Ligament Parameters Question

[Hojin Song]

Hi OpenSim experts!

I was trying to add knee ligament component, but quite not sure what are the ligament parameters exactly are.
I'm referring to the ligament parameter as shown in the image below (from the study of Blankevoort et al. https://www.sciencedirect.com/science/a ... 909190019J).

blakevoort.PNG (40.87 KiB) Viewed 55769 times

I see that for ligament component, we can set 'resting_length', 'pcsa_force', and 'force_length_curve'. Among these parameters, I could've make the 'force_length_curve' with reference to the curve shown in the same study (Blankevoort's) and I could also calculate the 'resting_lengths' for ligament bundles using initial strains. However, I'm not so sure what the 'pcsa_force' exactly is. My question is, is 'pcsa_force' corresponds to the stiffness that shown in the chart above? I've found several other studies for ligament parameters and they all gives stiffnesses and initial strain values, so I was wondering if I can use those stiffness values as 'pcsa_force' in opensim ligament component.

Thank you.

Best,
Hojin

[Thomas Uchida]

The "pcsa_force" parameter scales the normalized force obtained from the force-length curve. The Ligament::computeForce() method can be found here: https://github.com/opensim-org/opensim- ... t.cpp#L226. On lines 242-243, the tension is calculated by finding the normalized force that corresponds to the current normalized length (path length divided by resting length) and then multiplying by the "pcsa_force" parameter.