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Deltoid Muscle Anatomy

Quick Overview View Flashcards
Origin
  • Anterior head: Lateral quarter of clavicle & anterior third of lateral acromion
  • Intermediate head: Middle third of lateral aspect of acromion
  • Posterior head: Posterior third of acromion & spine of scapula
Insertion
  • Deltoid tuberosity of the humerus
Action
  • Shoulder abduction
  • Shoulder flexion
  • Shoulder extension
  • Horizontal adduction
  • Horizontal abduction
  • Medial rotation
  • Lateral rotation
Innervation
  • Axillary nerve (C5, C6)
Blood Supply
  • Thoracoacromial branch of the axillary artery (primary artery)
  • Posterior circumflex humeral artery
  • Deltoid branches of the profunda brachii artery
Last updated: Estimated Reading Time: 13 min

Location & Overview

The deltoid muscle, named for its distinctive triangular shape, is a large muscle located in the upper extremity. It covers the shoulder joint, giving the shoulder its rounded contour and playing an important role in various arm movements.[1,2]

This muscle, positioned in the lateral aspect of the shoulder, can be conceptually divided into three distinct portions based on their fiber orientation: the anterior (clavicular), lateral (acromial), and posterior (spinal) portions. Each of these divisions has specific roles in the movement and stabilization of the shoulder joint.[3] However, a more intricate view of the deltoid’s structure reveals that it consists of seven segments, each separated by their intramuscular tendons. This anatomical configuration suggests a more complex mechanism of action than just the contribution of the broadly defined three segments, thereby providing a more precise picture of the deltoid’s role in shoulder dynamics.[4] This is explored in further detail in the Origin & Insertion section.

The deltoid muscle is situated superficially, making it the most prominent muscle of the shoulder and upper arm. It envelops the shoulder joint, offering a protective layer to the deeper structures of the shoulder. Some neighboring muscles include the trapezius posteriorly, pectoralis major anteriorly, and latissimus dorsi inferiorly.[5,2]

A unique characteristic of the deltoid muscle is its extensive overlap with the rotator cuff muscles. These muscles work in a coordinated manner to balance the forces acting on the shoulder joint and provide stability during arm movements. The rotator cuff muscles are the supraspinatus, infraspinatus, teres minor, and subscapularis. These muscles can be remembered using the mnemonic ‘SITS’.[5,2]

anterior head of the deltoid
Pictured here we can see the deltoid muscle from an anterior view.
middle head of the deltoid
Pictured here we can see the deltoid muscle from a lateral view.
posterior head of the deltoid
Pictured here we can see the deltoid muscle from a posterior view.
deltoid muscle highlighted in red among the other muscles of the body, anterior view
Here we can see the deltoid muscle highlighted in red among the other muscles of the body, seen from an anterior view.
deltoid muscle highlighted in red among the other muscles of the body, posterior view
Here we can see the deltoid muscle highlighted in red among the other muscles of the body, seen from a posterior view.

Origin & Insertion

The deltoid muscle is known for its distinctive ‘U’-shaped origin, spanning across the clavicle, acromion, and the spine of the scapula. These points include the lateral third of the clavicle, the lateral acromion, and the spine of the scapula.[5,1]

The clavicle, also known as the collarbone, is a horizontal bone at the base of the neck. The acromion is a bony process on the scapula and serves as a connection between the clavicle and the shoulder blade, creating the acromioclavicular joint. The spine of the scapula is a prominent bony ridge that runs across the back of the shoulder blade, providing an attachment point for various muscles, including the deltoid.[5,1]

The deltoid muscle is typically further divided into three segments: the anterior, middle, and posterior segments. The anterior segment originates from the clavicle and the anterior third of the lateral acromion. The middle deltoid originates from the middle third of the lateral aspect of the acromion. The posterior deltoid originates from the posterior third of the acromion and the spine of the scapula. The deltoid muscle extends from these origins across the shoulder joint to insert onto the deltoid tuberosity of the humerus, which is a rough, raised area on the lateral aspect of the humerus.[4]

As previously noted, the deltoid muscle is technically composed of seven segments rather than the traditionally identified three. This segmentation is determined by the presence of three intramuscular tendons, each of which inserts into the deltoid tuberosity on the humerus. From this point of insertion, these tendons extend upward toward the defined origins.[4]

In the region of the proximal deltoid, the tendons associated with the anterior and posterior segments of the deltoid divide, producing three additional intramuscular tendons. These are then somewhat evenly distributed across the origins corresponding to their traditional segments. To clarify: the anterior deltoid presents three intramuscular tendons at the proximal end of the muscle, the middle deltoid typically displays one, and the posterior deltoid offers three. This culminates in a total of seven intramuscular tendons. However, at the distal end of the muscle, the six intramuscular tendons of the anterior and posterior deltoid segments consolidate into two intramuscular tendons and join the single middle deltoid tendon to insert on the deltoid tuberosity.[4]

