The primary muscles used in inspiration include diaphragm and external intercostal muscles.
The secondary muscles used in inspiration include the sternocloidomastoid and scalini
The primary muscle involved in expiration is the diaphragm.
The secondary muscles involved in expiration include the internal and external intercostals, external oblique, internal oblique, transverse abdoninal and rectus abdominis. These are the muscles which become fatigued in the severe asthmatic patient.