Final answer:
The teres major and posterior deltoid are agonists for shoulder extension, while the pectoralis major acts as the antagonist.
Step-by-step explanation:
The agonist for shoulder extension is the teres major muscle which extends the arm, and also assists in adduction and medial rotation. Another muscle involved in extension, acting as a synergist, is the posterior deltoid. The antagonist for shoulder extension, which would perform the opposite action, would be the pectoralis major, specifically its clavicular head, since it's involved in flexing the humerus.