Pull-ups are one of the greatest tests of upper-body strength because they require nearly every muscle in the upper body to work in order to complete the movement. To do a true pull up, grip a bar shoulder-width apart and allow your body to hang. Your arms should be completely extended over your head. While you pull your body up, focus on keeping your body still, not swinging either your body or your legs to complete the motion.
Primary Mover
The primary mover in a pull-up is the latissimus dorsi (lats for short). The lats are big sail shaped muscles that attach on the humerus (upper arm bone), down along the ribs and vertebral column, as well as the hips. The lats are actually the only muscle that connects the upper body to the lower body. The lats are active in many movements: for the shoulder the lats adduct, extend, and internally rotate; at the scapula they adduct, depress, and rotate downwards.
Synergistic Muscles
Because pull-ups are such a compound movement, there are many muscles that work synergistically to accomplish the movement. Synergistic movers are those that are active in a movement, but are not the primary muscle needed to complete the movement. The synergistic muscles are: brachialis, brachioradialis, teres major, posterior deltoid, rhomboids, levator scapulae, middle and lower trapezius, sternal head of the pectoralis major, and pectoralis minor.
Dynamic Stabilizers
Every exercise requires at least one muscle to act as a stabilizer, that is, to keep the body, or a part of the body, still while the rest of it moves. This has to do with Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. If there were no stabilizing force, the movement could not be done. In the case of pull-ups, the stabilizing muscles are considered dynamic, because while their job in the movement is to stabilize, they are also moving throughout the exercise, making them dynamic stabilizers. These dynamic stabilizers are the biceps brachii and the long head of the triceps.
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