Hi,
I have been trying to research and learn how to create a muscle system in Houdini.
I looked at the tutorials and scenes ( like the dinosaur setup ).

I have found a weird behavior. I'm trying to make one tissue stiffer than the other but no matter how I change Shape Stiffness the behavior is the same. I have tried multiple scenes and different Houdini versions. It was the same.
Then I tried to use a regular vellum solver and changing stiffness works well.
To me, it looks like there is something wrong with the muscle solver or maybe I'm missing something.

I would appreciate some help here.
Thank you.

Hi
Please post a hip file so I can take a closer look at what you're attempting to do.
thanks!

Hi John,

You can use the dinosaur file from here https://www.sidefx.com/contentlibrary/muscle-and-tissue/ [www.sidefx.com]

If you change the stiffness of the muscles it will not affect the simulation results

That example file uses some significant Fiber Strength, and the muscles have a boosted lower end to the Fiber Scale range. (meaning that at their minimum muscletension, they still have fiber stiffness applied.) Fiber stiffness can make the muscles stiffer. Once you reach a high stiffness value, there may not be much visible difference. So maybe that's what you're seeing?

Hi, I tried to disable fiber stiffness the results of low and high muscle stiffness still match
Here is a simple example to show what I mean. There are 2 outputs for each muscle solver and regular vellum solver. For muscle solver shape stiffness is 10 and 1e+07 and results are the same. For the vellum solver same values ( as for the muscle solver ) for stiffness but the results are different.

Thanks for the example SEGAS. The key difference is in your regular Vellum setup, you are using the stiffness parameters without a Scale by modifier. In the MuscleSolver, we use attributes to scale constraint stiffness values.
The Scale By factoring is not a linear scale.
If you look at a geometry spreadsheet for the Constraint Primitives, and watch what happens to the stiffness attribute for tetrahedral stretch, (Scale By Value behaves just like Scale By Attribute):
Stiffness: 1 x 1000

No Scaling yields a stiffness of 1000.0
Scale By Value: 1.0 yields stiffness = 1000.0
Scale By Value: 1.5 yields stiffness = 31669.0
Scale By Value: 2.0 yields stiffness = 1e6
Scale By Value: 10.0 yields stiffness = 1e30
Scale By Value: 100.0 yields stiffness = 1e37

So getting back to your original question, try using much lower stiffness values to see a difference. Once you start getting into higher Muscle Properties Stiffness values, the constraint stiffness starts to max out.

Hey John,
Thanks for looking into that

Just to follow up

What would I need to do to make a tissue stiffer? E.g. almost rigid like.

I'm trying to find an alternative to Ziva and in our setups, we had to make some tissues much stiffer than others. How would I do that in Houdini?

If you want something really rigid... while it's not directly supported in the muscle solver, you could easily crack the HDA open and add a Vellum ShapeMatch constraint yourself.

Thank you!