Breathing is a reflex action. Unlike other reflexes however, there is a lot about breathing that is controllable. The Reflex comes into play when the brain / body feels oxygen deprived. Then, whether you want to or not, you must breathe.
Role of the Lungs
The lungs provide the space for transition of oxygen into the blood and carbon dioxide out. They are composed of thousands of tiny air sacs. The more full each one gets, the better this transportation of gasses can be.
There are many forms of breathing. Most are constrained breathing - where posture limits movement of the lungs and other structures normally used in breathing. Common constraints are rounded shoulders and posture, arms pulled forward, slumping in a chair, and excessive tension in neck, upper back or chest. Thankfully, effective breathing may affect alertness, creativity and energy, but is not usually important to survival.
Although it starts as a reflex taking in of air, breathing in doesn’t happen without muscular activity. The diaphram expands the chest cavity by pulling down into the lower chest. Breathing out can be a relaxation until a state of equal pressure with external air is reached. Then the diaphragm must activly push. The primary muscles for breathing are the diaphragm and the 22 plus intercostals.
The diaphragm is a thin, flat muscle that stretches from your spine and the back of your middle ribs (about the level of the top of your shoulder blades)angling downward to your your sternum (breast bone)in the front. It covers the entire surface of the bottom of your rib cage, side to side and front to back. The diaphragm goes under your major organs (heart, lungs, stomach, liver). It’s the floor of your upper body. Below the diaphragm are the intestines, bladder, etc. It separates the upper part of your trunk from the lower part.
Take a breath. Then force air out in short spurts, laughing “Ha, Ha, Ha, Ha, Ha”. With each ‘Ha” you can feel the contraction in your diaphragm as it pushes air out. You can tell it’s your diaphragm because the tension goes from front to back, across the body and pushes up. It is assisted by your abdominals that contract on the front and sides, and create the strong sides to the trunk cylinder. One loud loooooooooong “Haaaaaaaaaaaaaa” can give the feeling of the diaphragm in a static contraction. Of course, it is combined with other muscle contractions for stability and containment of pressure, particularly in the lower abdominals.
Role of the Intercostals
Intercostal means between the ribs. If breathing were accomplished with only your diaphragm and abdominals, there would be very little chest movement. Instead, good diaphragmatic breathing involved movement of the ribs as well. This movement is both out to the sides and minimally, up and down. If you find that you have lots of up and down chest movement when you breathe, you are probably substituting your neck and shoulder muscles for Intercostal movement.
Place your hands on your ribs in a sort of “I told you to….” position so that you can feel your ribcage open up. Relax your shoulders, chest and neck. Take in a deep breath, trying to minimize up and down movement, concentrating on developing a side to side expansion. As the intercostals begin to work, they change your posture, encouraging an upright and relaxed position.
Breathing while Working
Most work that people do involves using their hands. The most efficient place to use your hands is directly in front of your body. At the same time, you need to see what your hands are doing, so you often need to look down. These positions, especially if they are held for long periods, tend to encourage rounded shoulder and a contracted chest. Both of these work against good breathing.
If you have an electronic calendar on your computer, that can give you reminders, you may get some benefit from setting a ‘breathing appointment’ every hour or ½ hour, reminding you to correct your posture and take a couple of good breaths. Over time, this can become a habit.
It is easier to maintain good breath support with singing than with talking. A little music in your life may not only be good for the spirit, but good for your health.
I've attached a link to the most interesting breathing book I found on Amazon. You can browze from there and see other approaches, DVDs and Kindle breathing materials.