Posterior Shoulder Muscles: Anatomy and Function

Study Notes

Divided into three groups: superficial (extrinsic), deep (extrinsic), and scapulo-humeral (intrinsic) muscles

Trapezius
- Origin: medial third of superior nuchal line, external occipital protuberance, nuchal ligament, and spinous processes of C7-T12 vertebrae
- Insertion: lateral third of clavicle, acromion, and spine of scapula
- Innervation: accessory nerve (C3, C4)
- Action: acts on scapula
Latissimus dorsi
- Origin: spinous processes of inferior 6 thoracic vertebrae, thoracolumbar fascia, iliac crest, and inferior 3 or 4 ribs
- Insertion: floor of intertubercular groove of humerus
- Innervation: thoracodorsal nerve (C6, C7, C8)
- Action: extends, adducts, and medially rotates humerus

Levator scapulae
- Origin: transverse processes of C1-C4 vertebrae
- Insertion: medial border of scapula superior to root of spine
- Innervation: dorsal scapular nerve (C5)
- Action: elevates medial border of scapula
Rhomboids
- Minor: origin - nuchal ligament and spinous processes of C7 and T1 vertebrae
- Minor: insertion - smooth triangular area at medial end of scapular spine
- Major: origin - spinous processes of T2-T5 vertebrae
- Major: insertion - medial border of scapula from level of spine to inferior angle
- Innervation: dorsal scapular nerve
- Action: retraction and medial rotation of scapula

Deltoid
- Origin: lateral third of clavicle, acromion, and spine of scapula
- Insertion: deltoid tuberosity of humerus
- Innervation: axillary nerve (C5, C6)
- Action: anterior part - flexes and medially rotates arm, middle part - abducts arm, posterior part - extends and laterally rotates arm
Teres Major
- Origin: posterior surface of inferior angle of scapula
- Insertion: medial lip of intertubercular groove of humerus
- Innervation: lower subscapular nerve (C5, C6)
- Action: adducts and medially rotates arm

Four intrinsic shoulder muscles: supraspinatus, infraspinatus, teres minor, and subscapularis
They rotate and stabilize the shoulder joint

Supraspinatus
- Origin: superior facet of greater tubercle of humerus
- Insertion: suprascapular fossa of scapula
- Innervation: suprascapular nerve (C4, C5, C6)
- Action: initiates and assists deltoid in abduction of arm
Infraspinatus
- Origin: middle facet of greater tubercle of humerus
- Insertion: infraspinous fossa of scapula
- Innervation: suprascapular nerve (C5, C6)
- Action: laterally rotates arm
Teres Minor
- Origin: middle part of lateral border of scapula
- Insertion: inferior facet of greater tubercle of humerus
- Innervation: axillary nerve
- Action: laterally rotates arm
Subscapularis
- Origin: subscapular fossa (most of anterior surface of scapula)
- Insertion: lesser tubercle of humerus
- Innervation: upper and lower subscapular nerves (C5, C6, C7)
- Action: medially rotates and adducts arm