Acting together, flexes the neck, raises the sternum and assists in forced inspiration.
In human anatomy, the sternocleidomastoid muscle ( /ˌstɜrnɵˌkliːdɵˈmæstɔɪd/), also known as sternomastoid and commonly abbreviated as SCM, is a paired muscle in the superficial layers of the anterior portion of the neck. When acting together it flexes the neck and extends the head. When acting alone it rotates to the opposite side (contralaterally) and slightly (laterally) flexes to the same side.
It also acts as an accessory muscle of inspiration, along with the scalene muscles of the neck.
Read more about Sternocleidomastoid Muscle: Etymology, Origin and Insertion, Innervation, Variations, Relations, Cultural Significance, Additional Images