In the study of muscle movements, understanding the roles of agonists, antagonists, and synergists is crucial. When considering the action of abducting the arm, the deltoid muscle serves as the agonist. This is because the deltoid is positioned on top of the arm, and as it contracts, it pulls the arm away from the body. The teres major, located underneath the arm, acts as the antagonist since its contraction would pull the arm back towards the body, opposing the action of the deltoid. A synergist for this movement, which assists the deltoid in arm abduction, is typically found on top of the shoulder. The supraspinatus, part of the rotator cuff, is a prime example, as it works alongside the deltoid to facilitate this movement.

Next, when examining the action of smiling, the zygomaticus major is identified as the agonist. This muscle, which runs from the zygomatic bone to the lips, pulls the corners of the mouth upward when it contracts, creating a smile. Conversely, the mentalis muscle, located in the chin area, acts as the antagonist. Its contraction would pull the mouth downward, opposing the smiling action. A synergist for smiling would likely be positioned above the mouth, assisting in the upward movement. This synergistic action helps enhance the smile by providing additional support to the zygomaticus major.