Pull-ups remain a classic for a reason: They flat-out get you strong. Pull-ups tax the latissimus dorsi, or lats, those wing-like muscles running laterally down the back. The lats are primarily responsible for rowing and pulling motions, making this exercise a key component of Kalama's training regimen.
Most of us claim to be able to do more pull-ups than we can. To complete a true pull-up requires a one-second hang at full-extension in between each rep, as well as bringing your chin all the way above the bar. If you really want to do it, Kalama-style, then find a tree branch and don't cheat yourself.