Muscles of the Shoulder

1. Introduction
2. Extrinsic Muscles
3. Superficial Layer of Posterior Extrinsic Muscles
   
   • Trapezius
   • Latissimus Dorsi
4. Deep Layer of Posterior Extrinsic Muscles
   
   • Levator Scapulae
   • Rhomboid Minor
   • Rhomboid Major
5. Anterior or Pectoral Region
   
   • Pectoralis Major
   • Pectoralis Minor
   • Serratus Anterior
6. Intrinsic Muscles
   
   • Deltoid
   • Teres Major
7. Rotator Cuff Muscles
   
   • Supraspinatus
   • Infraspinatus
   • Teres Minor
   • Subscapularis
8. Quiz

The shoulder joint is incredibly complex and can perform a wide range of movements. To allow for this, there are a large number of muscles that act at the shoulder joint. These muscles can be split into intrinsic and extrinsic muscles:

• Extrinsic - the extrinsic muscles can be split into:
  • Superficial Posterior - trapezius and latissimus dorsi.
  • Deep Posterior - levator scapulae, rhomboid major, rhomboid minor.
  • Anterior - pectoralis major, pectoralis minor, serratus anterior. 
• Intrinsic - Deltoid, teres major, teres minor, subscapularis, supraspinatus, infraspinatus.

Four of the intrinsic shoulder muscles are known as the rotator cuff muscles, and these are teres minor, subscapularis, supraspinatus and infraspinatus.

Introduction

The shoulder joint is incredibly complex and is capable of a large range of movements. Therefore, to allow for maximum mobility, there are 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. 

Extrinsic Muscles

These muscles originate from the posterior torso and attach to the shoulder. They can be split into two layers; superficial and deep.

Superficial Layer of Posterior Extrinsic Muscles

The superficial layer is composed of large, powerful muscles that originate from the spine and ascend to attach to bones of the shoulder. There are two muscles in the superficial layer:

Trapezius 

Originates from the skull down, along the spinous processes of the vertebrae until T12. It attaches to the clavicle and the acromion and spine of the scapula. It is innervated by the spinal accessory nerve (cranial nerve XI), with proprioceptive fibres from C3-C4. Trapezius elevates the scapula, allowing for it to rotate during shoulder abduction.

Latissimus Dorsi 

Originates from the spinous processes of approximately T6 to T12 via the thoracolumbar fascia, in addition to the iliac crest. It attaches to the humerus, at the intertubercular sulcus. It is innervated by the thoracodorsal nerve. Latissimus dorsi acts to extend, adduct and medially rotate the humerus at the shoulder.

Deep Layer of Posterior Extrinsic Muscles

There are three muscles to know in the deep layer, they are all innervated by the dorsal scapular nerve:

Levator Scapulae 

Originates from the transverse processes of C1-C4, then it attaches to the medial scapula. It acts to elevate the scapula (as the name suggests).

Rhomboid Minor

Originates from the spinous processes of C7-T1. It attaches to the medial scapula and acts to retract and rotate the scapula.

Rhomboid Major 

Originates from the spinous processes of T2-T5 and attaches to the medial scapula, acting to retract and rotate the scapula. 

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)]