Ak-Knife, Knife kills

Where does the knife on the ak knife do a OHK, I know the foot and hands will do less than 50ish dmg. but ive recently been on an ak knifing spree, and didnt kill some ppl, when they kill me, it shows how much dmg i did on them and i see 64s 85s. Does hitting the arm kill in one? or does it have to be Torso/Chest/Upperarm/Upperleg