The muscle that helps you breathe and separates the chest from the abdomen is the diaphragm.
More precisely called the thoracic diaphragm, it is a broad, dome-shaped sheet of skeletal muscle at the bottom of the chest. Its upper surface forms the floor of the thoracic cavity, while its lower surface faces the abdominal organs. When the diaphragm contracts, it moves downward, increasing chest volume and lowering pressure so air flows into the lungs. When it relaxes, the lungs recoil and air is pushed out.
The diaphragm is the main muscle of quiet breathing, although the intercostal muscles between the ribs also assist. It is controlled largely by the phrenic nerves, which arise from spinal nerves C3, C4, and C5. This nerve route reflects the diaphragm’s embryonic development, because it begins higher in the neck before descending into the chest.
A common mix-up is confusing the diaphragm with the abdominal muscles. The abdominal muscles help force air out during vigorous exhalation, but they do not provide the primary driving action for ordinary inhalation. The diaphragm also helps with coughing, vomiting, childbirth, and preventing acid reflux.