what muscles pull ups do build
Your pectoralis minor does do some of the work, as do the biceps, shoulders and other muscles of the back -- but for the most part, the pull-up is a lat exercise. The pectoralis major, the larger of the two pectoral muscles, is not activated much at all by pull-ups.
Pull-Ups is the only exercise I do that focuses on the latissimus. Pull-ups are good for other muscles too, like your forearms, triceps and traps, but for me the latissimus is the muscle that changes your look most drastically. Over 5 weeks of training I put on 4.5% to my back width. Read on to see how.