i'm doing the Chris lvl 99 trick again and have always seen that without any armor worn, defend causes you to take exactly half from a physical attack. this is probably right.... if you have a defense of 0, and the Boar hits for around 109, defending gets around 55. when defending, the attack damage is probably halved FIRST, with armor reducing it afterwards. this would be able to reduce attacks by more than half.
with a defense of 50, you would take 59 from the Boar normally. when defending, it's halved first so its attack value becomes 55.. 55 - 50 = 5.