simtk.org

Hi,
i currently have some troubles with the editing of a model and was hoping that someone could help me out. I am trying to perform some inverse dynamic calculations on the wrist model that comes with OpenSim (https://simtk.org/home/wrist-model). Unfortunately only the thumb and the index finger have joints, so i needed to create the ones for the other fingers myself. The model is decent now but i had some difficulties with the positioning of the joints.

1) Is there an easier way to edit the position of the joints than to open the .osim file in an text editor, edit the position values, save the model and open the model in OpenSim to see your changes? I had no reference points inside the .osim file, so i didn't really know if i needed to move the joint e.g. 0.001 or 0.01 units. This made editing the joints rather difficult.
2) How can i display/calculate the length of a body (in m or cm)?

Furthermore, the model is missing all mass- and inertia tensor values and i was wondering how others got these values.
3) Is there a way to compute the inertia tensor values from the .vtp files?
4) Is there a way to change the size of the center of mass indicator in OpenSim? Currently it is bigger than the size of the finger phalanges so it is hard to position it correctly.
Which leads to question 5): Does anybody know, by any chance, where i can find values for the mass, center of mass and/or inertia tensors of the finger phalanges? I am currently using values based on the total mass of the hand.

Thanks in advance!

Regards,
Yannik

Hi Yannik,

There are many OpenSim models on simtk.org (including the wrist model you mentioned) , you should consult their documentation as to what they're good for. If the model you're referring to doesn't have mass properties I'd take it that it hasn't been used for dynamics, so I suggest you contact the model authors to see if they have a dynamic model or maybe other users on the forum who are developing wrist models can pitch in to collaborate with you and/or point you to where to get the mass properties.

As to the remaining issues, I'd suggest you try the beta version of OpenSim 3.0 which will be released soon (already available on the download page/release betas), as it supports live editing of various objects so it should help you greatly on item 1) on your list. You can also sign up to be a beta tester by going here:
https://simtk.org/mailman/listinfo/opensim-testers and give us feedback.

For 2) You can create markers and move them around by dragging in the GUI then you can calculate the length from the locations of the markers.

For 3) vtp files are used for visualization only as of now, they come from low res scans, you'll need to use another tool to compute mass properties if only based on vtp files. Normally segment masses would account for other none-bone tissues (muscles, ligaments etc.) as well so using a percentage of the mass of the whole hand may be a better approach.

Hope this helps and best regards,
-Ayman

I tried out the OpenSim 3.0 Beta and it made the placement of the mass centers and the general editing easier. Thanks for that!

Regards,
Yannik