What is the term for the action that occurs when a joint is straightened?

The action that occurs when a joint is straightened is referred to as extension. This term specifically describes the movement that increases the angle between body parts, effectively straightening the joint. For example, when you straighten your arm after bending it at the elbow, you are performing extension.

This understanding of extension contrasts with flexion, which is the action of bending a joint and decreasing the angle between the body parts. Abduction and adduction are terms that refer to movements of limbs away from or towards the body’s midline, respectively, and do not involve the straightening of a joint. Hence, the term extension is accurately applied to the action of straightening a joint.