Muscles of Breathing: Inspiration and Expiration in Normal Respiration

Quiet breathing (eupnea) relies on coordinated muscle activity to cyclically change thoracic volume.

Inspiration (Active)

• Diaphragm (primary): Contracts → flattens → increases vertical thoracic dimension → negative intrapleural pressure → air inflow.
• External intercostals: Elevate ribs → increase anteroposterior and lateral dimensions (bucket-handle & pump-handle motions).
• Accessory muscles (if needed): Scalenes, sternocleidomastoids, pectoralis minor—recruited during exercise or distress.

Expiration (Passive at Rest)

• Relaxation of diaphragm and external intercostals → elastic recoil of lungs/chest wall → volume decreases → air outflow.
• No muscle contraction required in healthy individuals at rest.

Forced Expiration (Active)

• Internal intercostals: Depress ribs.
• Abdominal muscles: Rectus abdominis, obliques, transversus abdominis → increase intra-abdominal pressure → push diaphragm up.
• Internal intercostals (interosseous part) and innermost intercostals assist.

Clinical Relevance

• COPD: Flattened diaphragm → reduced mechanical advantage; accessory muscle use at rest.
• Neuromuscular disease: Diaphragm weakness → orthopnea, hypercapnia.
• Understanding muscle roles guides ventilator management and rehabilitation.