Question

Muscles work in pairs. When one of them contracts. The bone is pulled in that
direction. The other muscle of the pair relaxes, to move the bone in opposite direction
the relaxed muscle contracts to pull the bone towards its original position, while the
first relaxes. A muscle can only pull. It cannot push. Thus two muscle have to work
together to move a bone.

Answer 1

Actually

Muscles move body parts by contracting and then relaxing. Muscles can pull bones, but they can't push them back to the original position. So they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint.