Many muscles are used for breathing. During deep expiration, the abdominal muscles squeeze the abdominal contents upward, forcing the chest cavity to become smaller. Muscles running from the vertebrae to the ribs, and muscles running in between the ribs flex to rotate the ribs and sternum up and out. This increases the volume of the chest cavity during deep inspiration. However, the principle muscle used for respiration at all times is the dome shaped diaphragm. The diaphragm serves as a barrier between the abdomen and thorax. When contracted it flattens and pulls itself down, thus making more room inside the thorax. Because there is more space available in the thorax, air rushes in to fill the space, passing through the trachea and blowing up the lungs. When the diaphragm is relaxed, pressure from the abdominal contents pushes the diaphragm back up, decreasing the space available in the thorax, causing the air to rush out, and thus deflating the lungs.
The best way to strengthen your diaphragm is through aerobic exercise. Check out our Aerobic exercise page.