Is it a matter of strenght, or just of the right technique? There are twinks in any gym who can do 15 of those almost effortless, and others, often jacked guys, who couldn't do a single one to save their life.
It's strength, bodyweight, and technique, I could bust them out pretty easy at #180 - 200 when I did some climbing and a lot of bodyweight stuff. Once I started creeping past #210 or so, it got much harder.