Medieval knighthood sounds like a romantic career in fiction. What a life of chivalry was really like doesn't seem all that great. A team of scientists tried to replicate the work of a knight clad in up to 110 pounds of metal by dressing up volunteer historical re-enactors from the Royal Armories in London, and putting them on treadmills. Even though they were used to wearing armor, you better believe they worked up a sweat!
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Sure enough, the researchers found, armor was exhausting. The men used 2.3 times as much energy to walk while wearing the armor than without it, and 1.9 times as much to run. Being outfitted for battle turned out to be even more tiring than hauling around a backpack of the same weight would’ve been. As it turns out, covering your legs with enormous, heavy metal plates makes moving around a lot harder.
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http://en.wikipedia.org/wiki/Battle_of_Agincourt#Terrain
Even the Spanish conquistador's ran into problems due to the hinderancy of their armour.
http://en.wikipedia.org/wiki/Spanish_conquest_of_the_Aztec_Empire#The_Aztec_response
The last recognized peice of metal plate armor for military use gorget, but it's purpose was more "bling" then to actualy offer any protection.
http://www.peterfiner.com/current-stock/item/1363/
Medieval armour - I wear it often - is heavy when you carry it in a sack, but surprisingly light when the weight is spread over your body. You tire faster, sure enough, but you are not really much slower. And tired is better than skewered. I can run, jump and turn cartwheels in mine. But no marathons please.
It does get very stinky, as do the clothes and padding you wear underneath. The best remedy seems to be a good bake in the sun while you enjoy a cold beer. :)