Everybody cracks. As a boxer i can tell you some people just have tougher bodies and resist even more. But punch it quite enough and they'll start to feel the pain.
If anyone gets close to Wlad he hugs them and leans on top of them which the shitty refs they have in his fights always allow. Wlad has no inside game and there's a good chance his body is glass just like his chin.
khan peterson... has a glass chin, but an iron body peterson destroyed his body, and khan kept up his pace and never seemed to lose any stamina over it.