L. McKibben, air muscles, or Pneumatic Artificial Muscles (PAM). In principle, they consist of an inflateable rubber bladder inside a chinese finger trap cylindrical, helically wound braid.

A few years ago we ran a piece on Artificial Muscles which mentions pneumatic artificial muscles that contract when air pressure is applied, and it appears that [Automaton Robotics] has employed ...

"Soft robots that are powered by fluid engines—such as hydraulic or pneumatic action—can ... causing the soft robot to act as an artificial muscle that is flexing and relaxing.