- The primary function of an antagonist muscle during a movement is to oppose the action of the agonist muscle.
- When a muscle called the agonist contracts to produce a specific movement, the antagonist muscle relaxes or lengthens to allow smooth and controlled motion.
- However, the antagonist can also contract to control or decelerate the movement, providing stability and preventing injury by opposing the force generated by the agonist.
- This balance between agonist and antagonist muscles is essential for coordinated and precise movements.

To clarify the other options:
- Option 1 suggests the antagonist assists the agonist, which is actually the role of a synergist muscle.
- Option 2 refers to opening a joint by reversing the agonist’s action, but this describes the antagonist’s effect indirectly; the primary role is simply opposition, not actively reversing.
- Option 4 mentions stabilizing the joint by isometric contraction, which is typically the role of fixator muscles or sometimes the antagonist, but it is not the antagonist’s primary function during movement.

Therefore, the correct and most fundamental role of the antagonist muscle is to oppose the action of the agonist muscle to ensure controlled and safe movement.

Reference: Muscles: Testing and Function with Posture and Pain, 5th Edition, Chapter 2 / Page 34