Russ Mitchell wrote:and the thousand-dollar question: what was the mannequin made of?
I've put arrows right through the same shirt that was bulletproof mounted on a mannequin...
Russ: would you expect a bulletproof shirt to stop a knife blade?
Different principle. For instance, it's been said that bullets won't travel fast enough through water to injure a person, while a crossbow bolt will.
For the record though, I agree with Alaric - it's not necessarily the first sword blow that gets you; it's the fourth or fifth, but when it comes to arrows, unless it hits a vital (or sensitive, like an eye) organ, it's just not going to matter in time to make a difference.
At this stage, though, I'm in favor of skipping the Mythbusters guys in favor of simply declaring knights to be wearing plate that's good enough to be proof vs. arrows except for face shots. It gives them a measure of protection concurrent with their real-world status which they currently lack because of the "across the board" definition we're currently using. I have accordingly so emailed the SEM.