Explanation:
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. Then, when the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint.
Answer:
the muscles are designed to only contract
Explanation:
the muscles would not be able to pull if the two protein molecules are not present.
