I've seen my teacher do the same things sith the cinder block and the sledge hammer. It wasn't to me, but to his youngest son and his son was laying on a bed of nails. Fairly sharp nails. That is common. My point is that many of these types of stories get amplified in the telling and the retelling. Its the nature of telling tales in general and with regard to Chinese martial arts specifically. It's the equivilent of saying "my dad is stonger than your dad" or "my teacher can beat up your teacher"