Manor Preboards Module 3 Practice Test 2026 - Free Practice Questions and Study Guide

The diaphragm is the main muscle that drives normal inspiration. When it contracts, it moves downward, increasing the vertical size of the thoracic cavity. This expansion creates a negative pressure inside the chest, so air flows into the lungs.

Intercostal muscles assist by raising the ribs to expand the chest, but they are not the primary engine for quiet breathing; they play a supporting role in increasing chest volume. The sternocleidomastoid and other neck muscles become active mainly during heavy or labored inspiration to help lift the chest further. Abdominals aren’t used to inspire air; they mainly act during forced expiration by pushing the diaphragm upward and increasing intrathoracic pressure.

So, the diaphragm is the primary inspiratory muscle because its contraction directly expands the thoracic cavity and initiates the airflow into the lungs during normal breathing.