Most people breathe without even thinking about it. The purpose of breathing is to bring oxygen into the body when you breathe in and move carbon dioxide out of the body when you breathe out. The diaphragm, a large muscle that separates the lungs from the abdomen, is the main muscle responsible for normal breathing. The outer layer of intercostal muscles -- small muscles located between each rib -- also plays a small role in normal breathing. When the diaphragm and outer intercostal muscles are unable to move enough air in and out of the lungs, then additional muscles -- the accessory muscles of respiration -- are called into action. Use of accessory muscles often indicates the presence of a disorder affecting a person's ability to breathe.
Accessory Muscles of Inspiration
The accessory muscles used when breathing in -- called accessory muscles of inspiration -- include the scalene, sternocleidomastoid, trapezius and pectoralis major muscles. These muscles are found around the shoulders, neck and upper chest. When they contract, the accessory muscles of inspiration lift up the breastbone, upper ribs and collarbones. This causes the upper part of the chest to rise, making the lungs bigger and allowing more air to enter. When a person is not taking in enough oxygen, the body automatically activates these muscles every time the person breathes in.
Accessory Muscles of Expiration
Accessory muscles may also be used when a person breathes out. Normal breathing out is a passive action that occurs naturally without the use of any muscles. It occurs when the diaphragm and outer intercostal muscles relax, which reduces the size of the lungs and forces air out. When a person is unable to breathe out enough air, the accessory muscles of expiration automatically become activated.
The main accessory muscles of expiration are the abdominal wall muscles. When they contract, the diaphragm is pushed up into the chest, forcing more air out of the lungs. The inner layer of intercostal muscles are also accessory muscles of respiration. While the outer intercostal muscles assist with breathing in, the inner intercostal muscles do the opposite. When they contract, the chest becomes smaller, compressing the lungs and forcing air out.
Causes of Accessory Muscle Use
Accessory muscles are not used during usual breathing. In a person with normal lungs, accessory muscles of inspiration become active when a person tries to take in a big breath. In a similar manner, the accessory muscles of expiration are activated when a person tries to forcefully breathe out, such as when trying to blow out a candle. In a person with a disorder that affects their breathing, the accessory muscles can be automatically activated, even during usual breathing.
Accessory muscles of inspiration may be used with conditions such as pneumonia, fluid buildup in the lungs or any blockage that does not allow adequate air to pass through the throat and windpipe and into the lungs. Accessory muscles of expiration are used in disorders preventing normal movement of air out of the lungs. People with severe chronic obstructive pulmonary disease (COPD) often use these muscles, as air that cannot be breathed out becomes trapped in the lungs.
Using accessory muscles during regular breathing is generally a sign of an underlying problem. If you or someone else notices that you seem to be working extra hard to breathe, see your doctor promptly. Treating the underlying cause will help reduce the use of these muscles. If you have COPD and are using your accessory muscles of expiration, your doctor may recommend a breathing technique called pursed lip breathing, which may help improve your ability to breath out and ultimately decrease your accessory muscle use.
Reviewed by: Mary D. Daley, M.D.