What actually levels up armor rating?

[SaveVsBedWet]

Just wondering what it is in game that raises up the Light Armor or Heavy Armor skill. I see Heavy Armor go up every now and then but I have no idea why. Is it simply walking around in it? Fighting in it? Running in it?

Any ways to speed up the process so I can access perks that will bolster the Light Armor rating of my Nightingale armor?

 

[Callow]

I think receiving damage is what ups the skill.