Which muscle assists in the expansion of the thoracic cavity during inhalation?

The diaphragm is a dome-shaped muscle that plays a crucial role in respiration. It separates the thoracic cavity from the abdominal cavity and is the primary muscle involved in the process of inhalation. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the thoracic cavity. This increase in volume decreases the pressure inside the thoracic cavity relative to atmospheric pressure, allowing air to flow into the lungs.

Additionally, while the intercostal muscles also assist during inhalation by elevating the ribs and expanding the chest wall, the diaphragm is the main driver of the initial phase of inspiration. The other muscles listed, such as the rectus abdominis and sternocleidomastoid, are involved in different functions and movements but do not play a direct role in the expansion of the thoracic cavity during inhalation. The rectus abdominis primarily facilitates bending and compression of the abdomen, while the sternocleidomastoid assists with head movement and elevating the sternum but is not essential for the expansion of the thoracic cavity during normal breathing. Thus, the diaphragm is rightly identified as the key muscle assisting in this process.