Now this has come back to a reasonable discussion...
I found an interesting article regarding the power of bullets to incapacitate. I think it's reasonable to extrapolate this to musket balls and arrows.
http://www.firearmstactical.com/pdf/fbi-hwfe.pdf
It concludes:
Kinetic energy does not kill
Impact "shock" (blunt force trauma?) does not kill
The only certain incapacitation is a hit to the brain
The critical element to incapacitation is penetration
The penetration must sever major blood vessels and/or organs - therefore death is by blood loss
Even a severely wounded person can function for a signifcant amount of time.
The way to increase the severity of the wound is by increasing the size of the projectile
From this one can conclude that any armour that prevents significant arrow penetration is effective.
This then raises the question: How was a fully-harnesses man-at-arms defeated? Blunt force trauma from a pole-arm is not effective (from the same arugument as the arrow); cutting through plate armour, mail and linen (under) armour is not practicable; which leaves penetration from thrusts as the only viable way to stop them.
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