Using pneumatic pistons and servos to power robots makes them fast and strong, but also bulky and extremely heavy. No one is going to mistake ATLAS for a real human being. To eventually create humanoid-looking robots like the Terminator we need to mechanically replicate every part of the human anatomy—starting with the muscles.
Source: Gizmodo – The Freaky Artificial Muscles on this Human Skeleton Are the Future of Robotics