In this lesson, you’ll find out the meaning and exercises behind flexion. You’ll also have the chance to learn about different joint movements and muscles of the body. When you are through, test your new-found knowledge with a quiz!

Not All Muscles ‘Flex’

Typically when we hear someone say ‘flex your muscles,’ we imagine a very strong person posing in an awkward stance, contracting his or her muscles so they bulge out and look very big. However, in medical and anatomical terms, only some of the movements the bodybuilder is doing would actually be considered flexing, or flexion. Before we get into exactly what flexion is, let’s briefly review body mechanics.

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When two bones come together they form a joint. Many joints of the body move and are called synovial joints. These include joints like the elbow, shoulder, knee, and ankle. Other joints, like those between your vertebrae (i.e., back bones), are semi-mobile and can only move a little bit. In order for a joint to move, a muscle pulls on one of the bones of the joint. For instance, your biceps brachii muscle pulls on your radius bone in your forearm. This moves your forearm closer to your upper arm, which is bending your elbow. In this case, the angle at your elbow (between your upper arm and forearm) is getting smaller. This is the classic example of flexion.

Flexion is a movement in which the angle between two body segments gets smaller. Take a minute to think of any two body segments next to each other. Examples are the hand and forearm, thigh and shin, arm and torso, etc. Now think of the angle that those two segments make when you are standing up with your arms at your side and your legs straight.

In most cases, that angle is 180 degrees since the segments are in a line with each other. Now think of bending the joints in between those two body parts. If the movement decreases the angle between the two, that’s flexion.

If the movement increases the angle, then the movement is the opposite of flexion, or extension.

Here is one way to remember the difference between flexion and extension. Extension and increase angle both start with a vowel. Flexion and decrease angle both start with consonants. Another way to remember is to think of what you would do when someone says ‘flex your arms.’ Subconsciously you would bend your elbow and contract your bicep muscles. Bending your elbow is flexion!

Two Types of Flexion

Think about all the movements your shoulder can do. The shoulder has the highest degree of movement of any joint in the body. It can move in all three dimensions – up, down, forward, back, out and in! In contrast, flexion occurs in just one plane (i.e., in two dimensions). This creates a problem of defining the angle that is getting bigger or smaller.

We get around this by using two types of flexion: flexion and horizontal flexion. Flexion occurs when the arm is hanging down next to the body and then the arm raises forward. Horizontal flexion occurs when the arms are held straight out from the shoulders (like making a T with your body) and then move toward the front of the body. This is termed horizontal flexion since the flexion occurs in the horizontal plane.

Examples and Exercises of Flexion

Because flexion is a movement, muscles are responsible for flexion. Each joint has at least one, but typically more, muscles that control the flexion movement. The muscles of flexion are sometimes referred to as flexors, and some even have it in their name. To become more efficient at flexion movements, you must strengthen the muscles responsible for the movement.

Let’s go over the major muscles involved in flexion and some exercises you could do to strengthen those muscles:

The elbow makes the forearm move towards the upper arm. The muscles involved are biceps brachii, brachialis, and brachioradialis. Bicep curls and reverse bicep curls work these muscles.

The wrist causes the hand to move towards the inner side of the forearm. The muscles here are flexor carpi radialis, flexor carpi ulnaris, and palmaris longus. The wrist can be worked by doing wrist curls.

The shoulder we already described and involves the pectoralis major, anterior deltoid, coracobrachialis, and biceps brachii . These muscles can be worked through chest presses and chest flies.

The hip allows the thigh to move up closer to the torso, like when sitting and uses the rectus femoris or quadriceps. Hanging leg or knee lifts work the hip.

The knee allows the heel to move towards the butt and utilizes the hamstring muscles – biceps femoris, semitendinosus, and semimembranosus. These muscles are worked with leg curls and dead lifts.

The vertebrae helps the torso and shoulders bend towards the legs and uses the rectus abdominis muscles, which are worked through sit-ups.

The neck moves the chin towards the chest. This involves the longus capitis. There are no exercises that properly work this muscle.

Lesson Summary

Flexion is just one type of movement that the body does every day. In flexion, the angle between two bones or body segments becomes smaller. The classic example is when the elbow bends – the upper arm and forearm get closer together. However, flexion can occur at most synovial (moveable) joints. Flexion can also occur at semi-mobile joints like those in the vertebrae. Because the shoulder can move in many directions, it is also capable of horizontal flexion. The muscles that cause flexion are often referred to as flexors. So the next time someone tells you to ‘flex your muscles,’ tell them you can’t. But you can flex your joints using your flexors!