Ligament Class - Trouble Scaling

Shaun Fickling · Post by **Shaun Fickling** » Wed Apr 03, 2013 8:30 am

Hi

I've edited the gait2392 model to include an additional 2DOF (varus/valgus and int/ext rotation) at the right knee as well as the Anterior Cruciate Ligament using the new Ligament class. I've manually (i.e. by inspection on the GUI) found the Pathpoint set for the ligament, which is as follows:

<PathPointSet name="">
<objects>
<PathPoint name="ACL_a-P1">
<location> -0.008 -0.405 0.008</location>
<body>femur_r</body>
</PathPoint>
<PathPoint name="ACL_a-P2">
<location> 0.02 -0.035 -0.007</location>
<body>tibia_r</body>
</PathPoint>
</objects>
<groups/>
</PathPointSet>

However, when I scale the model, the ligament attachment on the femur ends up far away from where it should be. I think OpenSim is keeping the same values that I specified in the PathPoint and not scaling their insertion points accordingly as it does for the muscles. Should I be adding the ligament to the Scale Tasks?

In addition, once I've sorted this out, I'd like to be able to view the change in length of the ligament during gait. It doesn't look like this can be done in the Plotter Tool (it only seems to be able to do this for muscles). Is this one of the analysis functions that can be implemented with Forward Dynamics Tool?

Kind Regards,
Shaun Fickling

Ayman Habib · Post by **Ayman Habib** » Thu Apr 04, 2013 9:58 am

Hi Shaun,

You're right that the Ligament class does not appear to scale its attachment points (only PathActuators do). Please file a bug report and we'll try to fix it in the next release, in the meantime you can output the scale factors for different bodies from the ScaleTool and then apply them manually to the PathPoints on the Ligament (or as a temporary workaround, change to use a muscle before scaling then change it back to Ligament afterwards). There are no tasks that I'm aware of that would scale the Ligament.

For reporting length, you'd need to write your own analysis to report the length. Alternatively you can use the PointKinematics analysis to report the trajectory of the points on the ligament and then compute the length outside of OpenSim.

Thanks for the find and best regards,
-Ayman

Shaun Fickling · Post by **Shaun Fickling** » Mon Apr 08, 2013 1:27 am

Hi Ayman

Thanks for the advice, I've got it to work now. I'll be sure to file a bug report.

Kind Regards,
Shaun

Ligament Class - Trouble Scaling

Ligament Class - Trouble Scaling

Re: Ligament Class - Trouble Scaling

Re: Ligament Class - Trouble Scaling