Which sheet of muscle is the main muscle of respiration?

• A. Intercostal muscles
• B. Abdominals
• C. Diaphragm
• D. Trachea

Answer: (C)

The diaphragm is the main muscle of respiration. It is a dome-shaped muscle that separates the chest from the abdomen. When it contracts, it moves downward and flattens, increasing the vertical size of the thoracic cavity and reducing pressure inside the lungs, which draws air in for inhalation. This action powers quiet, everyday breathing and is the primary driver of inspiration. Other muscles, like the external intercostals, assist by expanding the chest further during deeper breaths, but the diaphragm does the bulk of the work. Abdominal muscles help with forced expiration, not normal inhalation, by pushing the abdominal contents up to push air out. The trachea, meanwhile, is simply the airway and not a muscle.