When you are first in the space station having gone straight from earth you will feel stronger. All of the items still have mass but they have negligible weight.
Without constant exercise and strain though, your body will begin to break down your muscles and bones. You aren't using them as much as before because of the reduced gravity, so your body 'recycles' the materials used to make muscle and bone into other things it needs.
This is part of the cause of the pains some astronauts feel after extended periods out of gravity, though some of this is just through word of mouth than through scientific evidence.
Returning to Earth is a painful ordeal for some who didn't exercise enough as the extra stress on the body due to gravity no longer has the previous muscle and bone mass to support your own weight.
So the strength isn't increased in microgravity, but without excessive use it will be decreased over time.