Hi raclem !
Looks like self intersection and some twisting of muscles are causing that weird deformation on fat sim.
If you are going with separate pass sim approach (like muscle pass, than fat pass). Than make sure ur driver deformation is solid , stable and free from any self intersections.
As @Andrewstanley suggested play around with attachments of muscles.
here are are few things which I would look into.
a. By selecting verts sparingly and add attachment especially at the origin and insertion of muscles. I would also play and test with different attachment strength.
b. Adjust the tets size of muscles to get optimal deformation.
c. Apply loose/weak attachment to its surrounding muscles ( can be fixed or/and slide attachment). For muscles closer to bones can play around with slide attachment in order to avoid moving away far from its position.
d. In some cases add additional layer of material and paint on muscles tissue especially long and slender part which can be represented as tendons. Where young modules can be higher.
e. play with muscles fiber driven with LOA or any joint Angle driver (like pose Reader, Rbf , pose interpolator etc)
I would suggest experiment with Mus+Fat coupling sim u can take advantage of bi directional pull/push of muscles and fat tissue. Where self penetration issue is minimised.