Which muscle is the primary muscle of respiration?

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

Answer: (A)

The diaphragm is the primary muscle of respiration. When it contracts, its central tendon moves downward and the muscle flattens, increasing the vertical dimension of the thoracic cavity and creating negative intrathoracic pressure that draws air into the lungs. While other muscles like the intercostals help move the chest wall and assist with breathing, the diaphragm does the bulk of the work during normal, quiet inhalation, making it the main driver of ventilation. The abdominal muscles come into play mainly during forced expiration, coughing, or heavy breathing by increasing intra-abdominal pressure to push the diaphragm upward. The scalene muscles can aid inspiration by lifting the upper ribs, but they’re not the primary inspiratory muscle. The diaphragmatic movement is mainly controlled by the phrenic nerves (C3–C5).