Problem:

I study biology at school, and unfortunately for me, my program skips the muscles in humans chapter.

I know (and mainly, feel) that the movement in one direction isn't created by the same muscle as the movement in the opposite direction, e.g the Triceps ("front") and Biceps ("back").

I know that the triceps straightens the elbow, while the biceps contracts the elbow.

I also know that, instead of actually moving the arm, I can contract these two muscles without actually moving the arm. That area becomes hard. Both muscles, as I feel, are contracting. I cannot statically contract only one of them.

Question: Whether this action is something "special", or simply both muscles working against each other, resulting in zero movement?