National Geographic has a science of superheroes analysis of Superman’s strength:
As any fan knows, the original explanation for Big Blue's power and skyscraping leaps was gravity. He comes, the story goes, from the destroyed planet Krypton, where gravity was stronger than it is on Earth.
Under this theory, Superman on our planet is like a human astronaut bounding around on the moon, only more so. The same applies to any of his other feats of strength, Kakalios says.
The biggest problem with this idea, says Michael Dennin, a physics professor at the University of California, Irvine, is that he can also walk normally—rather than bounding around like an astronaut on the moon.
Another problem is that he remains powerful no matter how long he stays on Earth. Because weightlessness forces people to use their muscles less, "astronauts get weaker with time," Dennin says.