1. Diaphragm - contractions increase the volume of the thoracic cavity and draw air into the lungs.
2. External intercostals - elevate the ribs to transversely increase the thoracic air capacity.
3. Accessory muscles - supplement the external intercostals in elevating the ribs.
   • Sternocleidomastoid
   • Serratus anterior
   • Scalenes

Expiration is mostly a passive process caused by the elastic recoil of the lungs, but it becomes an active process when—as in COPD— increased respiratory effort is required. Intercostal muscles and abdominal muscles participate in the active exhalation process.

1. Internal intercostals - depresses the ribs to reduce thoracic capacity.
2. Transversus thoracis - reduces the thoracic diameter.
3. Abdominal muscles - assist the internal intercostals in expiration by compressing the abdomen to force the diaphragm upward.
   • External obliques
   • Internal obliques
   • Transverse abdominis
   • Rectus abdominis