ligament models

[Glen Lichtwark]

The ligament models are very sensitive to the resting length, and this needs to be scaled along with the origin and insertion points. If you take your model through the range of motion at the joint of interest and calculate the tension in the ligaments, then be careful to make sure this tension develops at the correct joint angle as per what you expect from literature or otherwise.

In inverse dynamics, the ligaments forces are calculated and included in the analysis, unless you have specified not to use these in the calculations (as is the case with Muscles - thanks Ayman for pointing this out to me). If your inverse dynamics solutions are not what you expect when including the ligaments, then it is likely that the ligaments are taking up tension at the wrong position and therefore you have large joint moments when you should not - this could possibly make it almost impossible to find a static optimisation solution.

I hope this helps.

[Luca Modenese]

Just as a follow up question to Glen's initial post, I was wondering if it is actually confirmed that

The ligament paths are scaled with the model.

I am asking because I am trying to scale a model including ligaments, but the GeometryPath of the ligaments does not seem be updated according to the scaling ratios. Thank in advance for any help!

Luca

[jimmy d]

Hi Luca and others,

The scaletool treats ligaments as a force and not as a path actuator, so the geometry doesn't get scaled. This is a bug in the scaletool and will be addressed in future releases.

In the meantime, a workaround is to treat the ligaments as muscles and then after scaling, convert them back into ligaments. This will ensure the geometry paths are scaled appropriately.

Cheers,
-james

[Michele Baldoni]

Hi all,
I see this is an old post but I'll give it a try...

I am currently trying to add some ligaments to the lumbar spine model, but I got really confused by the ligaments properties definition. My idea was to modify the .osim file of the lumbar spine with an .xml editor (Notepad++), adding the ligaments block but copying and pasting ligaments definition from the model with knee ligaments by Hang Xu. All the ligaments in this model have the same force_length_curve definition, and they differ in the value of pcsa_force. My questions are:

1) The force_length_curve starts increasing at 0, but I read this sentence in a post by Glen:

Hence the tension should start to raise at a value of 1, not 0 as is used in the examples and as the standard values for the ligament class

Is the curve wrong then?

2) In the same post by Glen I read:

Note that the peak value in the standard function provided reaches a value near 1 at an x-value of 1 (which is essentially 100%). So to utilise this PCSA value with some physiological meaning you should essentially calculate the failure strain (lets say 20% or a x-value of 1.2), then find the y-value of the function at this value and then divide the maximum failure force by this value. E.g. if failure strain is 20% and failure strength/force is 400, the y-value is 0.175 at 1.2, therefore a PCSA value of 400/0.175 = 2285 is reasonable. This will then produce a value of 400 at 20% strain.

Where does the value of 0.175 come from? Where do I find this function that gives 0.175 at 1.2?

3) For example, if I had a ligament with a stiffness of 39000 N m^-1 and failure strain of 32%, how do I define the force_length_curve and how do I calculate the pcsa_force?

Many thanks!

Michele