The breathing muscles in humans are the diaphragm, the Upper Airway (UA) muscles, the intercostal muscles between the ribs, the neck muscles, and the abdominal muscles. Human breathing muscles may operate under voluntary or involuntary control. The muscles have varying degrees of involvement depending on whether breathing is voluntary, involuntary, or more rapid, as in breathing during exercise.
For breathing to occur, the muscles of the chest cavity expand, causing a vacuum and drawing the lungs outward and downward. As the lungs expand, air rushes into the space created within them, filling the tiny airways.
The diaphragm is the primary breathing muscle and is the divider between the thoracic, or chest, cavity, and the abdominal cavity. This unique muscle is involved in breathing in and out, whether voluntary or involuntary, and draws the chest cavity downward. During inhalation, the diaphragm is helped by the external intercostal muscles, located on the outside of the ribs. The intercostal muscles help increase the size of the chest cavity by drawing the chest outward. Since the diaphragm is so effective at regulating pressure in the abdominal cavity, it helps in non-breathing functions, like vomiting and defecating.
When the body is at rest, exhalation is mostly a process of relaxation of the breathing muscles with little muscle movement. When the body is active or stressed and requires more air, the diaphragm and the external intercostals work in reverse to increase air pressure in the lungs and push air out. During exercise, the abdominal muscles become one of the primary muscles involved in breathing by pushing the internal organs of the abdominal cavity upward against the diaphragm.
During rapid breathing, either due to exercise or diseases such as emphysema, accessory muscles get involved. The scalene and sternocleidomastoid muscles, which lie along the sides of the neck, are skeletal muscles that help in breathing. Other accessory breathing muscles are in various parts of the neck and shoulders, but researchers do not agree on which ones are involved and to what degree.
There are three levels of control that the body has over its breathing processes. The autonomous level is the automatic, involuntary respiratory process that involves the fewest breathing muscles. When speaking or exercising, the adaptive levels of control take over. The muscles may also be controlled voluntarily, at the behavioral level.
Some diseases of the lungs can put extra stress on breathing muscles and overwork the accessory muscles. Pneumonia, asthma, chronic bronchitis, lung cancer, and emphysema can all affect the ability of air to be exchanged in the alveoli or bronchial passages. Cystic fibrosis is a genetic disease that causes build-up of mucous in the lungs. In severe cases of lung disease, oxygen or artificial respiration may be needed.