Both flexion and extension occur within the sagittal plane and refer to the angle between body parts. Flexion describes a motion that decreases the angle between two body segments. For example, sitting down flexes the knees. In joints that can move in multiple directions, flexion takes place in the anterior (toward the front) direction. In the neck or body, leaning forward is flexion. Lateral flexion is the motion of the neck or the body left or right. Moving the arms or legs forward is also flexion. Some parts of the body, such as the ankles and wrists, use unique forms of flexion.
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