During which phase of breathing does the diaphragm primarily contract?

The diaphragm primarily contracts during inhalation. When inhaling, the diaphragm moves downwards, increasing the volume of the thoracic cavity and reducing the pressure inside the lungs. This pressure gradient allows air to flow in from the outside environment into the lungs. The contraction of the diaphragm is a critical component of the respiratory process, as it assists in maximizing lung capacity and facilitating efficient gas exchange.

In the context of the other options, during expiration, the diaphragm relaxes, and the elastic recoil of the lungs and thoracic cavity helps push air out, making it less active in the expulsion of air. The resting state does not involve the active contraction of the diaphragm, as it is primarily engaged during inhalation, and both expiration and inhalation together do not accurately specify the phase that primarily features diaphragm contraction. Therefore, the correct answer is that the diaphragm chiefly contracts during inhalation.