Infrahyoid muscles

The infrahyoid muscles are a group of four paired muscles that are situated in the neck below the hyoid bone, mainly in the anterior neck compartment. Because of their appearance, they are also known as strap muscles.

Most of these muscles attach to the inferior aspect of the hyoid bone, except for the sternothyroid, which connects the sternum with the larynx. The rest of the muscles link the hyoid bone with the sternum, clavicle, thyroid cartilage of the larynx, or the scapula.

The infrahyoid group includes the following four paired muscles:

• Sternohyoid
• Sternothyroid
• Thyrohyoid
• Omohyoid

All infrahyoid muscles are named according to their origin and insertion sites, except for the omohyoid. The first part of the name indicates the origin site, while the second part refers to the insertion of the muscle.

While each muscle of the group has a distinct function, collectively, the infrahyoid muscles facilitate swallowing and speech. They are responsible for positioning the hyoid bone along with the suprahyoid muscles. They also play an important role in positioning the larynx during swallowing. Overall, all infrahyoid muscles are also classified as accessory muscles of mastication, as they assist during swallowing and chewing.

All infrahyoid muscles, except the sternothyroid, depress the hyoid bone. The sternothyroid depresses the larynx. However, when acting together with other infrahyoid neck muscles, it also indirectly contributes to the depression and fixation of the hyoid bone. When the hyoid bone is fixed, the thyrohyoid muscle elevates the larynx.

Additionally, the infrahyoid muscles can be grouped into two planes according to how close to the surface they are located. These planes are known as the superficial and deep. The superficial plane includes the sternohyoid and omohyoid muscles, while the thyrohyoid and sternothyroid muscles form the deep plane.

Sternohyoid

The sternohyoid is a thin and flat muscle that is found in the muscular triangle on either side of the midline of the neck. It arises from the manubrium of the sternum, with some fibers also originating from the sternal end of the clavicle. This muscle inserts onto the inferior aspect of the body of the hyoid bone. It is the most superficially located infrahyoid neck muscle.

Sternothyroid

The sternothyroid is a broad and flat infrahyoid muscle positioned in the muscular triangle of the neck, behind the sternohyoid muscle. It arises from the manubrium of the sternum and inserts into the thyroid cartilage of the larynx. Therefore, the sternothyroid is the only infrahyoid muscle that does not directly attach to the hyoid bone.

Thyrohyoid

The thyrohyoid is the smallest and shortest muscle of the group. It is positioned deep to the sternohyoid muscle. This muscle arises from the thyroid cartilage of the larynx and inserts into the inferior aspect of the hyoid bone at the junction of the body and the greater horn. It is considered a continuation of the sternothyroid muscle, as it begins where the sternothyroid ends.

Omohyoid

The omohyoid is the longest muscle of the group. It is a narrow and flat strap-like muscle that is composed of two bellies - inferior and superior. Both bellies are continuous with each other and connected by the intermediate tendon. This muscle spans from the scapula to the hyoid bone, linking them functionally.

The inferior belly originates from the superior border of the scapula and then courses through the posterior neck triangle, dividing it into the occipital and subclavian triangles. Some fibers of the inferior belly may also originate from the superior transverse scapular ligament, as this muscular belly arises just medially to the scapular notch of the scapula.

Behind the sternocleidomastoid, the inferior belly transitions into the intermediate tendon, which is anchored to the deep cervical fascia near the carotid sheath. The superior belly starts at the tendon and passes toward the hyoid bone, where it inserts. Additionally, it separates the carotid triangle from the muscular triangle. Both mentioned triangles are parts of the anterior neck triangle.