there are upto four functional groups of muscles acting on joints. 1 agonist: actively contract to make a movement. muscle length reduces.
2. antagonist: resists the muscle on opposite side, thereby controls the speed of the agonist muscle contraction.
that's why they say both agonist and antagonist muscles are working in pairs.
furthermore when the movement is reversed the original agonist becomes the antagonist and the original antogonist becomes the agonist. hence antagonist pairs. right?
3. stabilisers: some muscles will hold the joint area stable while other three types of muscles are making a movement.
4. modifiers: some muscles can slightly change the direction of force exerted by agonists dynamically.