The Intrinsic Muscles of the Shoulder

Original Author: Oliver Jones
Last Updated: March 24, 2017
Revisions: 23

The muscles of the shoulder are associated with movements of the upper limb. They produce the characteristic shape of the shoulder, and can be divided into two groups:

  • Extrinsic – originate from the torso, and attach to the bones of the shoulder (clavicle, scapula or humerus).
  • Intrinsic – originate from the scapula and/or clavicle, and attach to the humerus.

Note: there are other muscles that act on the shoulder joint – the muscles of the pectoral region, and the upper arm.

In this article, we shall look at the anatomy of the intrinsic muscles of the shoulder – their attachments, innervation and actions.


The intrinsic muscles (also known as the scapulohumeral group) originate from the scapula and/or clavicle, and attach to the humerus.

There are six muscles in this group – the deltoid, teres major, and the four rotator cuff muscles (supraspinatus, infraspinatus, subscapularis and teres minor).

Deltoid

Fig 1.0 - The deltoid and teres major

Fig 1.0 – The deltoid and teres major

The deltoid muscle is shaped like the Greek letter delta – Δ. It can be divided into an anterior, middle and posterior part.

  • Attachments: Originates from the scapula and clavicle, and attaches to the deltoid tuberosity on the lateral surface of the humerus.
  • Innervation: Axillary nerve.
  • Actions: The anterior fibres flex the arm at the shoulder, the posterior fibres extend the arm at the shoulder. The middle fibres are the major abductor of the arm – it takes over from the supraspinatus, which abducts the first 15 degrees.

Teres Major

The teres major forms the inferior border of the quadrangular space – the ‘gap’ that the axillary nerve and posterior circumflex humeral artery pass through to reach the posterior scapula region.

  • Attachments: Originates from the posterior surface of the inferior angle of the scapula. It attaches to the intertubercular groove of the humerus.
  • Innervation: Lower subscapular nerve.
  • Actions: Adducts at the shoulder and medially rotates the arm.

Rotator Cuff Muscles

The rotator cuff muscles are a group of four muscles that originate from the scapula and attach to the humeral head. Collectively, the resting tone of these muscles acts to ‘pull’ the humeral head into the glenoid fossa. This gives the glenohumeral joint a lot of additional stability.

In addition to their collective function, the rotator cuff muscles also have their own individual actions.

Supraspinatus

  • Attachments: Originates from the supraspinous fossa of the scapula, attaches to the greater tubercle of the humerus.
  • Innervation: Suprascapular nerve.
  • Actions: Abducts the arm 0-15o, and assists deltoid for 15-90o

Infraspinatus

  • Attachments: Originates from the infraspinous fossa of the scapula, attaches to the greater tubercle of the humerus.
  • Innervation: Suprascapular nerve.
  • Actions: Laterally rotates the arm.

Subscapularis

  • Attachments: Originates from the subscapular fossa, on the costal surface of the scapula. It attaches to the lesser tubercle of the humerus.
  • Innervation: Upper and lower subscapular nerves.
  • Actions: Medially rotates the arm.

Teres Minor

  • Attachments: Originates from the posterior surface of the scapula, adjacent to its lateral border. It attaches to the greater tubercle of the humerus.
  • Innervation: Axillary nerve.
  • Actions: Laterally rotates the arm.
Fig 1.2 - The rotator cuff muscles, which act to stablise the shoulder joint.

Fig 1.2 – The rotator cuff muscles, which act to stabilise the shoulder joint.

Clinical Relevance: Rotator Cuff Tendonitis

Rotator cuff tendonitis refers to inflammation of the tendons of the rotator cuff muscles. This usually occurs secondary to repetitive use of the shoulder joint.

The muscle most commonly affected is the supraspinatus. During abduction, it ‘rubs’ against the coraco-acromial arch. Over time, this causes inflammation and degenerative changes in the tendon itself.

Conservative treatment of rotator cuff tendonitis involves rest, analgesia, and physiotherapy. In more severe cases, steroid injections and surgery can be considered.

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Quiz

Question 1 / 8
Which nerve innervates the deltoid muscle?

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Question 2 / 8
What is the action of the anterior fibres of the deltoid?

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Question 3 / 8
Which nerve innervates teres major?

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Question 4 / 8
Which of these is an action of teres major?

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Question 5 / 8
Which nerve innervates supraspinatus?

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Question 6 / 8
What is the action of infraspinatus?

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Question 7 / 8
Which nerve innervates teres minor?

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Question 8 / 8
Which muscle is typically affected in rotator cuff tendonitis?

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