📖 Read-only demo. All content is browsable; accounts, saved progress, and quizzes need the full app — run it locally from GitHub.

Skip to content
Head & NeckSource-checked

Sternocleidomastoid

Sternocleidomastoid

Pronunciation: STER-no-KLY-doh-MAS-toyd

Simplified front and lateral view of the neck showing the sternocleidomastoid on the sides, the scalenes deeper within, and the upper trapezius connecting the neck to the shoulders. The cervical vertebrae are shown as a central axis.
Head & neck — sternocleidomastoid, scalenes, splenius, and upper trapezius (simplified). A beginner-level schematic of the neck's primary muscles: sternocleidomastoid, scalenes, splenius capitis, and upper trapezius.
Illustration: Anatomy Learning Platform (original) · CC0-1.0 (public domain dedication)

For beginners

This is the muscle you see popping out on the side of your neck when you turn your head hard. It flexes your neck (brings your chin down) and rotates your head side to side.

Show the advanced explanation

The sternocleidomastoid is a bilateral muscle — when both sides contract together, the neck flexes; when only one side contracts, the head rotates toward the opposite side. A short, tight SCM (common from slouching) pulls the head forward and can compress the nerves and blood vessels in the neck.

Structure & function

Location
A thick, cordlike muscle on the front-side of the neck, easily seen when you turn your head hard to one side.
Origin
Sternum (breastbone) and clavicle (collarbone).
Insertion
Mastoid process of the temporal bone (a bony bump behind the ear).
Main action(s)
Flexes the neck (brings the chin toward the chest); rotates the head to the opposite side (right SCM rotates the head left, and vice versa).
Joint(s) crossed
Cervical spine (atlantoaxial and cervical intervertebral joints)
Movement plane
sagittal (flexion), transverse (rotation)
Innervation
Spinal accessory nerve (CN XI)
In daily movement
Nodding, turning the head to look over the shoulder, and stabilizing the head during movement.
In exercise
Prime mover of neck flexion in isolation; highly active in any head-turning or posture-correcting movement.
Commonly confused with
Often confused with the scalenes, which lie deeper and run at a different angle.
Common misconceptions
The SCM is not primarily responsible for neck extension — that job belongs to the posterior muscles (splenius, upper trapezius, and the posterior neck extensors introduced in Spine).

Exercises that use this muscle

As a primary mover

⚠ Safety note

Excessive or ballistic neck rotation (whiplash-style movements) can strain the SCM and the cervical discs. A forward-head posture overstretches the posterior neck and shortens the SCM, leading to imbalance and pain.

Sources

“The sternocleidomastoid originates on the sternum and clavicle, inserts on the mastoid process, and flexes and rotates the neck.”

Anatomy and Physiology 2e — OpenStax, Rice University · Ch. 11.6 Muscles of the Head, Neck, and Spine

Peer-reviewed textbook · License: CC BY-NC-SA 4.0 · Last checked 2026-07-19

Quiz me on the Sternocleidomastoid

Sign in to bookmark this and save notes.