Final answer:
The patient's inability to retract the scapula is due to paralysis of the trapezius and rhomboids, which are the two primary muscles involved in this movement.
Step-by-step explanation:
The patient was unable to retract his scapula due to paralysis of the trapezius and rhomboids. These muscles work together to execute the retraction movement of the scapula, which is pulling the scapula towards the vertebral column. The trapezius muscle is a large superficial muscle that extends from the occipital bone to the lower thoracic vertebrae and out to the spine of the scapula. It is responsible for various movements, including the retraction, elevation, and rotation of the scapula. The rhomboid muscles, consisting of rhomboid major and minor, lie deep to the trapezius and attach from the vertebral column to the medial border of the scapula. When the rhomboids contract, they pull the scapula towards the spine, which is the retraction movement.
Rhomboids' function is essential for proper shoulder and upper limb function, as they stabilize the scapula and ensure smooth motion. In the context of the other listed muscles, the infraspinatus is important in rotating the arm laterally, and the supraspinatus is involved in abducting the arm. The levator scapulae elevates the scapula, while the serratus posterior superior has a role in elevating the ribs during respiration, rather than in scapula movement. Since the question specifically mentions the action of scapula retraction, the correct answer is the rhomboids, which are directly responsible for this function alongside the trapezius.