Next Lesson - Muscles of the Upper Arm
Core
The shoulder joint is incredibly complex and is capable of a large range of movements. Therefore, to allow for maximum mobility, there is a large collection of muscles found in the shoulder region. These can be split into extrinsic and intrinsic muscles.
It is also very important when studying muscles that you are comfortable with anatomical terminology, as a lot is used in these articles. For more information, please check out our Anatomical Terminology Article.
These muscles originate from the trunk (the spine, the base of the skull and the chest wall) and attach to the shoulder. The posterior (back) muscles can be split into two layers: superficial and deep. The anterior (chest wall) muscles are covered in the pectoral region section below.
Superficial Layer of Posterior Extrinsic Muscles
The superficial layer is composed of large, powerful muscles that originate from the spine (trapezius also arises from the skull) and attach to bones of the shoulder. There are two muscles in the superficial layer:
The trapezius originates from the skull (at its base) and from the spinous processes (the bony points you can feel down the middle of the back) of the vertebrae from C7 (the seventh cervical vertebra, in the neck) down to T12 (the 12th thoracic vertebra). It attaches to the clavicle and the acromion (the bony tip of the shoulder) and spine of the scapula. It is innervated (supplied with nerves) by the spinal accessory nerve (cranial nerve XI), with proprioceptive (position-sensing) fibres from the C3 and C4 spinal nerves (the third and fourth cervical nerves). Its upper fibres elevate the scapula, its middle fibres retract it (pull it towards the spine) and its lower fibres depress it. The upper and lower fibres also work together to rotate the scapula upwards during shoulder abduction.
The latissimus dorsi originates from the spinous processes of T7 to T12, the thoracolumbar fascia (the sheet of connective tissue over the lower back) and the iliac crest (the top edge of the hip bone). It attaches to the humerus, at the intertubercular sulcus (the groove between the greater and lesser tubercles, two bony bumps at the top of the humerus). It is innervated by the thoracodorsal nerve. Latissimus dorsi acts to extend, adduct and medially rotate the humerus at the shoulder. Because these actions pull the arm down and back towards the trunk, it is an important climbing, rowing and swimming muscle.
Deep Layer of Posterior Extrinsic Muscles
There are three muscles to know in the deep layer. All three are innervated by the dorsal scapular nerve (levator scapulae also receives branches from the C3 and C4 spinal nerves):
Originates from the transverse processes of C1-C4 and attaches to the medial scapula (from its top corner, the superior angle, down to the level of the spine of the scapula). It acts to elevate the scapula (as the name suggests) and to rotate it downwards, which tilts the glenoid cavity (the socket of the shoulder joint) down.
Originates from the spinous processes of C7-T1. It attaches to the medial scapula and acts to retract the scapula and rotate it downwards.
Originates from the spinous processes of T2-T5 and attaches to the medial scapula, acting to retract the scapula and rotate it downwards.
Diagram - Illustration of extrinsic muscles of the shoulder
Creative commons source by Häggström, Mikael (2014). "Medical gallery of Mikael Häggström 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008. ISSN 2002-4436. Public Domain, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
There are three main muscles in the pectoral region:
Pectoralis Major (commonly known as Pec Major) is an important adductor of the humerus, pulling it towards the torso. It also contributes to medial rotation of the humerus and flexion of the shoulder joint, and makes up the majority of the musculature behind breast tissue. It originates from the clavicle, the sternum, and the costal cartilages (the cartilage joining each rib to the breastbone) of the upper six or seven ribs. Because it arises from the trunk (the sternum and costal cartilages) as well as the clavicle, pectoralis major counts as an extrinsic muscle. Its muscle fibres form a fan shape that converges on its attachment to the humerus. Pectoralis major is innervated by the medial and lateral pectoral nerves. These arise from the medial and lateral cords of the brachial plexus respectively (the cords are large bundles of nerve fibres).
Pectoralis Minor is a smaller muscle that sits deep to pectoralis major. It depresses (lowers) the scapula by drawing it forwards and downwards against the chest wall. This action also stabilises the scapula. It also rotates the scapula downwards and helps to protract the shoulder (move the shoulder anteriorly). It is innervated by the medial pectoral nerve (a branch of the medial cord of the brachial plexus, with a smaller contribution from the lateral pectoral nerve). Pectoralis minor arises from the front surfaces of ribs 3 to 5 and attaches to the coracoid process of the scapula (a hook-shaped projection at the front of the shoulder blade).
