A pullup is a body-weight exercise that involves grabbing an overhead bar and pulling your body up to it. You can do this with your palms facing forward or rear. You can also perform pullups with your palms facing each other with either a narrow or wide grip. This exercise involves motion at both the shoulder and elbow, so muscles that cross each of these joints are worked.
Eleven muscles cross and act at your shoulder joint. Some of these originate on your trunk, while others are attached to your shoulder blade. All these muscles insert on your arm. When you perform a pullup, your arm is drawn down to your body for the most part by two muscles in your back. These are the latissimus dorsi and teres major. The muscles form the V-shape appearance that people who lift weights typically possess.
Seven muscles cross and act at your elbow joint. Three of these, the biceps brachii, brachialis and brachioradialis, work together to bend your arm at the elbow as you perform pullups. The brachialis is involved no matter what hand position you employ. The contribution of the brachioradialis increases when your palms face each other, and your biceps brachii is more active when your palms face rear. Your biceps brachii and brachialis form the ball that is displayed when you flex your arm to make a muscle. Your brachioradialis is predominantly situated on your forearm and helps give size to that section of your body.
The pectoralis major, or pecs, is another muscle that crosses your shoulder. This muscle is the predominant one of your chest. Your pecs have fibers angled in distinctly different directions, which means that the muscle can create a variety of arm movements. In all cases, your pecs draw your upper arm to your body, but their contribution is reduced when your arm is extended overhead. This means that your pecs won't receive much stimulation when you perform pullups, though facing your palms to the rear does increase their involvement.
Your pecs are a large, fan-shape muscle that is divided functionally into a clavicular and sternocostal section. The clavicular portion comprises fibers that attach your arm to your collarbone. These fibers are angled up and receive stimulation when you pull your arm across your body at an upward angle. The sternocostal section has fibers that are directed both horizontally and downward. These fibers are prioritized when you draw your arm in those directions. A body-weight exercise you can do to provide greater stimulation to your pecs is dips. This exercise, which emphasizes the sternocostal section, involves supporting yourself on parallel bars with arms extended downward, lowering your body until your arms are fully bent and pushing back to the starting position. Lean forward slightly while performing this exercise to incorporate more fibers of your pecs.