Answer:
Hamstrings and Quadriceps.
Explanation:
Opposing muscle groups also known as "antagonistic muscles" are those that oppose the action of the other. Typically the easiest way to match these opposing muscles are on the opposite sides of the body. Example, your quads are on the front of your leg and hamstring on the back. Conveniently the quads are hip flexors while the hamstrings are hip extensors. Quads are knee flexors and hamstrings are knee extensors. So depending on the joint that they cross they will perform different actions.
Another example is the pecs and rhomboids, biceps and triceps AND SOO ON.
The agonist is typically the muscle thats flexes while the antagonistic is the one that relaxes and stretches.
