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What is the difference between flexion and extension?

In anatomy, flexion and extension are opposite movements of approaching and retreating in an anteroposterior direction. They are typical movements of synovial joints, although it can also refer to movements of other joints and moving parts of the body.

More precisely, flexion and extension are defined as angular movements made in the sagittal plane with respect to the median frontal plane . Next we will see what these anatomical planes are and then explain the differences between flexion and extension.

Anatomical planes of the human body
Anatomical planes of the human body

All anatomical descriptions of the human body are made with respect to planes and axes defined in the so-called anatomical position . In this position, the body is positioned in a standing position, with the arms extended to the sides and turned so that the palms of the hands face forward.

Starting from this position, the sagittal plane is the plane perpendicular to the ground that cuts the body in the right half and the left half . Any movement made in the anteroposterior direction is said to be in the sagittal plane.

For example, if we are in the anatomical position, raising and lowering the leg in front of the body, bending and extending the knee, or bending and extending, are movements performed in the sagittal plane.

In addition to the sagittal plane that cuts the body just in the middle (mid- sagittal plane ), any plane parallel to it is also called the sagittal plane, which is why we generally speak of sagittal planes and not just one.

For example, the movement of bending and extending the knee occurs in a sagittal plane (plane in the anteroposterior direction and perpendicular to the floor) but does not occur in the mid-sagittal plane, it does not occur right in the middle of the body.

Flexion and extension

Examples of flexion and extension

The angular movements within sagittal planes are called flexion and extension . This means that flexion and extension are movements that modify the angle between two articulated segments of the body and are carried out in an anteroposterior direction.

The flexion is the movement angle decreases and approaches the two distal ends of the articulated segments. The extension is the opposite movement, increases the angle between the articulated segments and away from each other.

For example, flexing and extending the elbow decreases and increases the angle between the forearm and the arm respectively.

Bending movements are a consequence of the contraction of one or more flexor muscles . The opposite movement, that of extension, is a consequence of the contraction of one or more extensor muscles .

For example, the flexion movement of the elbow is a consequence of the contraction of the biceps brachii, which brings the arm and forearm closer together, and the extension is a consequence of the contraction of the triceps brachii.


The flexor muscles act as agonists and the extensor muscles as antagonists, so when the flexor muscles are contracted, the extensors extend , and vice versa.

Flexion and extension should not be confused with angular movements in other planes. For example, if we lift the arm away from the body laterally, the angle between both parts varies, not in the sagittal plane but in the frontal plane. In this case we do not speak of flexion and extension but of abduction and adduction .

There are other movements in the sagittal plane which, by convention, are defined as flexion and extension movements even though they are not angular movements.

For example, the elevation of the arm to the front is defined as a movement of flexion and backwards as extension, but they are produced by rotation of the shoulder joint, so they are not angular movements.




  1. Jones, O .. Anatomical terms of movement .





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