Rib Mechanics During Inspiration: Diaphragm, Intercostals, and Chest Wall Expansion

Inspiration is an active process driven by coordinated muscle action to increase thoracic volume.

Primary Driver: Diaphragm

• Contracts → flattens → increases vertical thoracic dimension (70–75% of tidal volume at rest).
• Domed shape descends, pulling central tendon down.
• Costal fibers pull lower ribs upward/outward ("bucket handle" motion).

Accessory Inspiratory Muscles

• External Intercostals: Run inferoanteriorly; contraction lifts ribs → increases anteroposterior diameter ("pump handle" at upper ribs, "bucket handle" at lower).
• Scalenes (Anterior/Middle): Elevate 1st/2nd ribs → ↑ AP diameter at apex.
• Sternocleidomastoids: Elevate sternum (forceful inspiration).
• Pectoralis Minor, Serratus Anterior, Latissimus Dorsi: Fix scapulae to allow rib elevation.

Rib Motions

• Pump Handle (Ribs 1–6): Anterior end rises → ↑ AP diameter.
• Bucket Handle (Ribs 7–10): Lateral shaft rises → ↑ transverse diameter.
• Caliper (Ribs 11–12): Floating ribs flare laterally.

Expiration

Passive at rest: elastic recoil of lungs/chest wall + diaphragm relaxation. Active in exercise: internal intercostals, abdominals (rectus, obliques, transversus), internal obliques pull ribs down/in.

Clinical Relevance

• Paradoxical breathing (diaphragm paralysis, flail chest).
• Accessory muscle use → respiratory distress sign.
• COPD: flattened diaphragm → mechanical disadvantage → recruitment of accessories.