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How do flexion and extension change the angle between body parts?

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Medical illustration of a human knee joint in a parasagittal section just lateral to the midline, labeling ligaments, cartilage, meniscus, patella, synovial structures, and surrounding muscles and fat pads. — joint The ligaments and cartilage in a knee joint.

Flexion and extension are angular movements that change the relationship between body parts at a joint.1

Flexion is a bending movement that decreases the angle between two body parts or bones at a joint, such as when bending the elbow or knee.231
Extension is the opposite movement, which increases the angle between body parts, effectively straightening the limb.241

These motions are categorized as "swing" movements, where the angle changes between the long axis of a moving bone and a reference line in a fixed bone. While flexion and extension are distinct actions, they are often performed by opposing muscle groups, such as the quadriceps femoris for knee extension and the hamstring muscles for knee flexion.14