The chin up has long been used as a test of strength in relation to one’s bodyweight. The exercise is great for developing the muscles of the upper back, as well as the arm flexor muscles.
The latissimus dorsi is a primary target of the chin up, responsible for bringing the upper arm back towards the torso as the athlete ascends. Secondary targets of the upper back include the rear deltoid head, teres major and rhomboids.
Due to the supinated grip, the biceps are placed in a mechanically advantageous position and therefore play a large role in the chin up. This differs somewhat to other pull up variations where the grip used is neutral (palms facing one another) or pronated (palms facing away from the body), and therefore the biceps play a lesser role. This explains why most people can perform more chin ups than pull ups.