The subclavius muscle is a small, triangular-shaped muscle located between the clavicle and the first rib. Together with the pectoralis major and pectoralis minor, it forms the anterior axioappendicular muscle group. These muscles form the anterior wall of the axilla and provide support for upper extremity movements.
Anatomy
Morphology and Connection Points
The subclavius muscle is a small but functionally important muscle located in the upper anterior portion of the thoracic girdle, between the clavicle and the first rib. It originates from a short, thick tendinous structure located on the superior surface of the first rib, at its bony junction with its cartilaginous portion. This origin is located just anterior to the costoclavicular ligament. From here, the muscle fibers extend obliquely superolaterally and insert into a groove on the inferior surface of the clavicle, known as the sulcus musculi subclavii. This location provides the subclavius muscle with both a mechanical and stabilizing function. It supports the stability of the sternoclavicular joint by connecting the clavicle to the rib cage. It also controls the shoulder girdle by limiting excessive movement of the clavicle. This structural location also allows the muscle to act as a protective layer over the neurovascular structures (brachial plexus and subclavian vessels) located in the thoracic outlet region. Therefore, the subclavius muscle is considered not only a movement muscle but also an important anatomical cushion and supporting structure.
Embryological Origin and Classification
The subclavius muscle develops embryologically from muscle groups of somitic origin derived from paraxial mesoderm. As a transitional muscle between the scapulohumeral region and the thorax, it shares functional relationships with both the upper extremity and trunk muscles. Anatomical variations are common. In some individuals, the muscle fibers may insert on the coracoid process instead of the clavicle, or on both the clavicle and the coracoid process. A rare variant, the sternoscapular fasciculus, causes the muscle to extend to the superior border of the scapula. Furthermore, a variant called the sternoclavicularis muscle can extend between the manubrium sterni and the clavicle, positioning itself between the pectoralis major muscle and the coracoclavicular fascia. The subclavius muscle may be completely absent in some individuals.
Nerve Innervation and Vascular Supply
The muscle is innervated by the subclavius nerve. This nerve originates from the upper trunk of the brachial plexus, specifically from the junction of the C5 and C6 spinal nerve roots. The muscle's vascular supply is provided by the clavicular branch of the thoracoacromial artery and branches of the subclavian artery. The presence of nerves and vessels surrounding the muscle increases the clinical sensitivity of this area.
Function and Clinical Relationships
The subclavius muscle is a small but functionally important muscle that plays a role in stabilizing the shoulder girdle. Its primary function is to stabilize the clavicle by providing balance on the structures connecting the clavicle to the sternum. It pulls the clavicle downward, limiting excessive upward movement of the clavicle, particularly during arm elevation, forward extension, or active use of the shoulder. This effect contributes to the biomechanical balance of the shoulder joint and reduces loading on the sternoclavicular joint. When contracted on a fixed clavicle, the subclavius muscle can pull the first rib upward. This function can serve as an accessory respiratory muscle, particularly during labored breathing. This feature may be more pronounced in individuals with chronic lung disease or those with increased respiratory demands due to exertion. Clinically, the subclavius muscle also plays a role in protecting important neurovascular structures. The brachial plexus and subclavian vessels, located just below the clavicle, can be cushioned by this muscle in traumatic situations. Particularly in clavicle fractures, the cushioning effect provided by the muscle can prevent or mitigate direct damage to vessels and nerves. However, in some cases, hypertrophy or spasm of the subclavius muscle can contribute to the clinical picture known as thoracic outlet syndrome (TOS). The muscle's pressure on surrounding tissues can cause compression of the brachial plexus or subclavian vessels. This can lead to neurological symptoms such as numbness, tingling, weakness, and pain in the upper extremity, as well as circulatory disturbances. Consequently, the subclavius muscle plays a crucial role both in coordinating movement in the shoulder girdle and protecting vital neurovascular structures. Due to its anatomical and clinical characteristics, it is a muscle that requires careful evaluation, especially in cases of trauma, postural disorders, and thoracic outlet syndrome.
