10. what do we call the action when one muscle works against another muscle?

Antagonistic.

Explanation:Muscles generally work in groups or pairs. Muscles are connected to bones via tendons, these tendons and muscles help in movement of a joint. When a muscle is put in contraction, usually only one bone moves.  For example on contraction of biceps of the arm contracts then the radius moves but the scapula does not move.This type of muscle action is called antagonistic muscle action. In which the muscle which is working is called the prime mover or agonist and the relaxing muscle is called the antagonist.  

Result: The muscle action in which a muscle works against another muscle is called antagonistic.