Question about Ligament Forces

[Yidan Xu]

Hi,

I was using the FE model of Generation one to simulate a squatting motion and I do care about the ligament forces with relationship to time. However, I have no idea how we can output the ligament forces of 3D-modelling ligaments. Does anyone know how to do that in FEBio? Any helps, I will appreciate it a lot!


Thanks,
Yidan

[Snehal Chokhandre]

Hi Yidan,

I think you can get reaction forces at the nodes that have a prescribed displacement (e.g. ligament-bone ties) to get the reaction forces at the ligament. Use the following edit in the log file section in the .feb file,

<node_data data="Rx;Ry;Rz"></node_data>

However, this might still be the same as the total reaction force at the rigid body the nodes are tied to. I have emailed one of the FeBio developers Steve Maas asking for guidance. I will update you once I hear from him. You can also reach out to the FeBio team on their website, https://forums.febio.org/

Thanks,
Snehal

[Yidan Xu]

Hi Snehal,

Thanks for your kind reply! I tried to plot the nodal reaction forces at the tied faces of the ligament. However, the force is the total reaction force of the femur/tibia. I hope I could examine each ligament force during simulation. I also posted a question on the FEBio forum and I hope they could provide guidance. Also, I will expect that you receive any suggestions from Dr. Maas. Any updates, please let me know and I will appreciate it a lot!

Thanks,
Yidan

[Snehal Chokhandre]

Hi Yidan,

Sorry about the delayed response. I heard from Steve and it looks like there is no way currently to separate nodal reaction forces from the rigid body forces in FEBio. He is looking into finding a solution to that.

Best,
Snehal

[Yidan Xu]

Hi Snehal,

Thanks for your kind reply! Now, I built four imaginary rigid bodies at the joint center and rigid lock each of them to the tibia (in my simulation, the tibia is moving, while the femur is fixed). Then I tied the tibial nodes of each ligament to each rigid body separately, instead of using "rigid nodes" tied to the tibia. As result, I checked the nodal reaction forces in the plot file and the results for each ligament (ACL, PCL, MCL and LCL) are different. I'm not sure if changing the tie contact in this way will affect the accuracy or not. But it's working well until now.



Best,
Yidan

[Snehal Chokhandre]

H Yidan,
Thank you for updating with the workaround.

Snehal