Hello! I'm a new Houdini user studying the Muscle & Tissue demo (the T-Rex one). I wonder if there's a way to deform the core and tissue layers using a custom force from inside? To my understanding, the current tissue pass takes the cached simulation result of muscles and bones to drive the deformation. Instead of doing that, can we supply the tissue with some directional force to explcitly pull part of it inwards?

I also noticed that on this tutorial page [www.sidefx.com] it says:

(Optional) You can supply custom poly lines for the core’s falloff with the Input 4 on the Tissue Solidify SOP node. These lines determine from where the sucking in force emanates to pull the tissue in towards the muscles and bones.

I wonder if these poly lines can be useful to achieve the effect mentioned above?

Any information will be appreciated!

hi, and welcome!
In H19.0 the core is treated simply as a means of attachment -in two senses. The core attaches to your input animation (muscles and bones), and, your tissue attaches to the core. Beyond the simple attachment constraints, the core doesn't really contribute in any dynamic way.
In regards to "inward" forces, there are a couple acting on the tissue layers:
-The core and tissue solids respond to the Shrinkage parameters on the Tissue Properties node and affect the Rest Scale of the tets. ie, shrinking the rest scale will effectively pull the tissue surface inward.
-The outer surface points on the tissue have an attachment rest scale (attaching to muscles and bones) that can also be adjusted to pull the outer surface inward. This constraint can offer the most when it comes to producing muscle definition.

The poly lines you referred to are used to generate the weight value that the core-to-animated_geometry attachment relies on. ie, core points closer to the reference lines will have a stronger attachment weight. There are Distance and Falloff parameters to control the weights on the Tissue Solidify Node.
What the quoted excerpt from the docs implies is that when you shrink the core rest scale, the points with a stiffer attachment weight will be constrained more rigidly, and the weaker weighted points will have more freedom to move and respond to the shrinking effect.