Wow great progress! Some nice poses in there!
My main comment is that it all feels very stiff. There's not much in the way of ballistic motion going on. I mean especially when he throws the axe down. You'd expect to see a ton of follow-through on the muscles from that impact. My guess is that your muscle material stiffness is too high, the system is over constrained (meaning too many attachments), tet resolution is possibly too coarse. Or any combination of those.
As a general rule, you get natural results when minimizing the amount of attachments. Start out with origin/insertions, and then only add soft attachments between muscles/bones if you really need them. For the most part you want to let collisions handle as much as possible so that the muscles are free to move naturally.
Don't forget about inertial damping to dial down world space inertia, instead of adding more attachments too. (found on the zTissue node) It's a powerful feature.
Cool stuff though! Are you using muscle firing at all?