The diaphragm alone at rest, with accessory muscles for forced inspiration and abdominals for forced expiration
Mnemonic
“The diaphragm does the work; everything else helps”:
- Quiet inspiration - diaphragm (about two thirds of tidal volume) and the external intercostals
- Quiet expiration - passive, by elastic recoil. No muscles at all
- Forced inspiration - accessory muscles: sternocleidomastoid, scalenes, pectorals, serratus anterior, and the alae nasi
- Forced expiration - abdominal wall muscles and the internal intercostals
“C3, 4, 5 keeps the diaphragm alive” for the phrenic nerve.
Expiration being passive at rest is the point most often forgotten, and it explains why expiration is prolonged and active in obstructive disease, and why patients use pursed lip breathing to generate positive airway pressure and prevent dynamic collapse.
Accessory muscle use is a clinical sign of increased work of breathing, and paradoxical abdominal movement, where the abdomen moves inwards on inspiration, indicates diaphragmatic fatigue and impending respiratory arrest.
Expansion
Quiet inspiration
- Diaphragm, responsible for about 70 per cent of tidal volume
- External intercostals
Quiet expiration
- Passive, driven by elastic recoil alone
Forced inspiration adds
- Sternocleidomastoid, scalenes
- Pectoralis major and minor, serratus anterior, latissimus dorsi (when the arms are fixed)
- Alae nasi, and pharyngeal dilators
Forced expiration adds
- Abdominal muscles: rectus abdominis, obliques, transversus abdominis, which raise intra-abdominal pressure and push the diaphragm up
- Internal intercostals
The diaphragm’s dominance explains several clinical observations: C3, C4 and C5 injury is fatal without ventilation, bilateral phrenic palsy causes orthopnoea and paradoxical abdominal movement, and patients with weak abdominal muscles cannot cough effectively even when inspiration is normal.