The bones that make up the joint allow movement, but it is the muscles that pull the bones that produce the movement. Muscles are attached to bones by structures called tendons. Tendons must be both strong to facilitate movement and compliant to prevent damage to the muscle tissues. Ligaments, which are stiff structures that connect bones, help to prevent excessive movement. Muscles, tendons, and ligaments are attached around each joint at very specific positions, with joint surfaces shaped in exact dimensions. Fluid within most of the joints lubricates the joint surfaces to reduce friction and allow for lifelong use.
