Your ability to breathe is largely a function of several key muscles that respond to chemical changes in your blood and nerve impulses from your brain's respiration center. Your breathing is under both voluntary and involuntary control, and involves two distinct phases: inhalation and exhalation. Inhalation typically is an active movement, and it involves muscle contraction from your diaphragm, abdominal muscles and intercostal muscles to be maximally effective. Exhalation typically is a passive movement, relying on the elastic recoil of your lung's air sacs or alveoli and other tissues, although some active movement may also occur, especially during vigorous activity.
Diaphragm
Your diaphragm is a thin layer of muscle extending across the bottom of the rib cage that separates your chest and abdominal cavities, and is your most important breathing-related muscle, especially during quiet breathing, according to Johns Hopkins University. When you inhale, your diaphragm contracts and moves downward, away from your chest cavity, and the pressure in your lungs drops. The resulting pressure gradient causes air to move from your nose or mouth down to your alveoli.
During exhalation, according to the National Heart Lung and Blood Institute, your diaphragm relaxes and shifts upward into your chest cavity. Your diaphragm also helps you vomit, expel solid and liquid waste and prevent acid reflux by placing pressure on your esophagus as it passes through your diaphragm's esophageal hiatus.
Abdominal Muscles
Your abdominal muscles are important for breathing, both when you're at rest and when you're exercising. These muscles help move your diaphragm during inhalation and give you more power to empty your lungs, according to the Cleveland Clinic. Medical-Look.com states that your abdominal muscles rely on your respiratory system during activities that require a high degree of exertion. Your abdominal muscles become more effective with the delivery of extra oxygen, which explains your natural desire to take a deep breath when pushing out your abdominal muscles or lifting something heavy.
The American Medical Student Association recommends that you practice abdominal or diaphragmatic breathing--also known as "belly" breathing--to improve your flow of blood and lymph, improve your heart function, reduce your stress and improve your stamina. Belly breathing is the most efficient method of breathing because it promotes a greater expansion your rib cage and allows ventilation to occur in the lower part of your lung, which is bathed in the greatest amount of blood.
Intercostal Muscles
Your intercostal muscles consist of several groups of muscles--internal and external intercostals--and are located between your ribs. These muscles help form your chest wall and assist ventilation, especially during exercise, by elevating or depressing your ribs.
A 2004 study published in the "Journal of Physiology" examined the effects on lung gas exchange of diaphragmatic breathing compared with intercostal muscle breathing. Researchers determined that diaphragmatic breathing is significantly more efficient than intercostal muscle breathing, which means that using your abdomen to assist your breathing is more efficient than using your chest cavity to breath.
Your intercostal muscles are important ventilatory muscles, but they should only be actively used during activities--such as vigorous exercise--that require significant rib cage expansion and a corresponding increase in oxygen intake.
