How to make sit-to-stand motion considering ground reaction force

[masahiro yokota]

Hi,everyone!! I'm very new to OpenSim and I'd like to solve basic problem.

I'd like to know how to add reaction force from the ground or how to fix feet to the ground.

My goal is to observe muscle activities when a human perform sit-to-stand motion from a chair.
I'll use the "FullBodyModel" and make the motion file(.mot)from the motion performed in exeperiments.

Now, I only have joint angle information for ".mot file" and I'm afraid that my model cannot stand exactly
because of the lack of the ground reaction force.

Please give me some basic idea to enable me to try basic analysis.
Thank you.

[Thomas Uchida]

I'd like to know how to add reaction force from the ground or how to fix feet to the ground.

Assuming the feet aren't moving, I think the best strategy would be to weld them to the ground using WeldConstraints. Since you are "very new" to OpenSim, I suggest adding the constraints by editing the .osim file in a text editor. You can find the syntax for the WeldConstraint component by opening the GUI and selecting XML Browser from the Help menu:

Code: Select all

<WeldConstraint>
 <!--Flag indicating whether the constraint is disabled or not. Disabled means that the constraint is not active in subsequent dynamics realization-->
 <isDisabled>false</isDisabled>
 <!--Specify first of two bodies welded together by the constraint.-->
 <body_1></body_1>
 <!--Specify second of two bodies welded together by the constraint.-->
 <body_2></body_2>
 <!--Location of the weld in first body specified in body1 reference frame.-->
 <location_body_1>0 0 0</location_body_1>
 <!--Orientation of the weld axes on body1 specified in body1's reference frame.Euler XYZ body-fixed rotation angles are used to express the orientation.-->
 <orientation_body_1>0 0 0</orientation_body_1>
 <!--Location of the weld in second body specified in body2 reference frame.-->
 <location_body_2>0 0 0</location_body_2>
 <!--Orientation of the weld axes on body2 specified in body2's reference frame.Euler XYZ body-fixed rotation angles are used to express the orientation.-->
 <orientation_body_2>0 0 0</orientation_body_2>
 </WeldConstraint>

You may be able to use the model from this course project: https://simtk-confluence.stanford.edu:8 ... ng+Balance (see their project page on simtk.org here: https://simtk.org/frs/?group_id=867). The Confluence documentation may be useful as well (e.g., the "OpenSim Models" page, https://simtk-confluence.stanford.edu:8 ... Sim+Models).

[masahiro yokota]

Tom,

Thanks for your reply.
I could weld feet to the ground using Weld Constraints.
I'm wondering if I could weld feet to the ground, could ground reaction force be applied to the model?
Do I need to add other force to the model?
And I'd like to know how to add reaction force from the chair.
Now, I'm trying to use <Contact Geometry Set>. Is it possible to add reaction force from the chair using this?

Thank you.

[Thomas Uchida]

I'm wondering if I could weld feet to the ground, could ground reaction force be applied to the model? Do I need to add other force to the model?

The WeldConstraint will compute the forces (Lagrange multipliers) necessary to keep the feet on the ground; you do not need to add another force to the model. This strategy is a modeling simplification, of course; you probably want to confirm that the corresponding Lagrange multipliers are applying upward forces only.

[masahiro yokota]

Thank you very much,Tom.

I'll use the WeldConstraint to weld the feet to the ground.
I'm sorry but I can't figure out how to add reaction force from the chair.
I know that I can't use WeldConstraint when I simulate sit to stand. Because when people stand, their hip cannot keep in touch with the chair.
Could you give me some ideas?

Masahiro

[Thomas Uchida]

You might try starting your simulations just after the subject has lifted off the chair. If you need to start the simulation before that point, I suggest measuring these contact forces experimentally. You could try adding contact elements, but may have trouble finding good contact parameters, etc.

[masahiro yokota]

Hello, Tom.

After that, I tried to simulate using "WeldConstraint" to consider ground reaction force.

I could weld feet to ground. But when I run SO, this weld model outputs error.
If I didn't weld the feet using "weldconstraint", SO could run without problem. So, I think the factor of this error makes by "weldconstraint".

Could you tell me if I should take care of things when I use "WeldConstraint"?

Thank you very much.

[Thomas Uchida]

Please see topic 8475: viewtopicPhpbb.php?f=91&t=8475

[masahiro yokota]

Tom,

Thank you for your reply.

Is it mean that "WeldConstraint" should not use when I simulate SO?
If I'd like to use this, should I use CMC?
I'd like to use "WeldConstraint" if it is possible because I don't have ground reaction force.

Thank you very much.