The glottic opening is largest during deep inhalation. This is the phase of breathing where the vocal cords are fully abducted, allowing maximum airflow into the lungs. When we take a deep breath, the muscles controlling the vocal cords, primarily the posterior cricoarytenoid muscles, pull the cords apart wider than at rest.
During normal breathing or expiration, the glottic opening is smaller as the vocal cords come together again to allow for sound production and to a certain extent, regulate airflow. In contrast, during deep inhalation, the need for increased air intake requires the glottic opening to expand significantly. This physiological adaptation ensures that we can take in adequate oxygen, making the glottic opening at its largest during this phase of respiration.