Final answer:
The trapezius and rhomboid muscles uniquely perform the action of retracting the scapula, which in turn pulls the shoulder and upper limb posteriorly.
Step-by-step explanation:
The action that only the trapezius muscle and the rhomboid muscles perform is the movement of the scapula medially, which pulls the shoulder and upper limb posteriorly. The rhomboid minor muscle attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae. This specific action of retracting the scapula is not performed by other muscles such as the rotator cuff, triceps brachii, gluteus maximus, tibialis posterior, peroneus longus, peroneus brevis, deltoid, or brachioradialis.