What structure is involved in reducing thoracic volume during expiration when active?

The abdominal muscles play a key role in reducing thoracic volume during active expiration by contracting and pushing the diaphragm upwards. During normal breathing, expiration can be a passive process where the diaphragm relaxes and the elastic recoil of the lungs occurs. However, during active expiration, such as during vigorous exercise, the abdominal muscles contract forcefully. This contraction increases intra-abdominal pressure, helping to elevate the diaphragm further and forcing air out of the lungs more rapidly.

Additionally, while the intercostal nerves are involved in facilitating the movement of the intercostal muscles between the ribs, they primarily assist with inhalation and the mechanics of breathing, rather than directly causing a reduction in thoracic volume during expiration. The cervical and brachial plexuses are more associated with innervating upper limb muscles and are not directly involved in the mechanics of respiration. Thus, the involvement of the abdominal muscles in actively compressing the thoracic cavity is crucial for effective and forceful expiration.