Movable joints are also known as synovial joints. This is because the space between the bones is filled with a thick fluid, called synovial fluid, that cushions and lubricates the joint. The synovial fluid itself serves to reduce the friction between joint articulations.
There are a variety of types of movable joints, classified by how they move. For example, a ball-and-socket joint, such as the shoulder, has the greatest range of motion (degrees of freedom), allowing movement in several directions. Other movable joints, including hinge joints such as the knee, allow only one degree of freedom, considerably less than the ball-and-socket joint.
