Which is better, Pullups or Chinups? Lets look at the facts.

Quick facts:

In regards to trap and lat activation, pull ups and chin ups are equally effective.
When it comes to latissimus recruitment, the most important factor is distance between your hands. Grip width makes a difference.
Pronated or supinated grip? Whether you use the underhanded chinups or the overhanded pullups will make all the difference in bicep stimulation.

The Difference Between Chinups and Pullups

First, lets make sure we're all on the same page. The difference between chinups and pullups refers to the grip used. Chinups refers to using a supinated or underhand grip. Pullups refers to a pronated or overhand grip. People debate endlessly on which is more effective, which should be done when, etc. Lets work on educating you a bit about each variation.

Pullups vs. Chinups -- Traps

Which is better for stimulating the lower traps, the pullup or the chinup? Let's simplify this right now, in regards to training the trap muscles, the relevant factor is scapular movement. Movement of the shoulder blades is far more important than grip orientation in this regard.

Anatomically, the main function of all three portions of the traps are to retract the scapula. The upper most fibers elevate the scapula (shrugs), and the lower trap fibers are meant to depress the scapula.

Note the difference in orientation of the trapezius muscle fibers

But what does the research say? When subjects were measured in a study, not much difference was noted. They did see a difference in that the traps were more activated during the standard pullup than a neutral pullup, and overall more trap recruitment than the chinup. However, this difference was attributed to the pullup being a more challenging movement requiring more overall effort than the chinup, and not a result of grip orientation differences.

Pullups vs. Chinups -- Lats

The age old debate, which hits the lats better? Shoulder extension is a primary function of the latissimus dorsi muscle, and occurs during both the pullup and the chinup. But a secondary function of the latissimus is to adduct the shoulder joint. It is this adduction of the arm to the body that causes most people to assume "wider grip = wider lats".

Due to this adduction, grip with has an impact on which fibers of the lats are recruited. Trying to hit those tricky lower lat fibers? Go with a wider grip. Upper lats lagging a bit? A closer grip is better. However, for the most part, these differences don't apply to you.

Researchers found that all pulling variations hit the lats the almost the same. When muscle activation was examined during the pullup variations, lat activity was nearly the same in the multiple different grips and styles.

Pullups vs. Chinups -- Biceps

And finally, how to best work on the gun show. The biceps, or the biceps brachii (short head and long head) are the ever popular arm muscle of beach bros everywhere. But when it comes to the pullup and the chinup, which provides better stimulation for this aesthetic masterpiece?

Seeing as how both heads of the biceps have the same primary functions of elbow flexion with supination and forearm supination, we can see that supination is present during the chin up and not the pullup. Anatomically, the chin up does hit the biceps better than the pullup. However, the biceps are still quite active in the pullup as well due to the elbow flexion of the movement.

But what does the research say? One study found that bicep activity was in fact greater in the chin up rather than the pullup. However, it must be noted that bicep activity is particularly high in both variations.

Snyder, B. J., & Leech, J. R. (2009). Voluntary increase in latissimus dorsi muscle activity during the lat pull-down following expert instruction. The Journal of Strength & Conditioning Research, 23(8), 2204-2209.
Snarr, R. L., Hallmark, A., Casey, J., Nickerson, B., & Esco, M. R. (2015). Electromyographic comparison of pull-up variations. Conference paper.
Youdas, J. W., Amundson, C. L., Cicero, K. S., Hahn, J. J., Harezlak, D. T., & Hollman, J. H. (2010). Surface electromyographic activation patterns and elbow joint motion during a pull-up, chin-up, or perfect-pullup™ rotational exercise. The Journal of Strength & Conditioning Research, 24(12), 3404-3414.