Calculate contact forces with external objects in OpenSim?

[Ana Diaz]

Hi!

My name is Ana and I am a PhD Student at MIT. Currently, I am working on astronaut injuries caused by the spacesuit. I am considering using OpenSim to model the human-spacesuit interaction and to perform a musculoskeletal analysis of the human inside the spacesuit.

Is it possible to import an object (for example representating of a particular spacesuit model)? Which formats are needed to do that? Is that format exportable from the most common CAD softwares (i.e. CATIA...)?

But most important, is it possible to calculate the contact forces between the musculoskeletal model and the object? I looked at the user guide but I didn't find the answer to my questions.

Thank you very much!
Ana

[Ayman Habib]

Hi Ana,

I'm sure OpenSim can perform a musculoskeletal analysis of the human inside the spacesuit. The interaction between the human and suit though is much more complicated and it depends on how you want to model it and how you can validate the answers (do you have experiments? sensors to measure the forces?) Our muscles are not 3-d objects that can measure contact forces applied to them. I think the most reasonable approach would be to abstract the contact out (mostly) and apply estimated or measured forces from the suit to the OpenSim model.

Please let us know if you have further questions.

All the best,
-Ayman

[Ana Diaz]

Thank you Aymanh,

Yes, definitely we are going to model the spacesuit as a set of external joint torques we have already measured previously in the lab.

In particular, I am working with the left knee right now, and I want to add an external torque to the knee joint. I know how to create the external forces file and also how to select to torque with the "create/edit external force window".

However, I am not sure WHERE exactly the torque is going to be applied. I know I can select the object where the torque is going to be applied to (i.e. tibia_l, or femur_l), but the window does not allow me to select the point of application (you can do it with the forces, but not with the torques)

I am afraid the torque is going to be applied to the center of gravity of each object. Is that true?? If this is the case, how can I applied an external torque to the knee joint (or more exactly, the the point (0,0,0) in the tibia_l reference frame)?

Thank you so much for your help,

Ana

[Michala Cadova]

Hi Ana,
I had an expression that when you define the .mot file with the external force (and torque) vector, you also define the point of application. Maybe I am wrong, but I expected the columns of the “force file” to be as

Force
Name_force_vx
Name_force_vy
Name_force_vz

Point of application
Name_force_px
Name_force_py
Name_force_pz

Torque
Name_torque_x
Name_torque_y
Name_torque_z

Michala

[Ana Diaz]

Hi Michala,

Yes you are right, the columns of the force file are the ones you said. But I have the impression that the point of application is only available for the forces and not for the torques. I think that because when we use the "create/edit external force window", we are able to select only a torque, without specifying any force or point of application.

So, where are the torques exactly applied?

Thanks!
Ana

[Michael Sherman]

Torques are applied to bodies; they do not have a point of application. You may be thinking about the torque produced by a pair of forces -- in that case it is the forces that are applied at points.

Regards,
Sherm

[Ana Diaz]

Thanks Sherm,

You are right, torque are applied to bodies. I guess I am then looking for the center of rotation of the body when a torque is applied to it. I want this point to be the knee joint, and I am not sure if this is what I am doing, when I select the tibia_l as the body of application.

Thanks again!
Ana

[PRUDHVI TEJ CHINIMILLI]

Hello Ana,

I am also working on the similar problem statement of yours applying torque at the left knee joint.

Could you please provide me the solution if you figure out anything related.


Best,
Prudhvi