Melee and knock down - how does it work?

[toores]

I'm looking the new perk trees and the most probably i'm gonna make agility build. The thing is though, I really dislike knives for melee. Small reach and not enough knock downs. Also I find sledgehammer to be too slow. Basically club is the best melee weapon IMO - fast enough and knocks down frequently enough to give me some room. So my only option is to start up with strenth guy and once my club feels decent I can proceed to agility.

 

Can anyone explain how knock down actually works? In A17 it seem like there is certain amount of damage the zombie has to take and then the club is guaranteed to knock him down with the next hit.