Pull-ups place a tremendous amount of strain on the biceps muscles of the arms. When executing a pull-up movement, the biceps are one of the main muscles utilized in the raising and lowering segments of the exercise.
The latissimus dorsi, which is the large muscle on the lateral sides of the back, is responsible for supplying much of the force needed to execute a pull-up. The latissimus dorsi muscle contracts and tightens throughout the entire phase, both lowering and raising, of a pull-up.
The muscles of the forearms are worked in a secondary, or supporting, manner when a pull-up exercise is performed. When a pull-up bar is grasped, the muscles of the forearms tighten and contract in order to help secure a strong grip on the bar.
The muscles of the front shoulders, or deltoids, provide significant lifting stability and are worked considerably during the execution of a pull-up exercise. The deltoids contract and supply significant lifting and lowering power during the entire pull-up movement.
Abdominals (Core Muscles)
The core muscles of the body, mainly the abdominal muscles, receive significant muscle stimulation during execution of a pull-up. During the lifting and lowering phases of a pull-up movement, the abdominal muscles contract in order to help stabilize the body core--which helps to make the pull-up easier and more efficient.