The sternocleidomastoid (SCM) is a large, paired muscle located on either side of the neck. It originates from the sternum (breastbone) and clavicle (collarbone). It inserts a bony projection behind the ear and the occipital bone at the back of the skull into the mastoid process. The SCM muscle is responsible for various movements of the head, including rotation of the head to the opposite side, lateral flexion of the neck, and extension of the head and neck. It also functions as an accessory muscle for respiration, aiding in elevating the ribcage during inspiration. The SCM muscle is innervated by the accessory nerve (cranial nerve XI) and works in conjunction with other muscles of the neck and shoulder girdle to produce efficient movements during various activities, such as turning the head, maintaining posture, or breathing. Dysfunction or weakness of the SCM muscle can lead to difficulties with head and neck movements and may contribute to neck pain, headaches, or postural imbalances.