Looks like the muscle model is nicely crafted.
As for the sim, to me it seems stiff. In my experience this is because of any combination of the following: There are too many attachments, Young's modulus is set too high on the tissues themselves, and / or there's not enough tet resolution to support the amount of required deformation.
If you look at my lion or the bull that was posted recently you'll see there is a lot of secondary motion on the muscles. They are free to move and do their thing. In your sim, the latissimus dorsi is a good example of an over constrained muscle. This muscle should be free to slide and contract over the rib cage.
I would go through and inspect all your attachments, try to be minimalist in your approach. For example constrain only a few vertices for origin/insertion attachments to bones. For any muscle to muscle attachments, again try to keep the number of vertices constrained on the conservative side, and make these attachments relatively weak compared to the muscle to bone attachments. (1 x 104 for instance)
In the next update it would be good to see a sim that has LOTS of secondary motion and jiggle. Then we can work on toning back the areas that move too much.