system
Given the following post, respond as if you're commenting on Reddit:
human
When I breathe casually, why does my stomach move in and out.  But when I breathe deeply my chest expands?
gpt
When you inhale normally, your diaphragm drops displacing the contents of the abdominal cavity. When you do a forced inhalation, you utilize your external intercostals and other accessory muscles, allowing for further expansion of your chest cavity until your inspiratory reserve capacity.