What is the primary action that occurs at a hinge joint?

The primary action that occurs at a hinge joint is flexion and extension. Hinge joints, such as the elbow and knee, allow movement predominantly in one plane, which involves bending (flexion) and straightening (extension) of the bones that form the joint. This uniaxial movement enables the angle between the two connected bones to decrease during flexion and increase during extension, effectively functioning like the hinge of a door.

In contrast, other movements mentioned, such as rotation, abduction, and adduction, are characteristic of different types of joints. For instance, rotation is a movement that occurs at ball-and-socket joints, while abduction and adduction typically occur in planes that involve lateral movements, which hinge joints do not accommodate effectively.