The ______ is the muscle responsible for inspiration during quiet breathing.

Internal intercostals are primarily accessory muscles of forced expiration, drawing the ribs downward and inward to decrease thoracic volume.

During quiet tidal breathing, the diaphragm contracts and descends, increasing the vertical dimension of the thoracic cavity. This expansion reduces intrathoracic pressure below atmospheric pressure, causing air to flow passively into the lungs. The diaphragm is responsible for approximately 70-80% of the work of quiet inspiration and is the only muscle required under resting conditions.

External intercostals assist with inspiration by elevating the ribs but are accessory muscles recruited during increased ventilatory demand, not the primary driver during quiet breathing.

The rectus abdominus is an abdominal muscle involved in forced expiration and trunk flexion and plays no role in quiet inspiration.