In regards to:
“No one is seriously asserting that people in armor were like turtles or couldn’t walk, or were useless on the ground.”
There were two posts:
“Someone will be in with more detail but they were indeed heavy as fuck (at least the plate-type stuff)-- you would not walk around in it, but would be basically lowered onto a horse with a crane mechanism. People were know to fall off into puddles and drown, unable to lift themselves.”
and
“No, they weren’t huge and yes, most knights were useless when unhorsed”
Definitely not true, as shown in the videos I posted. An unhorsed knight could recover nearly as quickly as someone without any armor, assuming no additional variables (ie “peasants with sticks”), which aren’t really relevant to a knight wearing armor or not as they would pose a problem to anyone unhorsed.
“Compared to modern man, they would not be as capable physically, as is evidenced by the steady forward progress of sports records just over the last century.”
How is a modern athlete who has access to ridiculous amounts of performance enhancing drugs in any way relevant to a knight? Since knights were compared to marines, did I miss the part where all of the sports records being set and broken are done by someone in the armed forces every time?
“And your assertion that they were in terrific shape is just wrong.”
Where is the proof that knights were in terrible shape? I find it hard to believe someone in terrible shape would have the mobility and endurance to fight in a suit of armor; especially for the duration of a medieval battle. There’s quite a difference between having a boil, std, whatever, and being in poor shape. Presence of an STD is not always equivalent to physical capabilities; and nothing has been cited to show that knights were ravaged by STDs either.
“Then there is a group of people claiming that people during the middle ages were shorter, many had health problems, and that armor does create a penalty to mobility and endurance. All of these points are backed up by reputable historical sources and modern investigation.”
What?
The references cited by others earlier in the thread, in addition to the physical size of existing pieces of armor, do indeed show that there is a negligible difference in height among people today and people from the 14th-16th century (when heavy armor was most prevalent).
I don’t think anyone is saying that 75 lbs of armor doesn’t cause a penalty to mobility or endurance. It certainly does. What is under dispute is that it doesn’t cause such an immense penalty that one is not able to stand up on their own, unable to mount a horse, or unable to get up after falling from a horse.
I’ve yet to see ANY proper cites to support that knights were disease-ridden, unhealthy, unskilled, peasant-killing baboons. It seems that many have made good references as to the skills and physical prowess required to battle under the constraints that armor creates, and yet, it is refuted by wild claims that claim to come from “reputable historical sources and modern investigation”, yet specific cites to exactly WHAT or WHERE that may be seems to be eluding us.
Seems like the guys who strap on armor over their beer guts and beat each other with sticks in a similar fashion to those on a battlefield have a better insight as to what is or isn’t possible for someone in armor than somebody who is making wild claims with zero supporting evidence. 
