Thank you Andy!
I've already done the last thing you pointed, I will show you. But after solving the other interesting thing you underlined.
For the second part of your analysis, yes I have deeper muscles but maybe not enough:
should I add more?
The first thing you notice is something I've fought hard against for many weeks. I have already animated the bones to do exactly what you say, but for as many different settings I've tried, the scapulas always cut and penetrate the muscles, like blades. You can only glimpse the effect of the scapulas, which I hid for aesthetical reasons, but after the shy muscle bending you would have seen the scapula breaking through.
The scapula movement is very sudden, which I think is one of the reasons. What would you suggest for this?
Here are the last settings I've tried (a little bit better than the video I posted here, but the problem persists)
zSolver settings: Max Newton Iterations 16, Substeps 4 (Collision Detection of course is on)
Thank you again!