Subclavius Muscle: Origin, Insertion, Action, Innervation

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The subclavius muscle (m subclavius) is a small muscle that runs from the first rib and its costal cartilage to the inferior surface of the clavicle, and it is innervated by the nerve to subclavius from the upper trunk of the brachial plexus (C5-C6). Its action is to depress and stabilize the clavicle, resisting upward displacement at the sternoclavicular joint.

This muscle matters out of proportion to its size. It sits in the costoclavicular space, the narrow gap between the clavicle above and the first rib below, through which the subclavian vessels and the brachial plexus pass. A muscle in that gap is a muscle that can compress those structures, and it can also be a landmark for anesthesia and venous access. In domestic mammals the picture is different from the human one, because the clavicle itself is reduced or absent in most species. A student who learns only the human anatomy of the subclavius will be confused the first time they dissect a dog.

Summary Table

Where the Subclavius Sits and What It Is Made Of

The subclavius is a short, flat, triangular muscle. It lies in the infraclavicular fossa, the depression you can feel just below the collarbone. Its fibers run roughly horizontally and slightly upward and laterally from the first rib to the underside of the clavicle.

The muscle is enclosed in the clavipectoral fascia, a sheet of deep fascia that also invests the pectoralis minor and the axillary vessels. This fascial envelope is one reason the subclavius is easy to overlook during dissection. It does not stand out as a separate fleshy mass the way the pectoralis major does. Instead it appears as a thin strip of muscle plastered against the deep surface of the clavicle.

The clavicle itself is a membrane bone with a curious developmental history. In humans, endochondral ossification occurs at both the lateral and medial ends of the clavicle, while in the mouse the medial half of the clavicle anlage ossifies through endochondral bone formation with a homogenous eosinophilic matrix [1]. The pig is different again. The pig clavicle appears to lose its endochondral parts because the manubrium sterni and the acromion are shifted caudally during development [1]. That single developmental difference helps explain why the pig ends up with a fibrous vestigial clavicle while still carrying a subclavius muscle that extends from the first rib toward the supraspinatus fascia [1].

For the veterinary student, the practical point is this. The muscle and the bone do not always change together. A species can keep the muscle while losing most of the bone.

Origin and Insertion in Detail

Origin

The subclavius arises from the cranial surface of the first rib and from the costal cartilage of the first rib, close to the sternal end. In most human descriptions the origin is a short tendinous attachment that blends with the periosteum of the rib and the perichondrium of the cartilage.

Variation at the origin is real and worth knowing. In a scoping review and meta-analysis of 47 articles, the subclavius posticus variant originated from the sternal end of the first rib in most reported cases [2]. A separate case report described an aberrant subclavius posticus arising anteromedially from the costoclavicular ligament, running posterolaterally over the trunks of the brachial plexus, and inserting with the omohyoid [3]. The costoclavicular ligament is the strong band that binds the clavicle to the first rib at the sternoclavicular region.

Insertion

The main insertion is into a shallow groove on the inferior surface of the clavicle, in the middle third of the bone. This groove is sometimes called the subclavian groove or the groove for the subclavius.

Beyond the clavicle, there are accessory attachments that are more common than most textbooks suggest. In 52 upper extremities from 26 human cadavers, fibrous structures derived from the subclavius and inserting into the coracoid process were found in 49 of the 52 specimens [4]. The coracoid process is the hook-shaped bony projection on the scapula that serves as an attachment point for several muscles and ligaments. The authors argued that this coracoid attachment is more common than generally appreciated and that the function of the subclavius is therefore more complex than simple clavicular depression [4].

A Thai dissection study of 128 upper limbs classified subclavius insertion into four types. Type I, the classic insertion into the groove on the inferior clavicle, accounted for 64.06 percent. The remaining 35.94 percent showed progressively more extensive insertions reaching toward the conoid ligament, the coracoid process, the superior transverse scapular ligament, and the superior border of the scapula near the omohyoid insertion [5]. The conoid ligament is the medial part of the coracoclavicular ligament.

