During inspiration I am aware that the diaphragm and the external intercostal muscles contract, thus increasing the volume of the thoracic cavity.
However, I read on my textbook that during inspiration:
the diaphragm contracts, and at the same time the external intercostal muscles and one set of abdominal muscles help to raise the rib cage.
What I do not understand is the movement of the abdominal muscles, as I thought that they help in ventilation during exhalation (contract during exhalation, but not during inhalation). Is my assumption incorrect?