Final answer:
The muscles involved in expiration are the internal intercostal muscles, which contract to reduce the rib cage volume, and during forced exhalation, the abdominal muscles assist in expelling air. The diaphragm relaxes during this phase, aiding in decreasing thoracic volume.
Step-by-step explanation:
The muscles used for expiration primarily involve the internal intercostal muscles, which are situated just beneath the external intercostals and have the role of drawing the ribs together to constrict the rib cage. In addition to these muscles, during forced exhalation such as during strenuous activity or voluntary hyperventilation,
the abdominal muscles including the rectus abdominis, obliques, and transversus abdominis also contract to increase the pressure on the lungs and push air out more forcefully. The diaphragm, although it is the primary muscle for inspiration, also relaxes during expiration, contributing to the reduction in the thoracic cavity volume.
Different muscles are engaged in forced breathing, such as the scalenes for forced inspiration, and various abdominal muscles alongside the internal intercostals for forced expiration. The process of expiration can be passive during relaxed, quiet breathing, as it primarily relies on the recoil of the lungs and relaxation of the inspiratory muscles, or active during forced breathing, involving deliberate muscular contractions.