Which one is the primary muscle of breathing?

The largest and most efficient muscle is the diaphragm. The diaphragm is a large muscle that lies under the lungs and separates them from the organs below. As the diaphragm moves down or flattens, the ribs flare outward, the lungs expand and air is drawn in. This process is called inhalation or inspiration. It is attached to the spine, ribs and sternum and is the main muscle for respiration, playing a very important role in the breathing process.