As expressed before on a long, but interesting, research web site on Ye Olde Culture, Arms N' Shiz(e) that is in one of the suggestion stickies, Worbah's right about the plate.
It says that a fighter in that armor could indeed do cartwheels, leap straight from the ground, and even sprint for little bit. If armor was so crappy as to make you trip on your own boot, then they would stick with good old mail. So yeah; the game is realistic by allowing you to jump, etc. while packing the black armor.
You still need lots of endurance to be able to fight long enough to not look like a pansy in that armor, though. But it doesn't matter, since everyone was trained from childhood to do this crap. There's some stuff that limits movement, not weight, but the plate itself. Some plate armor, like the "Milanese" (I don't know either) design are heavier and have big pauldrons, which prevent you from moving your freakin' shoulder a bit. Others like the German design let you move around a lot like an unarmored guy, but the loss of the big-arse pauldrons means lower shoulder and a bit of lighter rib protection. That stuff is much thinner than we think (but it's still able to protect you from, like, all cuts), plus they do this stuff for exactly one childhood, so yeah. Even weapons weighed not much more than 4 pounds. Any retarded model sword you have that weighs like 16 pounds would be a bit of an awkward swing.
If anyone says I'm ever not detailed enough in my posts, he's going straight to hell, because he's a freaking liar.
And yeah, the horse jump is pretty nifty, since you can kill people by leaping off of a hill and cr-r-r-r-rushink their heeed! And jump over some pike-wielding wankers, I guess.