During which phase does the relaxation of the inspiratory muscles occur?

The relaxation of the inspiratory muscles primarily occurs during the phase of quiet expiration. During this stage, following the active contraction of the inspiratory muscles (like the diaphragm and intercostal muscles) that facilitate inhalation, these muscles relax, allowing the elastic recoil of the lungs and chest wall to drive air out of the lungs passively. This process does not require additional muscular effort and is a natural consequence of the lung mechanics. It highlights the significance of the mechanics of breathing, where inhalation involves energy expenditure, while exhalation during quiet breathing is a passive process where relaxation of those muscles aids in the return of the thoracic cavity to its resting position.

In contrast, the other options involve different muscular activities or phases. For instance, during forced expiration, active contraction of abdominal muscles occurs to expel air more forcefully. Spontaneous inspiration typically involves the active engagement of inspiratory muscles, and deep breathing consists of deeper and often more forceful inhalations and exhalations, which also contribute additional muscle activity.