Intercostal muscles

External and internal muscles between the ribs essential to breathing (see figure below).  Preterm infants have difficulties in coordinating these muscles during breathing such that the diaphragm moves out on inspiration and in on expiration (so-called paradoxical breathing).

Action of the intercostal muscles during inspiration and expiration. A: Ribs, B: Sternum, C: External intercostal muscles, D: Internal intercostal muscles, E: Diaphragm. 

See Fetal breathing movements, Preterm infant