The sternocleidomastoid is the dominant muscle of the anterior neck, dividing the neck into anterior and posterior triangles. Its combined actions of ipsilateral lateral flexion and contralateral rotation explain why a tight or shortened SCM produces the torticollis posture with head tilted to the ipsilateral side and rotated away. It is innervated by the spinal accessory nerve with proprioceptive input from C2 and C3, making CN XI palsy an important cause of SCM weakness alongside trapezius paresis.
| Origin | Anterior surface of the manubrium sterni; Medial third of the superior surface of the clavicle |
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| Insertion | Mastoid process and lateral half of the superior nuchal line |
| Nerve Supply | Spinal accessory nerve (CN XI) for motor supply; Cervical plexus (C2, C3) for proprioception |
| Blood Supply | Occipital and superior thyroid arteries |
| Actions | Ipsilateral lateral flexion of the head and neck; Contralateral rotation of the head and neck; Bilateral: flexion of the cervical spine and extension of the head; Accessory muscle of forced inspiration (elevates the sternum and clavicle when the head is fixed) |
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Bilateral SCM contraction during spine flexion with the neck extended (cervical flexion but atlantooccipital extension) produces the chin-tuck movement used to correct forward head posture and is the basis of SCM-targeted deep cervical flexion exercises.
Congenital torticollis from SCM fibrosis or ischaemia in infancy produces the classic head tilt with a palpable SCM mass or hard cord that gradually remodels with physiotherapy stretching. CN XI accessory nerve palsy from posterior triangle surgery produces combined SCM and trapezius weakness, producing the characteristic drooped shoulder with weak neck rotation and shrug. The SCM is also a site of referred pain from trigger points producing unilateral frontal headache.
The SCM is the most visible and palpable neck muscle, easily seen as the diagonal cord from the mastoid to the sternum-clavicle junction during neck rotation, becoming prominent during resisted contralateral rotation.
SCM fibrosis from birth injury producing head tilt and rotational asymmetry, managed with physiotherapy stretching and occasionally surgical SCM release for resistant contracture.
Accessory nerve damage during posterior triangle surgery producing SCM and trapezius weakness with shoulder drop and weak neck rotation, managed with nerve repair or physiotherapy.
