what is the diaphragm and intercostal muscles
1 the intercostal muscles contract, expanding the ribcage. 2  the diaphragm contracts, pulling downwards to increase the volume of the chest. 3  pressure inside the chest is lowered and air is sucked into the lungs.hen you inhale inhale: breathe air in 1  : the intercostal muscles contract, expanding the ribcage. 2  the diaphragm contracts, pulling downwards to increase the volume of the chest. 3  pressure inside the chest is lowered and air is sucked into the lungs.