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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?

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    $\begingroup$ In inspiration just the external intercostal muscles contract and in expiration the abdominal muscles contracts, and the external intercostal muscles relax thus the diaphragm relaxes and curves upwards. Furthermore, also in expiration, the internal intercostal muscles contract $\endgroup$ – Ebbinghaus Apr 27 '16 at 18:05
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    $\begingroup$ Also note that during Eupnea, in inspiration only the diaphragm and external intercostal muscles are "required" $\endgroup$ – Ebbinghaus Apr 27 '16 at 18:10
  • $\begingroup$ @JordiZambrino Okay, so what is written on the textbook is incorrect right? And during expiration the diaphragm relaxes? Thanks. $\endgroup$ – city7lights Apr 28 '16 at 8:08
  • $\begingroup$ Not necessarily, its just that my textbook states something different. And for your second point: The Diaphragm relaxes and curves upwards, the abdominal muscles contract, thus pushing the Diaphragm upwards, consequently the External intercostal muscles relax and the ribs fall, in the end, the Internal intercostal muscles contract, hence pulling the ribs downwards $\endgroup$ – Ebbinghaus Apr 28 '16 at 8:15

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