Wikipedia: "It is a common misconception that the plate armour of European soldiers adversely affected mobility in a significant manner, but in fact plate armour was less heavy and featured more even weight distribution than a full complement of a modern firefighter's gear" 

Also, they usually only weighed 30-40 lbs. And that's all over your body. That's not heavy at all.

Fun Fact: 
[youtube]kU9BoOJxpbE[/youtube]

Paper armor is a thing.