Final answer:
The infraspinatus muscle is the primary agonist for shoulder external rotation, but it is not listed among the given options.
Step-by-step explanation:
The muscle considered an agonist during shoulder external rotation is the infraspinatus. This muscle is located inferior to the spine of the scapula and functions specifically to laterally rotate the arm at the shoulder. Another muscle that assists in this movement is the teres minor, which not only serves in lateral rotation but also helps extend the arm.
While the deltoid muscle can contribute to lateral rotation when the arm is abducted, it is primarily known for its role in arm abduction and is not the main agonist for external rotation. Therefore, the best choice in the provided options would be none of the above, as the infraspinatus is responsible for this action, and it is not listed among the options.
The agonist muscle during shoulder external rotation is the infraspinatus.