In this anatomy lesion, I’m going to demonstrate flexion and extension, which are body movement terms that either decrease or increase the angle between two structures or joints, bringing them closer together or moving them further apart.
Flexion and extension are considered angular movements. I find that it’s really helpful for me to think in terms of angles, because if you think about the angle getting smaller or bigger between the two surfaces that you’re looking at, that will help you determine whether flexion or extension is occurring.
Flexion vs Extension
Flexion decreases the angle between two structures or joints as they bend or move closer together, whereas extension increases the angle between them as they straighten and move apart.
Elbow Flexion and Extension
Elbow flexion (also called forearm flexion) occurs when the angle between the forearm and arm decreases, allowing the ulna of the forearm to move closer to the humerus bone of the arm.
In contrast, elbow extension (forearm extension) occurs when the forearm moves away from the arm, increasing the angle between those bones.
Shoulder Flexion and Extension
Shoulder flexion, also called arm flexion, occurs when the angle at the humerus of the arm and the scapula decreases as the arms move anteriorly. In contrast, shoulder extension (or arm extension) occurs when the angle at the humerus of the arm and the scapula increases, causing the arm to move posteriorly. The joint here allows movement past the anatomical position. Some anatomists call arm movement beyond the anatomical position extension, whereas some call it hyperextension.
Wrist Flexion and Extension
Wrist flexion (also called hand flexion) occurs when the angle between the palm of the hand and the anterior surface of the forearm decreases, while wrist extension (or hand extension) is moving the palm of the hand away from the anterior surface of the forearm, hence the angle increases. This is another joint that can continue to move past the anatomical position in a posterior direction, which some anatomists call hyperextension.
Finger Flexion and Extension
Finger flexion occurs when the angle between the fingers and the palm decreases, as the fingers move toward the palm. When the angle between the fingers and the palm increases, finger extension occurs.
Flexion and extension also occur with the interphalangeal joints of the fingers (digits 2-5), including the distal interphalangeal joint (dip) and proximal interphalangeal joint (pip).
Thumb Flexion and Extension
The thumb (pollex) can confuse people because thumb flexion and extension occur in the frontal plane, which is a different direction than flexion of the fingers, which occurred in the sagittal plane. Thumb flexion moves the thumb toward the pinky finger, whereas extension moves the thumb away from the pinky finger. Think of your palm as a windshield and your thumb as the windshield wiper for this movement.
Flexion and extension can also occur at the interphalangeal joint of the thumb.
Hip Flexion and Extension
Hip flexion (or thigh flexion) occurs when the angle between the femur of the thigh and hipbone decreases as the thigh moves anteriorly (forward). Hip extension (thigh extension) occurs when the angle between the femur and the hip bone increases, as the hip joint straightens. This joint also allows posterior movement past the anatomical position, which some anatomists call hyperextension.
Knee Flexion and Extension
Knee flexion (leg flexion) occurs when the tibia bone moves toward the femur, causing the angle to decrease between those two structures. Knee extension (or leg extension) occurs as the angle between the leg bones increases, causing the leg to straighten.
Toe Flexion and Extension
Like the fingers, toe flexion and extension can also occur. Toe flexion involves bending the toes toward the sole of the foot, decreasing the angle between these two structures, while toe extension involves increasing the angle and straightening the toes.
Note: instead of using flexion and extension for the movement of the foot at the ankle joint, anatomists prefer to use the terms plantarflexion and dorsiflexion.
Neck Flexion and Extension
Neck flexion occurs as the angle between the head and the trunk of the body decreases as those two structures move closer together, whereas neck extension occurs as the head moves away from the trunk of the body, thus increasing the angle. The neck is another structure that can continue posteriorly, beyond the anatomical position, which some anatomists call hyperextension of the neck.
Vertebral Column Flexion and Extension
Vertebral column flexion at the trunk, (spine flexion) occurs when the angle between the trunk and the hip joint decreases. Vertebral column (spine) extension at the trunk occurs as the spine straightens and the angle between the hip joint and spine increases.
By the way, you might have noticed that most of these movements so far are occurring within (or parallel to) the sagittal plane. However, just like the thumb, flexion can also occur in the frontal (coronal) plane for the vertebral column. For example, if you bend the spine to the left or right, that’s called lateral flexion, and movement back toward the anatomical position is called lateral extension.
Note: you might want to watch our other lecture if you are unfamiliar with the different body planes.
Finally, when extension of a structure moves beyond a certain point, anatomists call it hyperextension. However, anatomists differ on what constitutes hyperextension when it comes to body movement terms.
For example, some anatomists say that when the arm, neck, wrist, or thigh moves past the anatomical position in a posterior motion, it becomes hyperextension. Other anatomists only consider these movements hyperextension if the movement exceeds the normal range of motion permitted by the joint. For test-taking purposes, follow your anatomy teacher’s definition!
Free Quiz and More Anatomy Videos
Take a free flexion and extension quiz to test your knowledge, or review our quick flexion vs extension video. In addition, you might want to watch our anatomy and physiology lectures on YouTube, or check our anatomy and physiology notes.