It is also worth mentioning that anatomical variations can occur, where some individuals may present an additional intramuscular tendon associated with the middle deltoid. However, such instances are relatively uncommon. Sakoma et al. (2011) found this occurrence to be 6.7% in their study of 30 cadavers (15 males and 15 females).[4]

deltoid intramuscular tendons
Pictured here we can see the seven intramuscular tendons of the deltoid muscle. The anatomy of these tendons is my interpretation of the images from the study conducted by Sakoma et al. (2011).[4] Full reference details are provided in the references section at the bottom of the page, click the numerical citation to go straight to it.
origin of the deltoid muscle
Here we can see the origins of the deltoid muscle highlighted in red. The two black lines divide the origins of the three heads of the deltoid, demonstrating their respective locations.
insertion of the deltoid on the deltoid tuberosity of the humerus
Here we can see the insertion of the deltoid muscle highlighted in blue, on the deltoid tuberosity of the humerus.

Action & Function

The deltoid’s actions can be divided into its three sections: anterior, middle, and posterior. The deltoid plays a crucial role in facilitating a wide range of shoulder movements. The deltoid can perform shoulder abduction, taking over from the rotator cuff muscles after the initial 15 degrees of movement to raise the arm from 15 to 100 degrees.[3] Shoulder abduction is primarily produced by the middle and anterior deltoid sections. The posterior deltoid also contributes, but to a lesser extent.[6] The anterior deltoid is slightly more active than the middle deltoid during abduction within the scapular plane, though both contribute a very similar amount to the movement.[6]

The anterior deltoid contributes to shoulder flexion and horizontal adduction, and also assists with medial rotation (internal rotation). In contrast, the posterior deltoid contributes to shoulder extension and horizontal abduction, while assisting with lateral rotation (external rotation).[7,8,9]

The deltoid’s role at the shoulder joint also includes stabilization. All of the deltoid’s sections also work together to prevent inferior displacement of the glenohumeral joint when they contract together.[3] An example of this is during a deadlift, when the humerus is pulled toward the bar by the heavy load. The deltoid assists in keeping the humerus in place in the glenoid cavity by pulling the humerus superiorly.

Image of a person demonstrating the movement of shoulder abduction.
This image demonstrates shoulder abduction. Shoulder abduction involves lifting your arm out to the side of your body. The opposite of abduction is adduction, which involves bringing your arm back toward your body. The deltoid muscle can abduct the shoulder.
Image of a person demonstrating the movements of shoulder flexion and extension.
This image shows both shoulder flexion and extension. Flexion is when the arm is moved forward and upward in an anterior direction. The opposite of this is shoulder extension, which occurs as the arm is moved backward in a posterior direction. The anterior portion of the deltoid contributes to shoulder flexion, whereas the posterior portion contributes to shoulder extension.
Image of a person demonstrating the movement of horizontal shoulder abduction and adduction.
This image demonstrates the action of horizontal abduction and adduction. Horizontal abduction involves moving your arm away from the midline of your body in the transverse plane, such as extending your arms out to the sides from a starting position in front of your body. Conversely, horizontal adduction involves moving your arm toward the midline of your body in the same transverse plane, like bringing your arms back in toward each other. The anterior section of the deltoid is involved in horizontal adduction, whereas the posterior section is involved in horizontal abduction.
Image of a person demonstrating the movements of shoulder internal and external rotation.
This image demonstrates the movements of shoulder internal and external rotation. Internal rotation involves rotating your arm toward the center of your body. In contrast, external rotation involves rotating your arm away from your body, toward the outside. The deltoid muscle is involved in both internal and external rotation of the shoulder. The anterior section is usually responsible for internal rotation, and the posterior section is usually responsible for external rotation.

Innervation

The deltoid muscle’s primary innervation stems from the axillary nerve, an essential peripheral nerve of the upper limb. The axillary nerve is primarily composed of nerve fibers originating from the C5 and C6 nerve roots of the cervical spine. These segments form part of the upper region of the spinal cord, colloquially known as the neck region. The neurons that emerge from these roots ultimately give rise to the axillary nerve, which branches off from the posterior cord of the brachial plexus, a complex network of nerves located in the shoulder area.[10,3,5]

The axillary nerve subsequently travels laterally, passing through the quadrangular space alongside the posterior circumflex humeral artery. Upon reaching the deltoid muscle, the axillary nerve divides into anterior and posterior branches, penetrating the muscle from its deep surface. This division allows the nerve fibers to spread throughout the muscle, contributing to its wide-ranging control of shoulder movement.[10,3,5]

