According to Western medicine, as well as Hatha yoga, Chinese medicine, Tai Chi, Qi Kwong, and other holisitic therapies, most of the breathing at rest is done using the diaphragm.
There are two great advantages of the diaphragmatic breathing in comparison with chest or costal breathing. The first one relates to natural massage of the lymphatic nodes located just under the diaphragm. The lymphatic system, or the sewage system of the body, does not have a pump and therefore, the lymph nodes are located in those parts of the body where natural massage is created. For example, we have large concentrations of lymph nodes on our necks. During the thousands of movements of our head, which we normally have throughout the day, the lymphatic fluid is pushed through valves in one direction creating natural drainage of the lymphatic system. Similarly, we have many lymph nodes around the under arms and adjacent areas, so that when we move our arms, the lymph nodes receive natural massage to remove waste products. In the groin area, there is another set of lymph nodes which is massaged by movements of our legs. There are no lymph nodes in those places of the body which do not experience natural massage.
However, the lymph nodes from the stomach, kidneys, liver, spleen, pancreas, both colons, and other organs are located just under the diaphragm so that nature expects us to breath using the diaphragm at rest.
The second reason relates to homogenous filling of lungs with air. When we breathe using the chest, only the upper parts of our lungs get fresh air since the diaphragm is almost immobile as it is the case when we slouch. When our posture is straight and we can use our diaphragm for breathing, the diaphragm moves as a piston in the cylinder of a car engine. In this situation, the whole lung is stretched down and all of the alveoli get normal gas exchange.
What is practically noticed, people, who have sufficient oxygenation of the body, naturally breathe using the diaphragm all the time, as it was over 100 years ago for most people. Oxygenation of ordinary people was equal to about 40-60 seconds.
In order to check oxygenation of the human body, the following test was suggested. After usual exhalation, hold your breath, but only until first stress. This test should be stress-free so that after breath-holding, the breathing pattern is unchanged.
Most modern people, even those who do not have chronic health problems, have about 20-25 seconds of oxygen in the body and their breathing is about twice bigger than the medical norm. overbreathing reduces carbon dioxide concentration in all muscles, including the diaphragm. Since CO2 is a relaxant of all smooth muscles of the human body, hyperventilation creates a state of spasm in the diaphragm. Moreover, deep breathing also predisposes us to slouching due to tension around the shoulders, neck, and other muscles in the upper part of the human body.
In sick people, body oxygenation is usually less than 20 seconds (see the Table with over 30 western medical research studies)
Russian medical doctors various developed techniques, breathing exercises, and methods how to normalize breathing, solve the problem with costal breathing, and increase body oxygenation so that people have straight posture and diaphragmatic breathing all the time without conscious attention. This therapy is known as the Buteyko breathing self-oxygenation method.