In human anatomy, the sternocleidomastoid muscle, also known as sternomastoid and commonly abbreviated as SCM, is a paired muscle in the superficial layers of the side of the neck. It is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the accessory nerve.
The Exercise Of The Month is the Neck Stretch, which stretches the sternocleidomastoid muscle. You can find out how to do it at https://youtu.be/aEYW8_wbZ14.