematics would be easier.

You need your model, and the IK solution to run this.

Hope that helps,

Stephen

[Zhen Yang]

Hi Stephen,

thanks for your help!

I have another question regarding CMC.

CMC changes kinematics in solution file states.sto, but when I check solution file Kinematics_q.sto the values follow the experimental motion. Why does this difference exist? And what should I do to make states.sto to follow the experimental kinematics?

Regards,
Zhen

[Stephen Riutta]

Hi Zhen,

RRA may change the kinematics to be more dynamically consistent with the ground reaction forces. If you're using the RRA kinematics in CMC, that may explain why there is a difference. If you're using the original kinematics file, I would suggest increasing the tracking weights in your CMC setup file. You may also need to increase the strength of the muscles in the model.

There is a lot more information on how RRA changes the kinematics, and how CMC tracks the kinematics in the documentation. http://simtk-confluence.stanford.edu:80 ... d+with+RRA

Best,

Stephen