Pullups are an ideal choice for exercising most of your upper body, including your biceps, latissmus dorsi and back. Pullups don't specifically target the shoulders, but because pullups strengthen the muscles around the shoulders, many people find that their shoulders get stronger with pullups. You can increase the shoulder-strengthening potential of a pullup by altering your grip.
Pullups and Shoulders
When performing a pullup, your biceps and latissmus dorsi are engaged for most of the exercise. Pullups cause shoulder adduction, which strengthens the shoulder joint as you bend your elbows, but the muscles in your shoulders are not the primary muscles propelling your movement or bearing your weight.
Most people do pullups with their hands shoulder-width apart. When you close your grip, however, you engage your shoulder muscles directly. Position your hands so that they face away from you and are only a few inches apart -- positioned approximately at each of your pecs -- then pull up on the bar. For maximum benefits, begin the pullup with your body hanging from the bar rather than jumping off of the ground or beginning with your elbows bent.
Pullups vs. Chin-Ups
While pullups are done with your hands facing away from you, chin-ups are done with your hands gripping the bar and facing toward you. Chin-ups engage the shoulder muscles slightly more than pullups, and can be a better option for people with a history of shoulder problems, as the grip of a chin-up puts less strain on the shoulder joint.
Doing a Pullup
Most people can't do a pullup without lots of training beforehand, so you will probably need to gradually work up to the exercise. Try lowering the bar. If you're using a fixed bar, simply hanging from the bar and pulling yourself up as far as you can go, then holding the position, can help you build strength. Bicep-strengthening exercises such as bicep curls can be particularly helpful. The upper body-strengthening workout provided by push-ups works many of the same muscles as a pullup, and can help you gain sufficient strength to do a pullup.