Flexion and extension describe changes in the angle at a joint, whereas abduction and adduction describe movement away from or toward the body's midline. These movements are defined from the anatomical position.
Movement Differences
| Movement | Definition | Sporting example |
|---|---|---|
| Flexion | Decreases the angle between articulating bones, usually in the sagittal plane | Bending the elbow during the upward phase of a biceps curl |
| Extension | Increases the angle between articulating bones, usually in the sagittal plane | Straightening the knee when kicking a football |
| Abduction | Moves a body part away from the body's midline, usually in the frontal plane | Raising the arms sideways during a star jump |
| Adduction | Moves a body part toward the body's midline, usually in the frontal plane | Lowering the arms to the sides after a star jump |
Flexion and extension are opposite movements. At the elbow, flexion brings the forearm toward the upper arm; extension returns the elbow toward the anatomical position. Movement beyond the normal anatomical position is called hyperextension, such as moving the shoulder backward past the trunk.
Abduction and adduction are also opposites. They commonly occur at joints such as the shoulder and hip. For example, moving the leg sideways away from the body is hip abduction, while bringing it back toward the body's midline is hip adduction.
A common misconception is that adduction must move one limb all the way across the midline. It only needs to move toward the midline. Similarly, flexion does not always mean moving forward; it is identified primarily by the change in joint angle and the specific joint involved.
IB Exam Technique
For IB SEHS subtopic B.1.1, identify the joint, name the movement, and support it with the correct anatomical reference. In movement-analysis questions, use phrases such as “elbow flexion decreases the joint angle in the sagittal plane” rather than simply stating that the limb “bends.”