An example of an antagonist muscle is?

An antagonist muscle is one that opposes the action of another muscle during a movement. In the context of a biceps curl, the primary mover, or agonist, is the biceps brachii, which is responsible for flexing the elbow. The triceps brachii acts as the antagonist in this scenario since it performs the opposite action—extending the elbow. As the biceps contract to lift the weight, the triceps must relax and lengthen to allow that movement, thus performing the role of the antagonist.

By understanding this relationship between muscle actions, it's clear that the triceps are engaged in opposition to the biceps during the curl, contrasting with the other options where the given muscles do not operate in an antagonistic capacity. For instance, the biceps are not antagonistic to themselves, and the internal obliques do not act as antagonists during a crunch; instead, they assist in torso flexion.