Back of a 20 year old male patient with a winged left scapula (shoulder blade). Winged scapulae occur as a result of paralysis or weakness of the serratus anterior muscle or the long thoracic nerve that operates it. The lack of action of the muscle means that the rhomboid and levator scapulae muscles are unopposed. This leads to the inferior angle of the scapula being pulled inward across the back. The patient is unable to raise the arm above the horizontal as a result.