Final answer:
The latissimus dorsi is the primary agonist during an upper-body vertical pulling movement.
Step-by-step explanation:
The shoulder agonist during an upper-body vertical pulling movement is primarily the latissimus dorsi. This broad, triangular muscle is located on the inferior part of the back and plays a crucial role in the extension, adduction, and internal rotation of the shoulder joint. Other muscles involved in the movement include the teres major, which assists in extending the arm, and the deltoid, which facilitates flexing and medial rotation, as well as extension and lateral rotation of the arm. However, the primary muscle that acts as the agonist during such movements is the latissimus dorsi.