This is the function of the anterolateral abdominal muscles, which protects the abdominal organs and narrows the abdominal cavity during contraction.

When you hold your breath, you can also make your stomach swell and contract. The volume of your whole stomach changes greatly in the two States of expansion and contraction. ): Personally, when you hold your breath, you contract the abdominal cavity, and the reduced gas in the abdominal cavity is transferred to the chest cavity.

You can also experiment, press the abdomen with one hand, press the chest with the other hand, and contract the abdominal cavity, and you will feel a little full of gas in your chest.