The external intercostal muscles occupy the posterior and lateral intercostal spaces, with fibres running inferomedially analogous to the external oblique in the abdomen. They elevate the ribs during inspiration by pulling the lower rib upward toward the upper rib, increasing both the transverse and anteroposterior chest diameters. The external intercostals are the primary respiratory muscles after the diaphragm.
| Origin | Inferior border of each rib from the tubercle to the costochondral junction |
|---|---|
| Insertion | Superior border of the rib below — fibres run inferomedially (downward and forward) |
| Nerve Supply | Intercostal nerves (T1 through T11) |
| Blood Supply | Posterior intercostal arteries |
| Actions | Elevation of the ribs during inspiration; Stiffen the intercostal spaces to prevent paradoxical inward movement during inspiration |
|---|
The external intercostal fibres run in the direction that produces rib elevation from the bucket-handle and pump-handle movements that increase chest volume during inspiration. Their orientation is the opposite of the internal intercostals (which lower the ribs during expiration).
Intercostal nerve blocks target the intercostal nerve in the neurovascular bundle at the inferior rib margin between the internal and innermost intercostal muscles, providing analgesia for rib fractures, thoracotomy pain, and herpes zoster. Intercostal flaps based on the posterior intercostal artery are used in chest wall reconstruction. Fractured ribs produce paradoxical chest wall movement (flail chest) when multiple consecutive ribs are fractured in two places.
The external intercostal muscles fill the rib spaces laterally and posteriorly, palpable during deep inspiration as the spaces stiffen against inward pressure.
External intercostal space injury from direct trauma producing localised rib pain with chest wall tenderness and restricted inspiration, managed with adequate analgesia.
