Muscle · Neck

Sternocleidomastoid

Musculus sternocleidomastoideus

The sternocleidomastoid is the thick rope that stands out on the side of the neck when you turn your head. It runs from the top of the breastbone and the inner end of the collarbone up to the bony bump behind the ear. Both sides together draw the head forward; one side alone side-bends the head toward that shoulder and turns the face to the opposite side. It has one important quirk: because it passes behind the joint between skull and atlas, it flexes the neck while extending the head — precisely the geometry of a poked-chin posture. It is also an accessory breathing muscle, working overtime whenever breathing becomes shallow and high in the chest, which is why it turns up in stress, headaches, jaw problems and neck pain alike.