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What is Flexion?

By Shelby Miller
Updated: Mar 03, 2024

Flexion is an anatomical term that refers to a movement produced at a joint by a muscle or muscles that causes the angle of the joint to decrease relative to its anatomical position. For example, when a person is standing normally, the hip joint is considered to be in a neutral or 180-degree position. Flexion at the hip occurs when that person raises her knee into the air, thus bending the hip and decreasing the joint angle at the hip from 180 degrees to 90, if the knee is brought to hip height. In order to produce this movement, the muscles being flexed require the coordination of tendons, ligaments, cartilage, and bones, all structures around the joint working together.

A large number of joints in the human body are capable of flexion and its opposite movement, extension. The elbow joint flexes when the arm bends at the elbow, the shoulder joint flexes when the arm is raised straight in front of the body, and even the joints between the vertebrae can be flexed, causing the spine to curl forward as it does during an abdominal crunch. While there are exceptions, most flexion and extension occurs in the sagittal or front-to-back plane of movement. Joint movements that occur in the frontal plane, or to the side of the body, such as raising one’s leg to the side, are typically known as abduction and adduction.

At each flexible joint in the body, there is a primary and often secondary muscle group responsible for flexion at that joint. In the hip, for instance, the primary muscles are the iliopsoas, the tensor fasciae latae, and the rectus femoris, collectively known as the hip flexors. Secondary muscles involved in hip flexion include the sartorius, gracilis, and adductors longus and brevis.

When a person raises her knee from a standing position to hip height, flexing the hip to 90 degrees, the brain first sends a signal through the motor neurons of the peripheral nervous system that tells the hip flexor muscles to fire. The hip flexors then contract, or shorten in length. Because they are attached to the hip and femur bones via tendons at either end, much like a series of pulleys and cables, they move the bones around the joint, pulling the femur bone forward as the knee lifts. This complex system results in a seemingly simple action: flexion of the hip joint to move the leg forward in space, just as occurs during walking, running, climbing, and other forward movements.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon988082 — On Feb 08, 2015

I just want to know: What is one joint in the human that clearly does not form a 180 degree angle when standing in anatomical position?

By lighth0se33 — On Oct 08, 2012

@Oceana – Sitting at a computer desk all day can place a strain on many of your muscles. I've noticed that my neck gets really tired and sore, so I started doing neck flexion exercises to strengthen it. This helps me get through the day without having to put my head down for awhile to rest.

By Oceana — On Oct 07, 2012

I do wrist flexion exercises to keep my hands and wrists in good working condition. I do a lot of typing and clicking with the mouse at work, so it is important that I do everything I can to stave off carpal tunnel syndrome.

I hold my arms straight out and bend my wrists downward. After five seconds of flexion, I do the opposite. I bend my wrists upward and hold for the same amount of time.

I do this exercise about five times in a row whenever my wrists or hands are feeling weary. It's a nice stretch, and it seems to loosen everything up a bit.

By orangey03 — On Oct 06, 2012

My cousin is into weightlifting, and he always uses elbow flexion to show off his biceps. He probably doesn't know this, though.

He just thinks he's flexing his bicep muscles. He's probably never thought about the fact that in order to do this, he has to reduce the angle between the upper and lower arms.

By DylanB — On Oct 06, 2012

I never really thought about knee flexion until I injured mine in a car accident. Suddenly, I could no longer bend it with ease. I couldn't flex or extend the muscles.

Recovery took a few months, but I eventually regained full use of my leg and my knee. I find myself thinking about the period when I couldn't flex my knee joint at times while I'm doing a dance workout, and I have a new appreciation for this ability.

Flexion occurs in our body so many times throughout the day that it just seems as natural as breathing. We don't miss it until it is impossible.

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