In anatomy, flexion and extension are two opposing movements that muscles can perform about a joint. Flexion is a motion in which the angle of the joint involved decreases, as in bending the elbow so that the forearm is brought toward the upper arm. Extension is a movement that increases the angle of the joint, as in 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 several kinds of joints and are initiated by many of the body’s muscles.
Like all movements, flexion and extension are performed in a plane that is determined relative to a body position referred to as anatomical position. In anatomical position the body is upright with arms at the sides and palms facing forward. Any movement that occurs in a front-to-back direction relative to anatomical position, as in lifting and lowering the leg straight out 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, though flexion is not always in a forward direction and extension is not always in a backward direction. For instance, the knee flexes backward while the elbow flexes forward.
While many of the body’s joints can produce movements in multiple planes, some allow only flexion and extension and therefore only bend and straighten in the sagittal plane. Examples of these joints, called hinge joints, are the knee and elbow. At the knee, flexion occurs when the knee is bent, while extension is the act of straightening the knee; the same goes for the elbow, though they occur in opposite directions. Other examples of hinge joints performing 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 sideways away from the body; adducted, which involves drawing the limb sideways back toward the body; circumducted, which involves circling the limb; and rotated, which involves turning the limb from side to side. On the neck, the head can be flexed, bringing the chin toward the chest, and conversely extended, tipping the chin upward, as well as abducted, adducted, circumducted, and rotated. Even the intervertebral joints of the spine are capable of flexion and extension on a segmental level, with flexion causing the trunk to bend forward and extension causing the trunk to straighten. These are also capable of rotation and a motion known as lateral flexion, 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 group of muscles called the agonist and resisted by an opposing muscle or group of muscles called the 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 on the front of the upper arm produces flexion while opposed by the triceps muscle on the back of the arm, which must lengthen in order for the biceps to contract. Conversely, the triceps is the agonist during extension, contracting to straighten the elbow while the biceps lengthens in opposition.