Joycek
Hey there thanks for sharing your project!
Making great progress
Have circled some stuff of note
That brachiochephalicus should probably pull straighter when the front limbs comes forward. You can see the skin pinching here because it's pinned too tight in the middle. Will make a nicer silhouette if you straighten it out.
That triangular tissue at the back seems to make it into a lot of quadruped muscle models. It's in the occasional anatomical drawing but I don't think you'll find it in in a veterinary medicine book. I'd be happy to be corrected on that, but we generally deal with this area in the skin pass. I can see you're getting a buckle when the back leg comes forward too which probably isn't quite right. I have the same buckle in my lion and it was the thing that was criticized the most, haha. I actually have a new version where it's resolved but I haven't rendered it.
I like the silhouette you're getting on the forearm. Often this tends to buckle in, but yours is looking pretty good.
In the problematic area you've circled, on my lion, I have a different material here. It has a lower amount of pressure and tighter surface tension. It allows the skin to web across between the tricep and the lats. That should reduce the amount of trapping that you're seeing because the points don't get sucked up into that valley.
Hope that helps, looking forward to seeing more stuff!