Whether your stomach bulges or contracts when you inhale depends on how you breathe. With abdominal breathing, the stomach bulges when you inhale, and with thoracic breathing, the stomach contracts when you inhale.
1. Abdominal breathing: It refers to the improvement of ventilation and control of air in and out of the lungs through the descent of the abdominal muscles and diaphragm, increasing the upper and lower diameter of the thorax. During inhalation, the diaphragm descends and squeezes the organs downward, thus the stomach expands rather than the chest. When exhaling, the diaphragm rises and the belly contracts. Abdominal breathing can be a good way to build lung capacity as well as exercise the abdominal muscles.
2. Thoracic Breathing: The chest muscles, mainly the intercostal muscles, control the flow of air in and out of the lungs during the entire breathing movement. During inhalation, the intercostal muscles contract, the thorax enlarges, and the abdomen contracts. During exhalation, the intercostal muscles expand, the chest narrows, and the abdomen expands. Chest breathing is used for short periods of time when a large amount of air is needed for strenuous exercise.