Muscles of Respiration - Diaphragm

The diaphragm is crucial for breathing and respiration. It's responsible for 45% of the air that enters the lungs during quiet breathing. During inhalation, the diaphragm contracts, thus enlarging the thoracic cavity (the external intercostal muscles also participate in this enlargement). This reduces intra-thoracic pressure: in other words, enlarging the cavity creates suction that draws air into the lungs. When the diaphragm relaxes, air is exhaled by elastic recoil of the lung and the tissues lining the thoracic cavity in conjunction with the abdominal muscles which act as an antagonist paired with the diaphragm's contraction.

It is not responsible for all the breathing related to voice. One has more control over the abdominals and intercostals than the actual diaphragm, which has relatively few proprioceptive nerve endings. By training proper posture and balance in the rest of the body, the diaphragm naturally strengthens and works in concert with surrounding structures rather than in isolation.

The diaphragm is also involved in non-respiratory functions, helping to expel vomit, feces, and urine from the body by increasing intra-abdominal pressure, and preventing acid reflux by exerting pressure on the esophagus as it passes through the esophageal hiatus.