To perform a pull-up, this exercise takes a certain amount of strength and is mainly used to target the latissimus dorsi and the teres major. When the shoulder blades come together at the top of the movement, the rhomboids and the middle and lower portions of the trapezius get involved; helping add thickness and strength to the back.
Pull-ups are one the most effective and demanding exercises you can do to develop overall back and hand-grip strength. According to Frederic Delavier, author of "Strength Training Anatomy," several synergistic muscles in the arm and back are used to carry out a pull-up, though only the latissimus dorsi and the teres major are the primary movers.These two muscles insert into the humerus, which gives them the angle or leverage to pull the body upward.
The latissimus dorsi, a large triangular shaped muscle located in the posterior, originates on the lower spine, last four ribs and inferior angle of scapula. The muscle the inserts into the intertubercular groove of humerus after spiraling around teres major. This allows the latissimus dorsi to extend, adduct and medially rotate the arm. The latissimus dorsi helps with forced expiration and deep inspiration as it assists in lifting the rib cage.
The teres major is a small muscle that originates on the lateral inferior angle of the scapula. Since it inserts to the intertubercular groove of the humerus, similar to the latissimus dorsi, it also assists with medial rotation and adduction of the arm. The teres major is known for its strength and stabilization of the shoulder joint, as it is one of the four muscles of the rotator cuff.
A few variations of the pull-up exist that can make an impact on your muscle development. Both a neutral grip and underhand still targets the two primary movers, but they tend to involve more synergistic muscles such as the biceps. In addition, Delavier states by pulling the elbows back to raise the chin to the bar, mainly solicits the upper and central fibers of the latissimus dorsi; excellent for developing the bulk of the back.
Heavy training of the back may potentially cause injury; partially tearing the long head of the triceps brachii. According to Delavier, the triceps is not used much when training the back, but the long head of the triceps is the most frequently injured muscle during lat pull-downs with heavy weight or pull-ups with added weight. Injury tends to happen when the latissimus muscle becomes over fatigued, which then shifts tension to the triceps long head.