What does "extension" refer to in joint movement?

In the context of joint movement, "extension" specifically refers to the straightening of a joint or the increase of the angle between two body parts. This movement typically involves moving a limb away from the body's midline, which aligns with the concept of straightening a joint after it has been flexed. For instance, when you straighten your arm at the elbow after bending it, that action is considered extension.

The first option describes "adduction," which is the movement toward the center of the body. The third option defines "flexion," which is the opposite of extension and denotes bending of the joint, thus decreasing the angle between the parts. The fourth option refers to a specific form of movement that involves rotation, not extension. Understanding these distinctions can clarify why the correct definition of extension is associated with moving a limb away from the midline and straightening a joint.