I think I know what the problem is. I clicked on this thread and got inspired by that art so I sketched this guy for fun.
Basically Ill illustrate the problem. The reason why your pose looks so stiff is due to a very very subtle problem with the ribcage and pelvis.
I think the reason why you had trouble with this is because of the references extreme foreshortening, which skewed the crap out of his main masses/forms.
Also, I sort of messed up the 3 pairs of pectoral muscles and used one. Sorry for that. Don't take my gesture drawing as an actual representation, it's more of an indication of what's going wrong with the model.