Athletes and soldiers have used bodyweight exercises since the beginning of history. They build a strong body without a great deal of bulk, and most of the exercises work more than one muscle group at once, so they can provide a full body workout in less time. Since they use the natural weight of your body, the resistance level is more in line with your own personal weight tolerance. Bodyweight exercises form the core of the movements of Pilates and yoga.
For the upper back, the pull-up does the job. It primarily works the latissimus dorsi, which are the large wing-shaped muscles in the upper back. Pull-ups also work the shoulders, particularly the deltoids, which form the cap of muscle over the shoulders. Simply grip the edge of a strong door or a chin-up bar shoulder-width apart and pull yourself up until your chin reaches over the bar. You could also pull up from behind your head so that your upper shoulders touch the bar. Start with one rep, using a stool or chair to help hold your weight at first.
The plank (also known as the bridge) is a move that works all the muscles that run along the back and spine. It is a good overall exercise for the entire core, since it works the abdominal muscles as well. The shoulders get a bit of a workout, too. It is much gentler for many people than crunches, and is one of the few that work all of the core muscles, front and back, at the same time. Lie on the floor with the elbows bent at 90 degrees and forearms resting on the floor. Hands should rest crossed under your forehead. Lift your body, making sure to keep it straight. Abdominals are held tight, and the hips are neither dropped or sticking up in the air. Hold for 30 seconds, doing two reps. Work up to one minute.
The back extension works the muscles of the lower back, which is a common problem area. Proper support of the lower spine through strong muscles is one of the best ways you can avoid back pain and injury. This move is deceptively simple, but highly effective. Lie on the floor face down, arms out and legs straight. With abdominals held in, lift the torso up as though a string is pulling you between your shoulder blades. Try to raise yourself as high as you can without arching your back, and hold for 30 seconds. As you get stronger, try lifting the legs at the same time as the torso and holding it.
Even though it may seem easy, bodyweight exercise is hard work and requires downtime, just like traditional weightlifting. The body needs to rest and rebuild muscle to get stronger, so do bodyweight routines no more than two to three times a week, with at least one day of rest in between.