The takeaway is that the subclavius is not a tidy two-point muscle in every body. It has a spectrum of scapular attachments, and those attachments are the anatomical basis for the subclavius posticus variant.

The Subclavius Posticus Variant

The subclavius posticus is a supernumerary muscle that lies posterior to the normal subclavius. It typically runs from the first rib to the superior border of the scapula. In a meta-analysis of 47 articles, the pooled prevalence was 11 of 2069 cases, or 4.9 percent. Cadaveric studies gave 10 of 1369, or 5.1 percent, and MRI studies gave 1 of 700, or 5.0 percent [2]. The muscle averaged 12 cm in length and 1 cm in width in that pooled analysis [2].

A separate MRI study of 350 patients found the subclavius posticus in 29 patients, or 8.3 percent, with bilateral occurrence in 6 patients, or 1.7 percent. The mean muscle thickness was 6.10 mm with a range of 2.5 to 10 mm. In 21 sides, or 60 percent, the muscle and the brachial plexus were touching. In the remaining 14 sides, the mean distance between muscle and plexus was 2.32 mm, ranging from 1.7 to 3.6 mm [6].

Those numbers explain the clinical interest. A muscle that touches the brachial plexus in most of the people who have it is a muscle that can compress the plexus.

Innervation of the variant is the key to understanding what it is. In a reported case, a subclavius posticus arising from the first costal cartilage and inserting on the upper margin of the scapula was innervated by a branch from the nerve to the subclavius [7]. That shared nerve supply is why the variant is classified with the subclavius rather than with the omohyoid, even though it sometimes intermingles with the omohyoid insertion [8].

Action and Function

The classic description of subclavius action is depression and stabilization of the clavicle. The muscle pulls the clavicle downward and forward and resists upward displacement at the sternoclavicular joint.

Electromyography supports the stabilizing role. In a study of 31 different movements in 12 subjects, action potentials suggested that the subclavius acts mainly on the stability of the sternoclavicular joint, with intensity varying according to how much the clavicle interacts with movement of the more distal parts of the upper limb. The authors concluded that the muscle seems to act as a substitute for the ligaments of the sternoclavicular joint [9].

The word substitute is the important one. The sternoclavicular joint is a saddle joint with a fibrocartilaginous disc, and it is stabilized primarily by ligaments. The subclavius adds active, adjustable support on top of that passive support.

The Dynamic View

A study in spider monkeys challenged the purely static view. Electromyographic monitoring during brachiation, vertical climbing, pronograde quadrupedalism, and leaping suggested that the subclavius acts as a dynamic element in movements of the pectoral girdle rather than a purely static stabilizer [10]. The same study found complementary activity between the subclavius and the caudal fibers of the trapezius. The subclavius was recruited when the animal needed to depress or resist cranial displacement of a protracted shoulder girdle, while the caudal trapezius was recruited when the girdle was retracted against the chest wall [10].

Protraction means movement of the shoulder girdle forward, and retraction means movement backward. The pairing makes mechanical sense. A muscle that pulls the clavicle down and forward is useful when the girdle is already forward and something needs to keep it from riding up.

For veterinary students, this comparative finding is a reminder that small muscles rarely have only one job. The static description is a starting point, not the whole story.

Innervation

The subclavius is innervated by the nerve to subclavius. This is a small branch that arises from the upper trunk of the brachial plexus, carrying fibers from the C5 and C6 spinal roots. The upper trunk is formed by the union of the C5 and C6 ventral rami. The nerve to subclavius is one of the first branches given off, and it runs downward and laterally to reach the deep surface of the muscle.

The nerve is small and easily destroyed during dissection. In a practical anatomy lab, the standard approach is to identify the upper trunk first, then look for a fine filament passing down toward the first rib. If the filament is lost, the muscle can still be identified by its position under the clavicle.

