Dexter has a different, more human-like body on which ZMP control does not work. Its joints, driven by air cylinders, are springy and flexible like human muscle. There are no stable postures that it can be put in where it can balance without active feedback, so it has to constantly adjust based on its sense of balance, the robot equivalent to your inner ear. It walks and balances the same way humans do, even wearing the same shoes humans wear. (via About the Robots)