The extraordinary range of motion of human hands, arms, and legs is made possible by movable joints. Joints are subdivided based on their structure. All freely movable joints are synovial joints, meaning joints in which the bones are not directly connected. Synovial joints can be further subdivided into six types of freely movable or diarthrosis joints, all of which are listed below.
A ball-and-socket joint features a bone with a ball-shaped end that fits into a cuplike socket on the other bone. This arrangement permits the widest range of motion of any freely movable joint. The shoulder and hip are ball-and-socket joints.
In a condyloid joint, a rounded projection on one bone fits into an elliptical socket on the other, permitting motion in two planes but preventing rotation. The joints between the bones in the palm of the hand and the fingers are condyloid joints, as are the joints between foot bones and toes.
In a saddle joint, each bone has a concave region and a convex region, and the shape of each bone is complementary to the other. Saddle joints permit a wide range of movement. The only saddle joint in the human body is found where the thumb joins the hand.
A hinge joint has an outward-bulging projection on one bone that fits into an inward-bulging depression in another. Rather like a door hinge, a hinge joint can only move in one plane--forwards and backwards. Elbows are hinge joints.
A pivot joint has a projection on one bone that fits into a ring-shaped structure of another. Pivot joints permit rotation and are found in parts of the body that rotate, like the atlas-axis joint in the neck.
Bones in gliding joints have flat surfaces that move parallel to each other to allow sliding or twisting. The carpals of the wrist are an example of a gliding joint.