Mostly physics. I will not spoil the fun of creating such character yourself. I'll just give the references that gave the most influence: look up for a talk on procedural animation in Overgrowth, PID controllers is another topic worth researching. Initial builds were PID+IK based, but I was not completely happy. It's iterative development - analyzing what I don't like at each iteration I came up with my own force response algorithm (non PID based) trying to resemble muscles.