What primary motion do hinge joints allow?

Hinge joints primarily allow for flexion and extension, which are the movements characterized by bending and straightening a limb or body part along a single plane. This is similar to the way a door hinge operates; it swings in one direction to open and another to close, allowing for movement back and forth without additional rotation.

In the human body, hinge joints can be found in the elbows and knees, where they permit flexion when the angle decreases and extension when the angle increases. This specificity in movement is what distinguishes hinge joints from other types, such as ball-and-socket joints, which allow for a wider range of motion including rotation.