Because muscles can only contract, many of the joints in the body require antagonistic pairs of muscles arranged as lever systems.
Levers have a pivot point, the fulcrum, and a lever arm to each side.
Sometimes the fulcrum can be at one end, and both lever arms extend to the same side. The distance from where the muscle attaches to the bone and where the joint pivots is the lever arm. The general principle of levers is that both sides of the fulcrum must be equal, or the force on one side times its length is equal to the force on the other side times its length. If the lever arm used for applying the force is longer than the lever arm used to transmit the, the arrangement is called mechanical advantage. If the lever arm used for applying the force is shorter than that used in the transmission of the force, then the arrangement is called mechanical disadvantage.