The shared innervation is also the reason the subclavius posticus is grouped with the subclavius. In one case the variant was innervated by a branch from the nerve to the subclavius [7]. In another, a hybrid muscle with features of both subclavius and subclavius posticus was supplied by the nerve to the subclavius [8]. One reported case described innervation from the suprascapular nerve instead [11], which shows that the variant is not perfectly consistent.

Species Presence in Domestic Mammals

This is where veterinary anatomy diverges sharply from human anatomy.

Dogs and Cats

The clavicle in the dog is a small, thin, rudimentary bone or cartilaginous nodule embedded in the brachiocephalic muscle, and it does not articulate with the sternum or the scapula. In the cat the clavicle is slightly better developed but still small and functionally unimportant for weight support. Because the bone is so reduced, the subclavius is variably present and often vestigial. When present, it is a thin slip running from the first rib toward the region of the clavicular intersection.

The practical consequence is that you cannot rely on finding a well-defined subclavius in every dog or cat. Its presence varies between individuals, and in many specimens it is a few pale fibers that are easy to mistake for fascia.

Pigs

The pig carries a fibrous vestigial clavicle, but a subclavius muscle is present and extends between the first rib and the fascia over the supraspinatus muscle [1]. This is a useful comparative example because it shows the muscle persisting in a species where the bone has become a fibrous band.

Humans

In humans the clavicle is a full bony strut and the subclavius is consistently present and well developed. Most of the clinical literature on the subclavius, including fracture management and thoracic outlet syndrome, comes from human medicine.

Why the Difference Exists

The developmental study comparing human, pig, and mouse fetuses offers an explanation. In all three species, the initial subclavius extends between the cartilaginous first rib and a mesenchymal clavicle. At the early stage, human and mouse fetuses have a mesenchymal manubrium sterni above the heart bulbus and an acromion above the humeral head. In pig fetuses, the manubrium remains far caudal to the first rib and the acromion sits on the laterocaudal side of the glenohumeral joint. In place of a well-positioned acromion, the pig has a large supraspinatus that covers the humeral head. By midterm, the human and mouse subclavius attaches to membranous bone of the clavicle, while the pig clavicle appears to lose its endochondral parts [1].

The lesson is that clavicle reduction and subclavius reduction are related but not locked together. Comparative anatomy rewards attention to both.

How the Subclavius Is Studied and Observed

Dissection

The standard approach is to reflect the pectoralis major and then the clavipectoral fascia. The subclavius appears as a thin muscle on the deep surface of the clavicle. Its costal origin is confirmed by following it to the first rib. The nerve to subclavius is traced back to the upper trunk.

Electromyography

Fine-wire or needle electrodes placed in the muscle allow recording during defined movements. This is how the stabilizing and dynamic functions were characterized [10][9]. In veterinary practice this is a research technique, not a routine diagnostic test.

Imaging

High-resolution magnetic resonance imaging can identify the subclavius and, more importantly, the subclavius posticus. One case of suprascapular nerve compression in an athlete was diagnosed by MRI, and the authors recommended high-resolution MRI for evaluating isolated suprascapular neuropathy [12]. The suprascapular nerve supplies the supraspinatus and infraspinatus muscles and carries sensation from the shoulder joint.

Computed tomography angiography and MRI have both been used to identify an aberrant subclavius posticus compressing the subclavian vein [13]. Dynamic magnetic resonance angiography, which images the vessels during movement, has been used to show compression that only appears in certain arm positions [14].

Ultrasound

Ultrasound is used clinically to guide needle placement near the subclavius. The anatomical relationship between the muscle and the subclavian vein is close enough that ultrasound guidance matters, as discussed below [15].

