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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.

Hey thanks! Sorry for getting back late but that really helped and I made my own cute little guy that works the exact same as yours! Thanks!