Rhmboids are two muscles rhomboids major and rhomboid minor.scapula muslce, downward rotation
The two rhomboids lie deep to trapezius to form parallel bands that pass inferiorly from the vertebral to medial border of scapula. Rhomboid major is thinner but twice as wider to the thicker minor. Usually there is small space between two muscles however in some cases single blended muscle .

Origin and insertion :

Rhomboid major-
Originates from Spinous processes T2-T5
Inserts into medial border of scapula from level of spine to interior angle

Rhomboid minor
Originates from McHale ligament Spinous processes C7 & T1
Inserts into triangular area medial end of scapular spine

Nerve supply:
Dorsal scapular nerve C5

Action:

Based on anatomical considerations, the rhomboid muscles adduct (draw medially) and elevate the scapula. The attachment of the rhomboid major fibers to the lower vertebral border of the scapula tends to rotate the scapula medially, turning the glenoid fossa down. These muscles assist forceful adduction and extension of the arm by stabilizing the scapula in the protracted position.

ELECTROMYOGRAFFICALLY, rhomboid muscle exhibited steadily increasing activity throughout abduction and similarly during flexion. In another study, the electrical activity of the rhomboids rapidly increased in intensity between 160° and 180° of either movement. This activity is not predicted by
any of the anatomically-based actions listed above.

Rhomboids are active in both forward and backward arm swings during walking. Probably to stabilize scapula.

Synergic action:

The rhomboid muscles act synergistically with the levator scapulae and with the upper trapezius for elevation of the scapula. They are synergistic with the levator scapulae and latissimus dorsi  in rotation of the scapula, but oppose the upper trapezius. Basmajian and DeLuca2 consider the rhomboids synergistic with the middle trapezius for assisting abduction of the arm to 90° and in early flexion of the arm at the shoulder joint.

Scapular adduction by rhomboids and middle trapezius muscles is opposed directly by the sarratus anterior and indirectly but powerfully by pecs major.

 

Contribution in postural dysfunction:

No obvious restriction is caused by myofascial trigger points in the rhomboids. However, the examiner should observe for rounded shouldered posture that may indicate tightness and shortening of pacs major and put sustain pressure on rhomboid and middle trapezius fibers. Such overstretching has given the rhomboids muscles the reputation of being prone to get weakness and inhibition. But there is lack of emg data that there is weakness of rhomboids, or it is contracting but overpowered by strong antagonist.

These nearly horizontal fibers act synergistically with the rhomboid muscles to adduct the scapula.The rhomboid muscles are important synergists for elevation and medial rotation of the scapula.These muscles assist forceful adductiexercise may not stretch the pectoralis minor muscle, and the contraction of the rhomboid muscles can reinforce the downward rotation of the scapula, which is a common problem.on and extension of the arm by stabilizing the scapula in the retracted position.

References :

  1. https://www.yoganatomy.com/rhomboid-muscle/