Strap muscles are a distinct group of skeletal muscles characterized by their long, flat, and ribbon-like appearance. They are primarily located in the neck, where they act as thin, elongated stabilizers and movers for delicate structures. Their unique morphology allows for a broad and gentle application of force over a significant distance. These muscles perform fundamental actions that maintain the body’s basic functions.
Defining the Muscle’s Morphology
The classification of a muscle as a “strap” muscle is based on its distinct architecture, which contrasts sharply with the fusiform shape of muscles like the biceps. Strap muscles are defined by having all muscle fibers running parallel to the long axis. This parallel arrangement gives them their characteristic long, thin, and ribbon-like look.
Unlike muscles with a prominent central belly, strap muscles maintain a relatively uniform cross-sectional area along their entire length. This structural uniformity allows the muscle to exert a consistent pull. The parallel fiber orientation maximizes the range of motion, enabling a long, uninterrupted contraction that is particularly suited for fine, controlled movements rather than generating immense power.
Primary Locations and Specific Naming
The most prominent group of strap muscles is found within the anterior cervical region, commonly known as the neck. They are categorized into the infrahyoid group, meaning they are situated below the hyoid bone, a small, U-shaped structure. The four main muscles in this group are:
- Sternohyoid
- Omohyoid
- Sternothyroid
- Thyrohyoid muscles
These muscles are named according to their attachment points, providing a clear anatomical map of their course. The sternohyoid muscle extends from the manubrium of the sternum up to the hyoid bone. This long, superficial muscle is generally the easiest of the group to identify.
Running deep to the sternohyoid is the sternothyroid muscle, which originates from the sternum and inserts onto the thyroid cartilage of the voice box. The thyrohyoid muscle is a short, superior continuation, traveling from the thyroid cartilage up to the hyoid bone. This muscle bridges the gap between the voice box and the hyoid, completing the chain of downward-pulling muscles.
The omohyoid muscle is unique, consisting of two bellies, superior and inferior, connected by a central tendon, which spans from the scapula (shoulder blade) to the hyoid bone. This complex, diagonal arrangement helps to depress and retract the hyoid.
Essential Roles in Laryngeal Movement
The primary function of the infrahyoid strap muscles is to control the position of the hyoid bone and the larynx, or voice box. Their general action is to depress and stabilize these structures following elevation, ensuring the smooth and rapid operation of the upper aerodigestive tract.
In the process of swallowing (deglutition), the hyoid bone and larynx must first be rapidly pulled upward by other muscles to close the airway. Immediately following this upward movement, the strap muscles pull these structures back down to their resting position. This controlled depression is necessary to reopen the airway for breathing and to quickly prepare the pharyngeal space for the next action.
The thyrohyoid muscle specifically acts to raise the thyroid cartilage relative to the hyoid bone, which is important for adjusting the overall length and tension of the vocal apparatus. The strap muscles work in direct opposition to the suprahyoid muscles, which are located above the hyoid and primarily function to elevate the structure.
This antagonistic partnership creates a stable, adjustable platform for the delicate intrinsic muscles of the larynx to operate. Controlling the vertical height of the larynx directly influences the length of the vocal tract, a factor that significantly affects the resonance and pitch of the human voice.
Clinical Assessment and Surgical Considerations
The location and structure of the strap muscles make them relevant in clinical practice, particularly regarding physical trauma and surgical procedures in the anterior neck. Due to their superficial position, they are susceptible to injury from direct impact or rapid acceleration-deceleration forces, such as whiplash. Damage can result in localized pain and difficulty with swallowing and voice control, necessitating physical therapy.
In surgery, the strap muscles serve as anatomical landmarks and are the initial layer incised during many anterior neck operations. Surgeons often split the sternohyoid and sternothyroid muscles along their midline to gain access to deeper structures, such as the thyroid gland during a thyroidectomy or the trachea for a tracheostomy. This longitudinal approach is preferred because cutting parallel to the muscle fibers minimizes damage to the tissue and promotes faster healing.
Their proximity to major neck vessels, including the carotid arteries and jugular veins, means they must be carefully retracted during deep neck dissection procedures. The omohyoid muscle specifically helps to define the borders of the carotid triangle, an area that contains significant neurovascular structures.
Furthermore, the activity of the strap muscles can be monitored using electromyography (EMG) as part of a diagnostic workup for swallowing disorders, known as dysphagia. Changes in the timing or strength of their contraction following a swallow provide objective data regarding the functional integrity of the pharyngeal phase of deglutition.