The shapes of the bone surfaces that meet at a joint determine the range of motion at that joint. Joints range from simple hinge joints that allow one axis of rotation to ball-and-socket joints that allow rotation in three axes (Fig. 37.20). The human elbow joint and the ankle joint of a dog are examples of hinge joints. The shoulder and hip joints are examples of ball-and-socket joints, which allow the widest range of motion, as when you throw or kick a ball. The joints at the base of each finger are intermediate: They allow you to flex and extend your fingers as well as spread them laterally or move them together when making a fist or grasping objects.

Joints with a broader range of motion are generally less stable. The shoulder is the most mobile joint in the human body, but it is also the most often dislocated or injured. In contrast, the ankle joint of dogs, horses, and other animals is a stable hinge joint that is unlikely to be dislocated, but its range of motion is limited to flexion and extension. Because muscles are arranged as paired sets of antagonists to move a joint in opposing directions, hinge joints are controlled by as few as two antagonist muscles (generally referred to as a flexor and an extensor). In contrast, ball-and-socket joints have at least three sets of muscle antagonists to control motion in three different planes. As a result, a more complex organization of muscles is needed to control the movements of the arm at the shoulder joint or the leg at the hip joint.