Which of the following pairs of muscles are not agonist / antagonist?

The internal oblique and external oblique muscles are not considered classic agonist/antagonist pairs. Instead, they work synergistically to facilitate trunk rotation and lateral flexion and function in opposite directions. The internal obliques on one side of the body work with the external obliques on the opposite side to produce rotation of the torso. This cooperative action differentiates them from typical agonist/antagonist relationships, where one muscle primarily causes movement while the other resists or opposes that movement.

In contrast, the quadriceps and hamstrings exhibit a textbook agonist/antagonist relationship where the quadriceps extend the knee while the hamstrings flex it. Similarly, the biceps brachii functions to flex the elbow, while the triceps brachii extends it, demonstrating their opposing roles. The tibialis anterior also opposes the gastrocnemius and soleus, which are responsible for plantar flexion at the ankle, confirming that this pair also aligns with the agonist/antagonist classification.