Final answer:
During heavy breathing, expiration involves active contractions of the accessory muscles like abdominal muscles and internal intercostals, forcing air out of the lungs as pressure within the thoracic cavity increases.
Step-by-step explanation:
During heavy breathing, also known as forced breathing or hyperpnea, the process of expiration is no longer passive but rather an active process requiring the contraction of additional muscles. While the diaphragm relaxes and moves upwards, and the rib muscles relax for a normal expiration, during heavy breathing other accessory muscles like the abdominal muscles including the obliques, and the internal intercostal muscles contract.
This active contraction increases the pressure in the thoracic cavity, forcing the air out of the lungs more forcefully and quickly than during normal expiration. The elasticity of the lungs also plays a role, as the lung tissue recoils for the air to be expelled.