Manually Modifying a jnt file

[Gregory Philips]

I would like to modify a jnt file. Specifically I would like to manually enter the length of the bones, the moments of inertia and the masses.

I have found where to input the mass. However if I want to define the center of mass as a certain distance away from one end of the bone where to I make the change? In the joints section?

And similarly where do you modify the lengths of the bones?

[Gregory Philips]

I want to manually enter these values as they change from subject to subject. And since there is no simple way to scale between subjects it makes sense to manually alter the length of the bones, the masses and center of mass.

[Ayman Habib]

Hi Gregory,

jnt files are SIMM's (not OpenSim) so I'd suggest you contact SIMM's customer support for questions regarding them.

As to OpenSim, you can still use the scaling tool to scale segments without using markers if you use the Manual scaling option then you can type in how much you want to scale each segment and OpenSim will take care of scaling the mass properties, com and muscle attachment points etc. for you. Please consult the user guide or let me know if you need help following this route.

Good luck,
-Ayman

[Gregory Philips]

Thank you Ayman. I will do that. And for the remaining changes it is simpler to do it in the .osim file.

I had a question relating to the axes of individual components. I have set their visibility to on so that I can see them. I just wanted to know if the axes in the same direction are the same colour. I mean if the model has x in red, y in green and z in blue, will the individual components' axes also be of the same colour.

[Ayman Habib]

Hi Gregory,

The colors for body/joint axes are:
yellow for Y, red for X and green for Z

We should be more consistent since these are not the same colors we use for World Coordinate system (RGB for XYZ).

Sorry about the confusion,
-Ayman

[Gregory Philips]

Thank you Ayman for clearing that up.

[Gregory Philips]

I am trying to use the manual scaling. However in order to scale the model either up or down, I need to know what the initial values were. And the paper that Dr. Kate Saul has provided only provides length properties for the muscles.

[Gregory Philips]

I tried creating markers of my own. However when I try to manually input the length between the markers (I have them calculates), it will not permit me and says that the values are not found on file.

[Ayman Habib]

Hi Gregory,

If you want to use manual scaling, you select the bodies on the "Scale Factors" page of the Scale Tool, and enter the scale factors you computed. You will not be prompted for anything to do with markers (just make sure you don't select to Adjust Model Markers on the settings page of the scale tool).

Hope this helps,
-Ayman