Final answer:
Maximal forced expiration involves active muscle contractions that forcefully expel air from the lungs, decreasing thoracic volume and increasing intrapulmonary pressure beyond normal passive exhalation.
Step-by-step explanation:
During maximal forced expiration, which is a form of hyperpnea, various muscle groups are involved in forcibly expelling air from the lungs beyond the normal passive exhalation. In addition to the relaxation of the diaphragm and intercostals after inspiration, other muscles contract to further decrease the volume of the thoracic cavity. The accessory muscles of the abdomen, such as the obliques, contract to push the abdominal organs upwards against the diaphragm. This, combined with the action of the internal intercostals, results in a significant decrease in thoracic volume and increase in intrapulmonary pressure, forcing air out.
These muscular contractions increase the force and amount of air expelled from the lungs, which is measured as forced expiratory volume (FEV). The functional residual capacity (FRC), which is the volume of air remaining in the lungs after maximal expiration, is also affected through this process, and it is considered when measuring the resistance to airflow and potential lung obstruction.