Flexion and extension are opposite movements that occur in the sagittal plane around a joint. They are initiated by agonist muscles and resisted by antagonist muscles. Some joints only allow flexion and extension, while others can perform multiple movements.
In anatomy, flexion and extension are two opposite movements that muscles can perform around a joint. Flexion is a movement in which the angle of the involved joint decreases, such as bending the elbow so that the forearm comes closer to the upper arm. Extension is a movement that increases the angle of the joint, such as straightening the elbow. Both occur in a single front-to-back plane of motion known as the sagittal plane. Flexion and extension can be performed at various types of joints and are initiated by many of the muscles in the body.
Like all movements, flexion and extension are performed in a plane that is determined in relation to a position on the body called the anatomical position. In anatomical position, the body is upright with the arms at the sides and the palms facing forward. Any movement that occurs in a front to back direction relative to anatomical position, such as raising and lowering the leg directly in front of the body, is said to occur in the sagittal plane. In fact, flexion and extension are the only movements that occur in the sagittal plane, although flexion is not always forward and extension is not always backward. For example, the knee flexes backward while the elbow flexes forward.
Although many of the body’s joints can produce movement in multiple planes, some allow only flexion and extension and therefore only bend and straighten in the sagittal plane. Examples of these joints, called articulating joints, are the knee and the elbow. In the knee, flexion occurs when the knee is bent, while extension is the act of straightening the knee; The same is true of the elbow, although they occur in opposite directions. Other examples of hinged joints that perform these movements only are the interphalangeal joints within the fingers and toes.
Most joints that are capable of flexing and extending do so in addition to other movements. The shoulder and hip joints can be flexed and extended, as well as abducted, which involves lifting the limb laterally from the body; adducted, which involves dragging the limb sideways toward the body; circumduced, which implies encircling the limb; and twisted, which involves turning the limb from side to side. At the neck, the head can be flexed, bringing the chin to the chest, and, conversely, extended, tilting the chin up, as well as abducted, adducted, circumducted, and rotated. Even the intervertebral joints of the spine are capable of segmental flexion and extension, with flexion causing the trunk to bend forward and extension straightening it. These are also capable of rotation and a movement known as side bending or side bending.
Flexion and extension are also differentiated by the muscles that can produce them. At any given joint, flexion is initiated by a muscle or muscle group called an agonist and resisted by an opposite muscle or muscle group called an antagonist. The same is true for extension, only the role of the muscles is reversed. At the elbow joint, for example, the biceps brachii muscle in the front of the upper arm produces flexion, while the triceps muscle in the back of the arm opposes it, which must lengthen for the biceps to contract. In contrast, the triceps is the agonist during extension, contracting to straighten the elbow while the biceps lengthens in opposition.
Protect your devices with Threat Protection by NordVPN
