afan
Sidekick
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I think Justice Bringer was referring more to his anatomical endurance. Although he's from another planet, Superman's body functions much in the same way a human's will. He tires and suffers from the effects of fatigue if he pushes himself physically(at his level of strength).
You could compare Superman's saving of Metropolis in Superman Returns to helping your friend move to their new house across the street. No, it won't kill you, but you would be at a significantly lesser grade of physical performance if the neighborhood bully came to pick a fight with you.
I believe that's a fairly recent incarnation, didn't they invent that take for the "Death of Superman" series.