What is the Difference Between Flexion and Extension?

in anatomy, flexion and extension they are opposite movements of approach and removal in the anteroposterior direction. These are typical movements of synovial joints, although they 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 sagittal plane in relation to the median frontal plane . Next we will see what these anatomical planes are and later we will explain the differences between flexion and extension.

The sagittal plane and the frontal plane

All anatomical descriptions of the human body are made with respect to the 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 are facing forward.

Starting from this position, the sagittal plane is the plan perpendicular to the ground that cuts the body in right half and left half . Any movement made in the anteroposterior direction is considered 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 exactly in the middle ( midsagittal plane ), any plane parallel to it is also called a sagittal plane, so we usually speak of sagittal planes and not just one.

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For example, knee flexion and extension movement occurs in the sagittal plane (plane in the anteroposterior direction and perpendicular to the floor), but it does not occur in the midsagittal plane, it does not occur right in the middle of the body.

flexion and extension

You angular movements inside of 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 performed in the anteroposterior direction.

THE flexion is the movement that the angle decreases and approaches the two distal ends of the articulated segments. THE extension is the opposite movement, increase the angle between the articulated segments and apart from each other.

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

Bending movements are the result 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 movement of elbow flexion is a consequence of the contraction of the biceps brachii, which brings the arm and forearm together, and the extension is a consequence of the contraction of the triceps brachii.

Flexor muscles act as agonists and extensor muscles as antagonists, therefore, when the flexor muscles contract, the extensors extend and vice versa.

Flexion and extension should not be confused with angular movements in other planes. For example, if we move the arm away from the body laterally, the angle between the two parts varies, but not in the sagittal plane, but in the frontal plane. In this case, we are not talking about flexion and extension, but about abduction and adduction .

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

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

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