n.one of two thick muscles running from the sternum and clavicle to the mastoid and occipital bone; turns head obliquely to the opposite side; when acting together they flex the neck and extend the head
Inall flexion activities, symmetricalorasymmetricalsternocleidomastoidswere the majorflexormuscles, andthey had to balance the mechanics of the neck as agonists, synergists, or antagonists.