And let's not forget there is lots of it around. Lets work up a system using air power and see what falls out. There are lots of devices to transform pressure into movement. I am intrigued with the woven tubing muscle I thought of. I will have to experiment with some of these and see if they offer any real advantages.
Lets assume that the leg muscles are the strongest muscles in our robot.
The leg muscle will have to lift the entire weight of the robot and load. Assume a foot one-foot long. Lets hook the calf muscle to the heel two inches behind the ankle. If we use 100-PSI air pressure that works out to about 17 square inches. An air bag working through a lever may be the best choice for this.