Clinical Relevance, Limitations and Common Mistakes

Clavicle Fractures and Anesthesia

Clavicle fractures are common, and surgical fixation requires anesthesia of the region. Interscalene brachial plexus block combined with supraclavicular nerve block has been used, but it carries adverse events. A newer technique, the subclavius muscle plane block, targets the sensory nerves of the surgical region more selectively. A randomized non-inferiority trial protocol compares the subclavius muscle plane block plus supraclavicular nerve block against interscalene brachial plexus block plus supraclavicular nerve block in 76 patients scheduled for open reduction and internal fixation of clavicle fractures [16]. The primary outcome is the proportion of patients not requiring intraoperative sufentanil supplementation [16]. This is human clinical research, but it illustrates why the subclavius is a useful anatomical plane. It sits between the superficial tissues and the deeper neurovascular bundle.

Subclavian Vessel Relations and Catheter Pinch-Off

The subclavian vein runs anterior to the subclavius, and the subclavian artery and brachial plexus run posterior to it. That arrangement has direct procedural consequences.

Catheter pinch-off syndrome occurs when a central venous catheter is crimped between the clavicle and the first rib, or entrapped where it penetrates the subclavius. In a cadaver study of 28 adults, the subclavius and subclavian vein were tightly adherent in 72 percent of specimens, partly adherent in 14 percent with a mean distance of 4.5 mm, and loosely connected in 14 percent with a mean distance of 6.1 mm [15]. The authors concluded that the lateral insertion technique may not prevent penetration of the muscle and that real-time ultrasound guidance may prevent it [15].

The clinical message is straightforward. If you are placing a line below the clavicle, know where the subclavius is, and use ultrasound.

Thoracic Outlet Syndrome

Thoracic outlet syndrome is compression of the neurovascular bundle as it leaves the thorax. The subclavius posticus is one recognized cause.

Neurogenic compression has been reported repeatedly. One case described a 32-year-old man with weakness in radial deviation of the left hand and an extra muscle in the costoclavicular space identified as subclavius posticus on MRI [17]. Another described an athlete with incapacitating suprascapular nerve compression from a subclavius posticus, relieved by decompression and excision of the muscle [12]. A third described a 30-year-old woman with thoracic outlet compression from a subclavius posticus accompanied by a caudally inserted middle scalenus muscle on the second rib, shown on CT angiography and MRI and treated surgically [18].

Venous compression is also reported. One case described a 40-year-old man who developed deep vein thrombosis of the left upper limb after repetitive overhead activity. Imaging identified an aberrant muscle from the first rib to the superior border of the scapula compressing the subclavian vein, consistent with subclavius posticus. He was treated conservatively with anticoagulants and physical therapy, and at final follow-up the thrombus had recanalized completely and he was symptom free [13]. Another report described two young overhead athletes with bilateral subclavius posticus in whom thoracic outlet syndrome appeared only in the dominant limb, where hypertrophy of the lateral cervical muscles from sport combined with the variant to narrow the outlet [14].

The normal subclavius itself can also participate. A retrospective study of patients with neurogenic thoracic outlet syndrome and MRI signs of nerve impingement compared those who had the subclavius spared during surgery with those who had it divided. The study investigated whether dividing or sparing the muscle affected outcomes [19]. This is the strongest evidence that the ordinary subclavius, not just the variant, can compress the brachial plexus in some patients.

Common Mistakes Students Make

The first mistake is assuming the subclavius is always present in dogs and cats. It is not. The rudimentary clavicle means the muscle is variable.

The second mistake is describing the subclavius as only a clavicular depressor. The coracoid attachments found in 49 of 52 specimens [4] and the dynamic activity patterns in primates [10] both argue for a broader functional description.

The third mistake is confusing the subclavius with the subclavius posticus. The posticus is a variant that runs to the scapula, not just to the clavicle, and it is the variant that most often causes compression symptoms.

The fourth mistake is forgetting the vessel relations. The vein is anterior, the artery and plexus are posterior. Getting that backwards will make you misread every cross-sectional image.

