Sternocleidomastoid Muscle Contraction: Causes, Effects & Functions

What happens when the sternocleidomastoid contracts?

The sternocleidomastoid muscle (SCM) is a paired muscle located on the anterior and lateral aspect of the neck. It arises from the sternum and clavicle and inserts into the mastoid process of the temporal bone. The SCM is responsible for a variety of movements of the head and neck, including:

- Flexion: the SCM flexes the head forward

- Rotation: the SCM rotates the head to the opposite side

- Lateral flexion: the SCM laterally flexes the head to the same side

- Extension: the SCM assists the other extensor muscles in extending the head

In addition to these actions, the SCM also helps maintain good posture and balance. When the SCM is contracted, it can help to:

* Keep the head upright and in alignment with the body

* Prevent the head from drooping forward or to one side

* Help to stabilize the neck and keep the airways clear

The SCM is innervated by the accessory nerve (cranial nerve XI). The SCM is often used to assess cervical spine mobility, balance, and posture. It is also one of the muscles commonly injected with botulinum toxin for cosmetic purposes, such as to reduce the appearance of neck wrinkles and platysma bands.