How can a robot move? Pneumatic motor, magnetic motor...???

[Captain Max]

I'm curios about robotic locomotion. I see some are pneumatic powered, some use regular servo motors, and some are in development to use that new Artificial Muscle polymer.

 

What else can they be powered by?

 

I did some reading on electrostatic motors. They're super compact, but not very powerful.

 

I also see that there are torque motors, but those are heavy.

 

What would be small, efficient and yield a large amount of torque? I guess speed is not important since you're only moving a limb/lever a short distance.