Sternal recession is a clinical sign of respiratory distress that occurs as high negative thoracic pressure causes indrawing of the chest. This phenomenon only occurs in children, since adults have much more bony thoracic features.
“Sternal recession is commonly seen in young infants with laryngomalacia, [collapse of the upper larynx during inhalation that causes airway obstruction]. Inspiratory stridor causes a paradoxical movement of the sternum. This deformity improves as the laryngomalacia settles and the skeleton becomes more rigid. Progression to a fixed pectus excavatum is rare in this group of children. In all other children with pectus deformities, there is no evidence that the deformity improves with time” (Williams & Crabbe, 2003).
Williams, A. M., and D. C. G. Crabbe. “Pectus deformities of the anterior chest wall.” Paediatric respiratory reviews 4.3 (2003): 237-242.