Eleven

"Don't push yourself too hard," Hopper warns. "Part of building a muscle is letting it rest, so it can heal. Rebuild stronger than before."

He's not sure of the comparison is apt, because they know so little about Eleven's powers. What he's saying might not apply to her at all or it might be deeply necessary, even more than it is for regular muscles. He has no idea. He just knows he doesn't want her getting hurt.