1. Inhalation & Exhalation
Antagonistic Muscle action in ventilation
External and internal intercostal muscles & diaphragm and abdominal muscles as examples of antagonistic muscle pairs
Ventilation involves 2 pairs of opposite movements that change the volume and therefore the pressure inside the thorax.

2. Inspiration
Diaphragm moves downward and flattens Ribcage moves upward and outward The volume inside the thorax increases, pressure decreases Pairs: Antagonistic Diaphragm contracts – moves down, pushes abdomen wall out Muscles in abdomen wall relax External intercostal muscles contract, move ribcage up and out Internal intercostal muscles relax

3. Expiration
Diaphragm moves up and becomes dome shaped Ribcage moves downward and inward Volume inside the thorax decreases, pressure increases Antagonistic Pairs: Diaphragm relaxes, pushed up into dome shape Muscles in abdominal wall contract pushing diaphragm up External intercostal muscles relax Internal intercostal muscles contract, pulling ribcage in