The anterior branch of the axillary nerve primarily provides innervation to the anterior and middle segments of the deltoid muscle. In contrast, the posterior branch mainly serves the posterior segment. Moreover, the nerve’s extensive branching within the muscle ensures an efficient distribution of nerve fibers, facilitating the deltoid’s various actions.[10,3,5]

Image of the axillary nerve highlighted in red
Pictured here you can see the axillary nerve highlighted in red. The deltoid muscle is innervated by the axillary nerve.
Image of the axillary nerve and the brachial plexus
Pictured here you can see the axillary nerve alongside the brachial plexus. The deltoid muscle is innervated by the axillary nerve.

Blood Supply

The deltoid muscle’s blood supply is primarily provided by the thoracoacromial artery, a branch that stems from the axillary artery. The thoracoacromial artery arises from the second part of the axillary artery, which lies posterior to the pectoralis minor muscle, before branching to supply multiple regions, including the deltoid muscle.[11,3]

In its course, the thoracoacromial artery gives off multiple branches specifically known as the deltoid branches. These branches run in close proximity to the cephalic vein, located within the deltopectoral groove, a visible depression between the deltoid and pectoralis major muscles. This vascular network ensures the deltoid muscle receives a steady and reliable blood supply.[11,1]

While the thoracoacromial artery is the primary arterial supplier to the deltoid muscle, it is not the only contributor. Additional blood supply comes from the posterior circumflex humeral artery and the profunda brachii artery. However, these vessels play a much lesser role than the thoracoacromial artery. The posterior circumflex humeral artery, although not the predominant provider, gives off terminal branches that bridge the gap between the deltoid muscle and the proximal humerus, enriching the muscle with the necessary nutrients and oxygen for its function.[11,1]

References

  1. [1] Standring S. (2015). Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 41st Edn. Amsterdam: Elsevier.
  2. [2] Peterson SL, Rayan GM. Shoulder and upper arm muscle architecture. J Hand Surg Am. 2011 May;36(5):881-9. doi: 10.1016/j.jhsa.2011.01.008. PMID: 21527142.
  3. [3] Elzanie A, Varacallo M. Anatomy, Shoulder and Upper Limb, Deltoid Muscle. (Updated 2022 May 15). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537056/
  4. [4] Sakoma Y, Sano H, Shinozaki N, Itoigawa Y, Yamamoto N, Ozaki T, Itoi E. Anatomical and functional segments of the deltoid muscle. J Anat. 2011 Feb;218(2):185-90. doi: 10.1111/j.1469-7580.2010.01325.x. Epub 2010 Nov 30. PMID: 21118198; PMCID: PMC3042752.
  5. [5] Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. 8th ed. Philadelphia: Lippincott Williams & Wilkins; 2017.
  6. [6] Ackland DC, Pak P, Richardson M, Pandy MG. Moment arms of the muscles crossing the anatomical shoulder. J Anat. 2008 Oct;213(4):383-90. doi: 10.1111/j.1469-7580.2008.00965.x. Epub 2008 Aug 6. PMID: 18691376; PMCID: PMC2644775.
  7. [7] Miniato MA, Anand P, Varacallo M. Anatomy, Shoulder and Upper Limb, Shoulder. (Updated 2022 Jul 25). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK536933/
  8. [8] Yasojima T, Kizuka T, Noguchi H, Shiraki H, Mukai N, Miyanaga Y. Differences in EMG activity in scapular plane abduction under variable arm positions and loading conditions. Med Sci Sports Exerc. 2008 Apr;40(4):716-21. doi: 10.1249/MSS.0b013e31816073fb. PMID: 18317372.
  9. [9] Kuechle DK, Newman SR, Itoi E, Morrey BF, An KN. Shoulder muscle moment arms during horizontal flexion and elevation. J Shoulder Elbow Surg. 1997 Sep-Oct;6(5):429-39. doi: 10.1016/s1058-2746(97)70049-1. PMID: 9356931.
  10. [10] Gurushantappa PK, Kuppasad S. Anatomy of axillary nerve and its clinical importance: a cadaveric study. J Clin Diagn Res. 2015 Mar;9(3):AC13-7. doi: 10.7860/JCDR/2015/12349.5680. Epub 2015 Mar 1. PMID: 25954611; PMCID: PMC4413059.
  11. [11] Smith CD, Booker SJ, Uppal HS, Kitson J, Bunker TD. Anatomy of the terminal branch of the posterior circumflex humeral artery: relevance to the deltopectoral approach to the shoulder. Bone Joint J. 2016 Oct;98-B(10):1395-1398.

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