Passive recoil of lung tissues expanding the thoracic cavity and drawing air in

Relaxation of the diaphragm leading to increased intrathoracic pressure and passive air movement into the lungs

Contraction of the diaphragm causing it to move downward and expansion of the chest cavity creating negative intrathoracic pressure

Contraction of the abdominal muscles to push the diaphragm upward, forting air into the lungs