Subsurf modifier+armature. Effect changes when applying the modifier.

Hi to all,

It’s been a while since my last time a used Blender and now I have a maybe very silly question.
I’m working with a head model that has a subsurf modifier (but not applied yet). I use an armature for the animations and everithing looks great. The problem arises when I apply the modifier to the head. The animations that where smooth don’t look smooth again.
Is there any way of maintaining the same or similar smooth effect after applying the modifier? I mean, I would like to make the rig without the modifier applied and then apply it to export the mesh.

Many thanks in advance.