Firstly, most people aren't carrying backpacks around. I dunno what you guys do, but really the only thing most of us have are a bottle of water, wallet, mobile and any other small crap like a jumper or whatever. It really doesn't way much at all. And if you can't run with a backpack on, I'm wondering how you're physically capable of doing Parkour at all. Because you must be seriously unfit and weak as piss. =/