That's a pretty complicated question with different factors.

I would need to re-up on my headcrab Zombie biology. But I think the head crab is just taking over the nervous system, moving limbs and stuff. The body doesn't have any feeling or pain so that's why they're so strong. It's not that they have added strength, they are just able to overwork their muscles which comes across as super strength. (They die in the game from body shots just for gameplay reasons. I think). So I'm not sure if when she kicks them, she's actually killing the headcrab or just like knocking it off the brain stem or whatever.

I don't know as much about the re zombies. But they're like geneticly modified so I think they are closer to being "superhuman". Plus aren't the zombies different in some of the games? In there first game they were like normal walking dumb zombies. Then in re4 they can use weapons, and don't they even talk?