Joints, or articulations, are very complex. There are 6 primary types of synovial or freely movable joints in the human body and each is capable of specific types of movement. For example the knee joint is a hinge joint and is capable of movement forward and back in the sagittal plane. It is not capable of side to side movement in the frontal plane. However, the hip is a very versatile ball and socket joint and is capable of movement in all planes of motion.

The bony structure of the joint primarily dictates its function. The surrounding joint capsule and connective tissue play a role in joint mobility.