Final answer:
The diaphragm is the most significant contributor to normal, resting inspiration as it contracts to increase thoracic volume and decrease pressure, allowing air to enter the lungs.
Step-by-step explanation:
The single muscle that contributes most to normal, resting inspiration is the diaphragm. During inspiration, the diaphragm contracts and flattens, increasing the volume inside the pleural cavities and decreasing the pressure within them, causing air to flow into the lungs. While the external and internal intercostal muscles aid in the process by changing the shape of the rib cage and thereby the volume-pressure ratio inside the pleural cavities, it is the diaphragm that plays the primary role in inspiration at rest. Contraction of the diaphragm leads to a larger thoracic cavity and more space for lung expansion, which decreases intra-alveolar pressure and allows air to enter the lungs due to the resulting pressure gradient.