Hey there --
Thanks for posting, great effort learning stuff on your own.
A common temptation when on the Ziva learning curve is to make things stiffer to solve problems 🙂
With muscle sims, if the solver scale is set correctly, it's pretty rare that you'll want to play too much with the stiffness of muscle materials, this is because the defaults are set to be pretty close to the real world equivalents.
Here's what I'd do if I was looking at your scene:
Reset all muscle materials to default.
Double check solver scale is correct for your character. (a pitbull would probably be, what, less than a meter high)
Look at every attachment you've made. Perhaps it would be best to delete all attachments and start again.
Try to be minimalist with attachments. So initially, just do the origins and insertions, and use as few vertices as possible to get the job done. This will help stop the system from becoming 'over constrained'; a common contributor to simulations that look stiff.
Make sure that your tet resolution is high enough to support the deformation you want to see. If you're looking at your muscle and it only has 6 tets, that's too low, but you don't want to go so high that you have more tets than vertices on your muscle mesh. Perhaps incrementally lower the tetSize attr on each muscle and re-sim to check results.
Add fibers to introduce tension. If you're finding that the muscles are too floppy at this stage, you can add muscle fibers on the muscles and give them a little bit constant excitation, so that they're 'on' all the time.
Adding line of actions to your muscles will make them perform correctly when compressed too.
Begin incrementally adding soft (stiffnesses in the range of 104 - 106) sliding attachments between muscles that are separating too much.
Simulate often after small changes to track how those changes are affecting your results
Let me know how that goes!