Muscle Anatomy of the Sternocleidomastoid

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

Sternocleidomastioid Muscle

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

  Antagonists: 

  • Splenius Capitis -  contralateral side

Recommended Beginner Anatomy Books
The Anatomy Coloring Book is one of the best study and reference books for beginning anatomy students. The diagrams are clearly labeled and allow you to see the relationship and placement of the various structures of the body.

The Trail Guide To The Body is another excellent book to help you learn the musculature of the human body. Though the book is geared toward massage therapists and physical therapist assistants, the book with its illustrations and text helps anyone gain a thorough understanding of the human musculoskeletal system and movement. I highly recommend this book for anyone studying anatomy.

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Information about cause and effect of Sternocleidomastoid pain:

Sternocleidomastoid Muscles:  Head, Eyes, Sinus, Ears, Throat Pain

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