What is the main muscle required for breathing that lines the thorax and abdomen?

The diaphragm is the principal muscle responsible for breathing. It is a dome-shaped sheet of muscle that separates the thoracic cavity from the abdominal cavity. When the diaphragm contracts during inhalation, it moves downwards, increasing the volume of the thoracic cavity. This action creates a negative pressure that draws air into the lungs.

In contrast, the intercostal muscles, while they play a supportive role in assisting with breathing by elevating the ribs and expanding the chest cavity, are not the primary drivers of the inhalation process. Similarly, the rectus abdominis and external oblique muscles are primarily involved in movements related to posture, trunk flexion, and rotation, rather than directly facilitating the mechanics of breathing.

Thus, the diaphragm stands out as the main muscle specifically dedicated to the breathing process.