The scalenus posterior, also called the posterior scalene, is the smallest of the scalene muscles in the neck. There is a posterior scalene on each side of the neck. These muscles move the neck and the second rib.
Each scalenus posterior muscle connects the lower two cervical vertebrae (of the neck) to the second rib. They insert onto the posterior-lateral surface of the rib, which means they attach to the back of the side of the bone.
The scalenus posterior flexes the cervical vertebrae when it acts unilaterally (with only one of its pairs of muscles working). This moves the head forward, as might occur when one is nodding. It also flexes the neck and raises the second rib when it acts bilaterally (when both posterior scalene muscles are working). The second rib is raised during inspiration, or breathing in. The scalenus posterior works with the longus capitis, longus colli, scalenus medius, and the scalenus interior muscles.
Its nerve supply includes the posterior branches C5, C6, C7, and C8. Injury to the scalenus posterior may result in scalenus anticus syndrome (also called thoracic outlet syndrome). This results in pain in the chest, shoulders, back, and arms. In extreme cases, the scalenus posterior may decrease blood flow to the hands, making them numb and cold. Scalenus anticus syndrome may be treated through myofascial release — a special type of tissue massage therapy — and neck adjustment by a chiropractor. Such treatment may relieve pressure on the nerves and blood vessels by reducing the tension in the muscles and abnormal motion of the neck.