The fifth mistake is treating the nerve supply as obvious. The nerve to subclavius is small and comes off the upper trunk, not the cords or the posterior division. Students who look for it in the wrong place will not find it.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Review

1. Origin: first rib and its costal cartilage, near the sternal end.
2. Insertion: inferior surface of the clavicle in the subclavian groove, with common accessory slips to the coracoid process and scapula.
3. Action: depresses and stabilizes the clavicle and resists upward displacement at the sternoclavicular joint.
4. Innervation: nerve to subclavius from the upper trunk of the brachial plexus, C5-C6.
5. Relations: subclavian vein anterior, subclavian artery and brachial plexus posterior.
6. Species: consistent and well developed in humans, variably present and often vestigial in dogs and cats because the clavicle is rudimentary, present with a fibrous clavicle in pigs.
7. Variant: subclavius posticus runs from the first rib to the superior border of the scapula and is a recognized cause of thoracic outlet compression.

Frequently Asked Questions

What is the origin and insertion of the subclavius muscle?

The subclavius originates from the first rib and its costal cartilage near the sternal end and inserts into a groove on the inferior surface of the clavicle. Accessory attachments to the coracoid process and superior scapular border are common.

What nerve supplies the subclavius muscle?

The nerve to subclavius supplies it. This branch arises from the upper trunk of the brachial plexus and carries C5 and C6 fibers.

What does the subclavius muscle do?

It depresses and stabilizes the clavicle and resists upward displacement at the sternoclavicular joint. Electromyographic evidence supports a role as an active stabilizer that supplements the joint ligaments.

Is the subclavius present in dogs and cats?

It is variably present and often vestigial. Dogs and cats have a rudimentary clavicle, so the muscle is reduced or absent in many individuals.

What is the subclavius posticus?

It is a supernumerary variant of the subclavius that runs from the first rib to the superior border of the scapula. It is present in roughly 5 percent of people in pooled anatomical studies and can compress the brachial plexus, subclavian artery, or subclavian vein.

Why does the subclavius matter clinically?

It sits in the costoclavicular space with the subclavian vessels and brachial plexus, so it is relevant to clavicle fracture anesthesia, central line placement and catheter pinch-off, and thoracic outlet syndrome.

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Sources

1. Comparative anatomy of subclavius muscle and clavicle: a histological study using human, pig and mouse foetuses.
2. Prevalence and anatomy of the anomalous subclavius posticus muscle and its clinical implications with emphasis in neurogenic thoracic outlet syndrome: Scoping review and meta-analysis.
3. An Aberrant Subclavius Posticus Muscle: A Case Report.
4. The anatomy and variation of the coracoid attachment of the subclavius muscle in humans.
5. Supernumerary subclavius muscle in Thais: predisposing cause of thoracic outlet syndrome.
6. The prevalence and characteristics of the subclavius posticus muscle in the adult population on MRI.
7. A case of muscle subclavius posticus with special reference to its innervation.
8. Bilateral variation of subclavius muscle resembling subclavius posticus.
9. Electromyographic study of the subclavius muscle.
10. Function of the subclavius muscle in a nonhuman primate, the spider monkey (Ateles).
11. A case with subclavius posticus muscle.
12. Subclavius posticus: an anomalous muscle in association with suprascapular nerve compression in an athlete.
13. Venous thoracic outlet syndrome in a patient with an aberrant subclavius posticus muscle: a case report.
14. Thoracic Outlet Syndrome in the Overhead Athlete: A Report of 2 Cases of Subclavius Posticus Muscle.
15. The Relationship of the Subclavius Muscle with Relevance to Venous Cannulation below the Clavicle.
16. Comparison of anesthesia efficacy between subclavius muscle plane block and interscalene brachial plexus block for open reduction and internal fixation of clavicle fractures: study protocol for a randomized non-inferiority clinical trial.
17. Neurogenic thoracic outlet syndrome due to subclavius posticus muscle with dynamic brachial plexus compression: a case report.
18. The subclavius posticus muscle: an unusual cause of thoracic outlet syndrome.
19. Improved functional outcome in NTOS patients following resection of the subclavius muscle with radiological signs of nerve impingement: indication of participation of the subclavius in brachial plexus compression.