Pull-ups are one of the best resistance exercises you can do. Few exercises draw as much attention as someone either doing lots of pull-ups or doing some variations of the basic pull-up. They require great upper body and core strength to complete. The difficulty of pull-ups can now be made easier or more difficult, thanks to weight belts that clip on to a weight plate or dumbbell (more difficult), and the assisted pull-up machine (easier).
The primary muscle used to do a pull-up is the latissimus dorsi (lat). The primary function of the lats is to either bring the arms into the body or the body to the arms. In the case of a pull-up, the arms are bringing the body in.
There are a handful of secondary muscles used in the pull-up, all of which are in the arms. The most important is likely the brachioradialis, the largest of the forearm muscles. The brachioradialis is crucial for elbow flexion, evidence of its use can be seen when you start a pull-up. The muscle on top of the forearms almost instantly bulges.
Biceps and Triceps
What comes as a surprise to many is that not only is the biceps a stabilizer muscle in the pull-up, but so are the triceps. Because the biceps are also involved in elbow flexion, they are clearly involved in the exercise, but not to the extent that many believe. The bigger surprise to many is that the triceps are also involved. But, because the long head of the triceps attaches to the scapula and the humerus, one of its actions will be similar to that of the lats.
A handful of other muscles are involved and are referred to as synergists, meaning they are needed to assist in the movement for maximum efficiency. The list includes the muscles in the forearm (brachialis), back (teres major, rhomboids, lower trapezius, levator scapulae), chest (pectoralis major and minor) and shoulders (posterior deltoids).