Strap muscles are a distinct category of skeletal muscles, characterized by their elongated, flat, and ribbon-like appearance. Their parallel fibers give them a uniform, band-like shape, allowing for precise, often linear, movements. Prominent in the neck and thigh, these muscles contribute to various bodily functions.
Anatomy of the Neck Strap Muscles
The neck features a prominent group of strap muscles known as the infrahyoid muscles, located in the front of the neck, below the hyoid bone. These four paired muscles—the sternohyoid, sternothyroid, thyrohyoid, and omohyoid—are organized into superficial and deep layers. The sternohyoid and omohyoid muscles form the superficial plane, while the sternothyroid and thyrohyoid muscles lie deeper.
The sternohyoid muscle extends from the sternum and clavicle to the hyoid bone, a U-shaped bone in the neck. The sternothyroid muscle originates from the sternum and inserts onto the thyroid cartilage (Adam’s apple). Situated beneath the sternohyoid, it is the only infrahyoid muscle not directly attaching to the hyoid bone.
The thyrohyoid muscle connects the thyroid cartilage to the hyoid bone, positioned directly on top of the sternothyroid muscle. The omohyoid muscle, unique among the infrahyoid group, has two bellies (superior and inferior) joined by an intermediate tendon. It originates from the scapula and inserts onto the hyoid bone, traversing obliquely across the neck.
Function of the Neck Strap Muscles
The neck strap muscles collectively control the position and movement of the hyoid bone and the larynx (voice box). Their primary action involves depressing the hyoid bone, pulling it downwards after elevation.
During swallowing, these muscles help lower the hyoid bone and larynx, allowing food or liquid to pass smoothly into the esophagus. This depression also reopens the laryngeal inlet, which closes during swallowing to prevent aspiration, thereby re-establishing breathing. The strap muscles also contribute to speech by precisely adjusting the larynx’s position, influencing vocal pitch and resonance. They work with the suprahyoid muscles, which elevate the hyoid bone, to achieve fine motor control.
Clinical Significance and Common Issues
The neck strap muscles can be affected by various conditions, often leading to neck discomfort or pain. Muscle strains are common, frequently resulting from sudden movements like whiplash injuries or prolonged poor posture, such as “tech neck.” These strains can cause localized pain, stiffness, and muscle spasms.
Trigger points, hypersensitive spots within taut bands of muscle fibers, can develop in these muscles. When pressed, these trigger points can cause local pain and referred pain, radiating to areas like the head, jaw, or chest. For instance, trigger points in neck muscles can lead to headaches or discomfort around the ear.
In medical procedures, the neck strap muscles are important, particularly during anterior neck surgeries like thyroidectomies. Surgeons often retract or dissect these muscles to gain adequate exposure to underlying structures, such as the thyroid gland. While strap muscle division was once thought to impact voice or swallowing, studies suggest that transection does not significantly result in long-term voice or swallowing issues, and any temporary symptoms are often transient.
The Strap Muscle of the Leg
Beyond the neck, the term “strap muscle” also applies to other muscles due to their characteristic shape. The sartorius muscle, located in the thigh, is a prime example and is notably the longest muscle in the human body.
The sartorius muscle originates from the anterior superior iliac spine, a bony prominence at the front of the hip, and runs obliquely across the front of the thigh to insert onto the inner side of the tibia, just below the knee. Its actions include flexing, abducting, and laterally rotating the hip joint, as well as flexing the knee joint. When the knee is flexed, it can also medially rotate the leg.