Muscles

In animals muscle is the specialised tissue which allows movement of various parts of the skeleton. There are some variations on the muscle theme, such as the tongue and heart, but generally muscles are found in pairs acting on opposite sides of a skeletal joint. They act in pairs because they can only contract or apply force by pulling on the bone to be moved. For instance, in the arm the larger biceps raise the forearm and hand while the smaller triceps muscles lower it. The lowering muscle is smaller because the design assumes gravity assistance.

The most common equivalent to a muscle in a robot is the hydraulic ram

Pneumatic rams

servo drives

air muscles

flexators

electro-polymers