Final answer:
The prime movers of shoulder retraction are the rhomboid major, rhomboid minor, and the middle fibers of the trapezius muscle, which work together to pull the scapula toward the vertebral column.
Step-by-step explanation:
The prime movers of shoulder retraction are the muscles located at the posterior thorax, which include the trapezius, rhomboid major, and rhomboid minor. When the rhomboids contract, they pull the scapula medially towards the vertebral column, resulting in the retraction of the shoulder. The trapezius muscle also assists in this motion, particularly its middle fibers.