contraction of the internal intercostal muscles inspiration
Best Answer: During inspiration (breathing in) our external intercostal muscle contract and the internal intercostal muscle relax together with the movement of ribs and the thorax, this increases the volume of the throacic cavity.Air pressure in our lung decrease as the volume increases.ating Newest Oldest. Best Answer: During inspiration (breathing in) our external intercostal muscle contract and the internal intercostal muscle relax together with the movement of ribs and the thorax, this increases the volume of the throacic cavity. Air pressure in our lung decrease as the volume increases.