When do objects begin to get heavy for Superman?

[skatalite]

We've seen him lift a plethora of things: land masses, planes, cars, buildings, people, just to name a few. But has it ever been explored/explained when Superman begins to feel the weight of something?

For instance: If you or I pick up a piece of paper, it's easy-peasy. If we lift a TV, we feel our muscles working. If we lift furniture, we really feel it.

So, when does Superman begin to feel it? Is lifting a person or automobile the equivalent of us puny humans lifting a single sheet of paper?

I'm not an avid reader, but has this been explained in the comics?

[carabas]

It is entirely defined by the needs of the plot.

And some writers (notably Jeph Loeb) claim he has no upper limit.

[MythicBrawn]

Pulling the moon with WW and Green Lantern was a strain on his muscles.

[Primemao]

Pulling the moon with WW and Green Lantern was a strain on his muscles.

Never seen that one but, think this recent one show a pretty good limit strain.

http://i39.tinypic.com/2a9pni9.jpg

http://i43.tinypic.com/12479g9.jpg

http://i43.tinypic.com/33xts0k.jpg

[CYOTI]

We've seen him lift a plethora of things: land masses, planes, cars, buildings, people, just to name a few. But has it ever been explored/explained when Superman begins to feel the weight of something When his supply of solar energy begins to diminish. Otherwise Loeb is probably right he probably doesn't have an upper limit.