The rhomboid muscle consists of two parts. Rhomboid Minor and Rhomboid Major. The rhomboid minor is smaller and above the rhomboid major.
The rhomboids originate mostly in the thoracic spine (C7-T5) and insert into the medial border of the scapula (shoulder blade).
The primary function of both minor and major rhomboid muscles is to hold the scapula in place, and secondarily, hold the arm to the rib cage.
They work to retract (pull back) the shoulder blade, bring it closer to the spine, and to help elevate the upper arm. Weakness is the rhomboid muscle cause the shoulder blade to "wing" out causing the scapula to become unstable.
The rhomboid muscle also becomes weak and thin when the thoracic spine is chronically in flexion and responds by becoming overly tight.
The rhomboids are posture muscles.