The rhomboid muscles

The rhomboids are two bilateral, superficial muscles located in the upper back. They consist of two functionally similar muscles called rhomboid minor and rhomboid major. Together with trapezius, levator scapulae and latissimus dorsi, they comprise the superficial layer of the extrinsic back muscles.
The rhomboid muscles extend between the nuchal ligament, spinous processes of thoracic vertebrae and scapula. Under innervation from the dorsal scapular nerve, the rhomboids primarily retract the scapula superomedially and rotate the glenoid cavity. Therefore, the rhomboids are important to stabilise the scapula into position and reinforce the shoulder.
Rhomboid major is a broad quadrilateral muscle that originates from the spinous processes of the second to second to fifth thoracic vertebrae (T2-T5). The muscle extends obliquely in an inferolateral direction to insert into the medial border of scapula, between the inferior angle and root of the spine of scapula. The attachment takes place both on the dorsal and costal aspects of the medial border of scapula.
The main action of the rhomboid muscles is scapular retraction around the scapulothoracic joint. Scapular retraction is a simultaneous sliding of the scapula superiorly and medially along the trunk. This superomedial movement of the scapula rotates the glenoid cavity inferiorly, dropping the shoulder girdle. By opposing excessive scapular protraction, the rhomboids help to maintain a correct posture when sitting, standing and walking.