Why a pair of muscles are said to be antagonistic?

A muscle originates from one point of bone A,stretches over a moveable joint and attaches at another bone B.When it contracts,it shortens and thus brings the 2 bones together and so bend the joint like a hinge,
For a joint to be usefully mobile and thus executes a movement,it needs a muscle or a group of muscles to contract in synchony while another group working in the opposite way will relax.These antagonistic muscles are
1. Flexors and extensors of the limbs and digits
2. Adductors and adductors fingers and toes
3. supinators and pronators of the elbow
4.inverters and evertors of the ankle
5. internal and external rotators of the shoulders and hip
Any way,a muscle will contract and then relax to conserve energy called ATP or it will become fatigue.


etc.