As a respiratory physiologist, I can explain the process of how your stomach moves during breathing. The primary muscle involved in respiration is the Thoracic diaphragm, which separates the thoracic cavity from the abdominal cavity. When you breathe in, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity. This action creates a negative pressure within the lungs, drawing air in. As the diaphragm moves downward, it pushes the abdominal organs downward as well, causing your belly to move outwards. Conversely, when you breathe out, the diaphragm relaxes and moves upward, reducing the volume of the thoracic cavity. This causes the pressure inside the lungs to increase, which pushes the air out. As the diaphragm moves upward, the abdominal organs are no longer pushed downward, allowing your belly to move inwards. So, the movement of your stomach during breathing is a direct result of the diaphragm's action in creating the pressure changes necessary for respiration. read more >>

The main breathing muscle is your Thoracic diaphragm, a muscle which delineate the thoracic from the abdominal cavity, which when breathing in expands your lung by tightening, so going down and making your belly move outwards, when breathing out the diaphragm relaxes, going up, and your belly moves inwards.read more >>