Shoulder muscles

Pectoralis major

The pectoralis major muscle covers the anterior thoracic cage. It has three heads; clavicular, sternocostal and abdominal. Its clavicular head originates from the anterior surface of the medial half of the clavicle. The sternocostal head originates from the anterior surface of the sternum and rib cartilages 1-6, while its abdominal head originates from the anterior layer of the rectus sheath. 

By inserting into the crest of greater tubercle of the humerus, it enables adduction and internal rotation of the arm. Also, it draws the scapula anteroinferiorly. The pectoralis major muscle is innervated by the lateral and medial pectoral nerves.

Pectoralis minor

The pectoralis minor lies underneath the pectoralis major muscle. It originates at the rib cartilages 3-5 and inserts to the medial border and the coracoid process of the scapula. The medial pectoral nerve supplies the muscle. The pectoralis minor draws the scapula anteroinferiorly and anchors it to the thoracic cage.

Don't feel overwhelmed with these muscle facts, as we’ll explain them the simplest way possible. To make your student life even easier, we’ve created these muscle anatomy reference charts for you.

Subclavius

The subclavius muscle originates at the cartilage and the sternal end of the 1st rib. It inserts at the anteroinferior surface of the middle part of the clavicle. The nerve to the subclavius supplies this muscle. Its functions are to depress and hold the clavicle in position. 

Serratus anterior

The serratus anterior muscle is divided into three parts which differ in origins and insertions; superior, middle and inferior. 

• Superior part - originates from the ribs 1-2 and intercostal fascia; inserts at the anterior surface of superior angle of scapula
• Middle part -  originates from the ribs 3-6; inserts along the anterior surface of the entire medial border of the scapula 
• Inferior part - originates from the ribs 7-8/9 (sometimes also from rib 10 and the external oblique muscle); inserts along the lower medial border and inferior angle of the scapula.

The long thoracic nerve supplies the serratus anterior. Its functions are to draw scapula anterolaterally, to suspend scapula on thoracic wall and to rotate scapula (draws inferior angle laterally).

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Extrinsic muscles

Trapezius

The trapezius muscle is a large muscle that defines the nuchal region. It has three parts; descending, transverse and ascending. Same as in serratus anterior, the parts have different origins and insertions.

• The descending part originates at the medial third of the superior nuchal line, external occipital protuberance, spinous processes of cervical vertebrae or nuchal ligament. It inserts to the lateral third of clavicle.
• The transverse part originates from the broad aponeurosis at spinous processes of vertebrae T1-T4 (or C7-T3), while it inserts to the medial aspect of the acromion and to the superior crest of spine of scapula.
• The ascending part originates from the spinous processes of vertebrae T5-T12 (or T2-T12), and inserts to the medial end of spine of scapula.

The muscle is innervated by the branches of the spinal accessory nerve (CN XI) and the anterior rami of spinal nerves C3-C4 via the cervical plexus.
The trapezius functions are also part-specific. The descending part draws scapula superomedially, extends and laterally flexes the head and neck and rotates the head. The transverse part draws scapula medially, while the ascending parts draws it inferomedially. 

Latissimus dorsi

Latissimus dorsi is another large muscle at the posterior shoulder area. You already assume that this one also has some parts, and you’re correct; there are four parts of this muscle. They are the vertebral, iliac, costal and scapular. 

• The vertebral part originates from the spinous processes of vertebrae T7-S1 and the thoracolumbar fascia.
• The iliac part originates at the posterior third of the crest of the ilium, a part of the bony pelvis.
• The costal part originates from the ribs 9-12, while the scapular part originates from the inferior angle of scapula. All parts converge and insert together to the intertubercular sulcus of humerus.

The muscle is innervated by the thoracodorsal nerve. Its functions are the adduction, internal rotation and extension of the arm, while it also assists in the respiration.

Levator scapulae

The levator scapulae originates from the transverse processes of the vertebrae C1-C4. Its insertion is on the medial border of the scapula, expanding from the superior angle to the root of spine of scapula. It is supplied by the anterior rami of spinal nerves and dorsal scapular nerve.

The functions of the levator scapulae include the lateral flexion of the neck (ipsilateral), drawing the scapula superomedially and rotation of the glenoid cavity inferiorly.

Rhomboids

There are two rhomboid muscles; rhomboid minor and rhomboid major. The rhomboid minor muscle originates from the nuchal ligament and the spinous processes of the vertebrae C7-T1 and inserts to the root of the spine of scapula. The rhomboid major originates from the spinous processes of T2-T5 vertebrae and inserts to the medial border of the scapula from inferior angle to root of spine of scapula.

The dorsal scapular nerve innervates both muscles, and they both share functions of drawing scapula superomedially, rotating the glenoid cavity inferiorly and supporting the position of scapula.

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Intrinsic muscles

Deltoid muscle

The deltoid muscle has three parts; the clavicular, acromial and spinal part. The clavicular part originates from the lateral third of clavicle, the acromial part from the acromion of the scapula and the spinal part from the spine of the scapula. They all insert to the deltoid tuberosity.

Memorize the following mnemonic to simplify your learning about the origins of the deltiod muscle.

Deltoid helps you carry SACS

• Clavicle
• Acromion
• Spine of scapula
  

The axillary nerve supplies the muscle. The functions of the deltoid muscle are related to the shoulder joint and include the arm flexion, arm internal rotation (clavicular part), arm abduction (acromial part), arm extension and arm lateral rotation (spinal part).

Teres major

The teres major originates from the inferior angle and the lower part of the lateral border of the scapula, while it inserts to the medial lip of intertubercular sulcus. The lower subscapular or thoracodorsal nerves supplies it. The functions of the teres major are the arm adduction and arm internal rotation.

Rotator cuff

The rotator cuff is a group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) that enclose the glenohumeral joint by forming the cuff around it.

You can easily remember them with the following mnemonic!

Rotator cuff SITS on the shoulder

• Supraspinatus
• Infraspinatus
• Teres minor
• Subscapularis
  

They are innervated by the suprascapular (supraspinatus and infraspinatus), axillary (teres minor) and upper and lower subscapular nerves (subscapularis). Acting together, the rotator cuff muscles stabilize the shoulder joint. These muscles produce a large spectrum of arm movements which include internal and external arm rotation, arm abduction and arm adduction.

Test your knowledge on the intrinsic muscles of the shoulder with our custom quiz!