Which muscle is the primary inspiratory muscle?

• A. Intercostal muscles
• B. Abdominal muscles
• C. Diaphragm
• D. Scalenes

Answer: (C)

The primary driver of quiet inspiration is the diaphragm. When it contracts, it moves downward and flattens, increasing the vertical dimension of the thoracic cavity and drawing air into the lungs. This action accounts for most of the volume change during normal, resting breathing. The intercostal muscles assist by lifting the ribs to expand the chest, but they don’t generate the main inspiratory effort in calm breathing. Abdominal muscles aren’t involved in inspiration; they mainly help with forced expiration and increasing intra-abdominal pressure. The scalene muscles can lift the upper ribs to aid inspiration during labored breathing, but they act as accessories rather than the primary engine.