Instructables has a good tutorial up on how to make pneumatic muscles out of latex tubing and a braided sleeve. They work a bit like that extendable fist cartoon characters are so fond of. The latex expands and forces the lattice of the braided tube to turn the length into width, while preventing the latex tube from rupturing. They can be cheaply made out of readily available materials, and can lift 400 times their own weight. It occurs to me that since the air doesn’t have to be lost in order to flex the muscle, a closed system with an air bulb might be used to remove the requirement for an air compressor. It would also provide the potential for a certain amount of tactile feedback on the strain the muscle is under.