Final answer:
Extension is the movement that increases the angle between the bones of a joint, straightening the joint.
Step-by-step explanation:
Extension is the movement of a body part that increases the angle between the bones of a joint, straightening the joint. It is the opposite of flexion, which decreases the angle between the bones. For example, when you raise your arm up at the shoulder joint, you are performing extension. Extension is one of the types of angular movements allowed by synovial joints.