Flexion is the bending of a joint that decreases the angle between two body parts. Extension is the straightening of a joint that increases that angle. Together, they make up the most fundamental pair of movements your body performs, from bending your elbow to pick up a coffee cup to straightening your knee when you stand up from a chair.
Both movements happen in the sagittal plane, which is the imaginary flat surface that divides your body into left and right halves. Almost every joint in your body performs some version of flexion and extension, though the range varies widely depending on the joint.
How Flexion and Extension Work
The simplest way to picture flexion is any movement that folds a joint closed. Curl your arm so your hand comes toward your shoulder: that’s elbow flexion. The angle between your forearm and upper arm gets smaller. Now straighten your arm back out: that’s elbow extension. The angle opens up again.
The same logic applies throughout your body. Bending forward at the waist is spinal flexion. Leaning backward is spinal extension. Raising your thigh toward your chest is hip flexion. Swinging your leg behind you is hip extension. In every case, flexion brings body segments closer together on the front (ventral) side, and extension moves them apart.
Muscles That Drive Each Movement
Your muscles work in opposing pairs to control these movements. The muscle doing the primary work is called the agonist, and the muscle on the opposite side that controls or resists the motion is the antagonist. When you curl your arm, the biceps is the agonist driving flexion, while the triceps on the back of your arm acts as the antagonist. To extend your elbow, those roles reverse: the triceps becomes the agonist and the biceps becomes the antagonist.
The same pairing happens at the knee. Your quadriceps (the four muscles on the front of your thigh) extend the knee, straightening your leg. The hamstrings on the back of your thigh flex the knee, bending it. When you kick a ball, the quads are the agonists and the hamstrings slow the motion. When you bend your knee to sit down, the hamstrings take the lead and the quads control the descent. This push-pull relationship is what gives joints both power and stability.
Normal Range of Motion by Joint
Every joint has a typical range, measured in degrees, for how far it can flex and extend. These numbers matter when physical therapists assess injury recovery or when you’re trying to understand whether your mobility is normal. A CDC study on joint range of motion provides reference values for adults aged 20 to 44:
- Knee flexion: roughly 138 to 142 degrees (bending the knee fully)
- Hip flexion: roughly 130 to 134 degrees (bringing the thigh toward the chest)
- Hip extension: roughly 17 to 18 degrees (swinging the leg behind you)
- Shoulder flexion: roughly 169 to 172 degrees (raising the arm overhead)
- Elbow flexion: roughly 145 to 150 degrees (bending the arm fully)
These ranges decrease with age. Adults aged 45 to 69 typically lose several degrees across all joints. Knee flexion, for example, drops to about 133 to 138 degrees in that age group. Women tend to have slightly greater flexibility than men at most joints throughout life.
Clinicians measure these angles using a tool called a goniometer, which looks like a protractor with two arms. One arm aligns with the stationary body segment, the other with the moving segment, and the hinge sits over the joint’s center of rotation.
Hinge Joints vs. Ball-and-Socket Joints
The type of joint determines how flexion and extension play out. Hinge joints like the elbow and knee move primarily in one direction, opening and closing like a door. Their flexion and extension paths are straightforward and predictable.
Ball-and-socket joints like the hip and shoulder are multiaxial, meaning they allow movement in nearly every direction. They still perform flexion and extension, but they also allow you to move your limb sideways (abduction and adduction), rotate it, and swing it in circles (circumduction). Because of this extra freedom, shoulder flexion can take your arm almost 180 degrees overhead, making it one of the most mobile joints in the body.
Flexion and Extension in the Spine
Spinal flexion is forward bending, and spinal extension is backward bending. Unlike a single hinge joint, the spine achieves these movements through small contributions from many individual vertebrae stacking up. The lumbar spine (lower back) has a total flexion-extension range of roughly 50 to 65 degrees, with flexion accounting for the larger share (about 51 degrees forward) and extension contributing around 15 degrees backward.
This is why you can bend much farther forward than you can lean backward. The shape of the vertebrae, the thickness of the discs between them, and the tension of the surrounding ligaments all limit how far each segment can move. Losing range of motion in the lumbar spine is one of the earliest signs of conditions like disc degeneration or chronic low back stiffness.
Special Cases: The Ankle and Thumb
A few joints use different terminology for what is essentially flexion and extension. At the ankle, pulling your toes up toward your shin is called dorsiflexion, and pointing your toes downward is called plantarflexion. Dorsiflexion decreases the angle between the top of the foot and the front of the leg. Plantarflexion decreases the angle between the sole of the foot and the back of the leg. Both movements happen at a hinge joint, but the specialized names exist because the foot’s orientation makes “flexion” and “extension” ambiguous.
The thumb is another exception. While your other four fingers flex and extend in the sagittal plane (curling toward and away from your palm), the thumb moves in a plane angled away from the other digits. When you flex the thumb, it points generally toward the base of your pinky finger. This offset angle is part of what makes the thumb opposable, letting you grip and pinch in ways the other fingers cannot. The thumb also has one fewer joint than the other fingers, with only two bending points instead of three.
What Happens When Joints Move Too Far
When a joint is forced beyond its normal range of extension, the result is hyperextension. A hyperextended knee, for instance, occurs when the knee bends backward past its natural limit. This is a common injury in sports and can damage the ligaments that stabilize the joint. Severe hyperextension of the knee is one of the most frequent causes of ACL and MCL tears. If you hear or feel a pop inside the joint during this kind of injury, it’s more likely that a ligament has torn.
Hyperflexion works the same way in the opposite direction, forcing a joint into excessive bending. Whiplash injuries often involve hyperflexion of the cervical spine, where the neck is snapped forward beyond its safe range. Both hyperextension and hyperflexion can damage ligaments, tendons, cartilage, and the joint capsule itself, depending on the severity of the force involved.