Sternocleidomastoid Anatomy

Sternocleidomastoid Muscle Anatomy

Origin:
  Sternal Head: Upper part of the anterior surface of the manubrium
  Clavicular Head: Superior surface of the medial one-third of the clavicle

Insertion: Lateral surface of the mastoid process of the occipital bone, from its apex to its superior border, and by a thin aponeurosis to the lateral half of the superior nuchal line

Actions:
Bilaterally: Flexion of the head and neck, extension of the head and neck
Unilaterally: Rotation of head to opposite side, lateral flexion

Innervation: Accessory nerve: cranial nerve XII and ventral rami of the (C2, C3)

Blood Supply: Branches from the vertebral artery

Primary Actions of the Sternocleidomastoid

Primary Actions of the Sternocleidomastoid

1. Extension of the head and cervical spine when posterior fibers act bilaterally

  Agonists:

  • Longissimus Capitis
  • Spinalis Capitis
  • Splenius Capitis
  • Semispinalis Capitis

  Antagonists:

  • Longus Capitis
  • Sternocleidomastoid (anterior fibers)

2. Flexion of the head and cervical spine when anterior fibers act bilaterally

  Agonists: Longus Capitis

  Antagonists:

  • Longissimus Capitis
  • Spinalis Capitis
  • Splenius Capitis
  • Semispinalis Capitis
  • Sternocleidomastoid (posterior fibers)

3. Lateral flexion of the head and cervical spine when acting unilaterally

  Agonists: Splenius Capitis

  Antagonists: Splenius Capitis – contralateral side

4. Contralateral rotation of the head and cervical spine when acting unilaterally

  Agonists: 

  • Semispinalis Capitis
  • Trapezius (upper part)

  Antagonists: 

  • Splenius Capitis –  contralateral side
  • Trapezius (upper part)

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