I love this stuff. Why can Superman fly? When he lifts a car, why doesn't the car break apart? (No one in comics even tries to explain that kind of problem.) Back to powers - OK, he's from a heavy-gravity planet, so he should be able to bound around on Earth like we can on the Moon. (In original comics, he he could not fly, only leap.) But how could he walk like a normal human? Scientists and fans (often one and the same) never tire of speculation.
Here's some more:
I'm reminded of the hilarious argument on The Big Bang Theory about how Superman extending his arms to catch Lois Lane as she fell fro ma skyscraper would just (yuck) cut her in three pieces.