Final answer:
The major muscles that act at the shoulder girdle are the subclavius, pectoralis minor, serratus anterior, trapezius, rhomboid major, and rhomboid minor.
Step-by-step explanation:
The major muscles that act at the shoulder girdle are the subclavius, pectoralis minor, serratus anterior, trapezius, rhomboid major, and rhomboid minor. These muscles help to position and stabilize the pectoral girdle, which consists of the clavicle, scapula, and proximal end of the humerus. They play a crucial role in allowing the head of the humerus to move the arm in multiple directions.