The serratus anterior originates from the outer surfaces of the first eight or nine ribs and attaches to the medial border of the scapula. It protracts and upwardly rotates the scapula, holding it against the chest wall. It is supplied by the long thoracic nerve (C5-C7). During pushing, normal protraction moves the scapula forwards around the rib cage. Injury to the long thoracic nerve can weaken or paralyse serratus anterior, causing medial scapular winging. The medial border of the scapula lifts away from the chest wall, and this is often more obvious when the person pushes against a wall.
These are smaller muscles that originate from the clavicle or scapula and attach to the humerus.
There are six muscles to know, four of which are part of the rotator cuff:
The deltoid is a large muscle covering the shoulder joint. It can be divided into an anterior, middle and posterior region. It originates from the clavicle, the acromion and spine of the scapula. It attaches to the humerus via the deltoid tuberosity (a rough patch on the outer side of the humeral shaft) and is innervated by the axillary nerve (a branch of the brachial plexus).
- The anterior portion is responsible for flexion and medial rotation of the shoulder.
- The middle portion is responsible for the abduction of the shoulder.
- The posterior portion is responsible for extension and lateral rotation of the shoulder.
Teres major is a small muscle. It originates from the posterior surface of the scapula, near its inferior angle (the bottom corner) and lower lateral border. It attaches to the intertubercular groove (also called the intertubercular sulcus) of the humerus. It is innervated by the lower subscapular nerve. Teres major acts to adduct and extend the shoulder and to medially rotate the arm, working with latissimus dorsi.
The following four are classified as rotator cuff muscles:
As the name suggests, supraspinatus originates from the supraspinous fossa of the scapula (the hollow above its spine) and attaches to the greater tubercle (a bony bump near the top) of the humerus. It is innervated by the suprascapular nerve. Supraspinatus helps to abduct the arm. It is traditionally taught as acting over the first 15° before the deltoid takes over, and then assisting the deltoid. However, recordings of the muscles' electrical activity show that supraspinatus switches on at about the same time as the deltoid and several other shoulder muscles, so the 15° figure is best seen as a teaching convention, not a strict hand-over.
Once again, as the name suggests, this muscle originates from the infraspinous fossa of the scapula (the hollow below its spine) and attaches to the greater tubercle of the humerus. It is innervated by the suprascapular nerve. Infraspinatus acts to laterally rotate the arm.
This muscle originates from the posterior surface of the scapula, along the upper two-thirds of its lateral border, and attaches to the greater tubercle of the humerus. It is innervated by the axillary nerve. Teres minor acts to laterally rotate the arm.
This muscle originates from the subscapular fossa (the hollow on the front surface of the scapula) and attaches to the lesser tubercle of the humerus. It is innervated by the subscapular nerves (upper and lower). Subscapularis acts to medially rotate the arm.
It is important to remember the names of the rotator cuff muscles, as they are associated with key pathologies of the shoulder. Examples are rotator cuff tears and subacromial impingement (pinching of the cuff tendons beneath the acromion). They can be remembered by the mnemonic SITS (supraspinatus, infraspinatus, teres minor, subscapularis). In that order, their main actions are: assist abduction (supraspinatus), lateral rotation (infraspinatus), lateral rotation (teres minor) and medial rotation (subscapularis).
Diagram - The posterior view of the intrinsic muscles of the shoulder. Note that the subscapularis is not seen here as it is found anteriorly. It can be located just deep to the short head of the biceps brachii.
Creative commons source by OpenStax College, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Diagram - The anterior view of the shoulder, with the subscapularis highlighted
Creative commons source by OpenStax College, edited by Dr. Sachin Sudhakaran [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
For Pathologies and Conditions of the Shoulder, check out our article on Conditions of the Shoulder and Clavicle.
Reviewed by: Dr. Thomas Burnell
Edited by: Dr. Maddie Swannack and Dr. Marcus Judge
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