Which muscle expands the thorax and compresses the abdomen during respiration?

• A. Diaphragm
• B. Intercostal Muscles
• C. Pectoralis Minor
• D. External Oblique

Answer: (A)

During breathing, the diaphragm acts as the main inspiratory muscle. When it contracts, it moves downward, increasing the vertical dimension of the thoracic cavity and thereby expanding the chest. This expansion lowers intrathoracic pressure and draws air into the lungs. At the same time, the downward movement compresses the abdominal contents, elevating intra-abdominal pressure and causing the abdomen to be compressed outward. This combination— expanding the thorax while compressing the abdomen—fits how the diaphragm works during respiration. The other muscles either focus on moving the rib cage to expand the chest or on compressing the abdomen for forced expiration, but not both in this way.