Final answer:
The agonist muscle at the shoulder during an upper body vertical pulling movement is the latissimus dorsi, which extends, adducts, and medially rotates the humerus, supported by synergists like the teres major and the biceps brachii.
Step-by-step explanation:
The agonist at the shoulder during an upper body vertical pulling movement primarily involves the latissimus dorsi muscle. This broad, triangular muscle is one of the two axial muscles that play a significant role in moving the humerus. During a vertical pulling movement, such as a pull-up, the latissimus dorsi works by extending, adducting, and medially rotating the humerus.
In addition to the latissimus dorsi, other muscles like the teres major and the biceps brachii act as synergists, helping in the movement. The function of these muscles in conjunction with the latissimus dorsi allows for efficient and controlled motion during the exercise