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Discussion Starter · #1 · (Edited)
A high yield topic in USMLE step 1 is the muscles of respiration and which one does what
Here's a quick encounter

Important to differentiate between quiet breathing and active breathing such as during exercise or in asthmatic patients

In Quiet Breathing;
Inspiration is accomplished by the diaphragm as it flattens downward towards the abdomen pushing the viscera down and causing the rise in the abdominal wall during inspiration.
Expiration is simply a passive relaxation of the diaphragm.

In Active Breathing;
Inspiration is effected by external intercostals (pulling the ribs up and outwards), scalene, and sternomastoids (pulling the first two ribs) so as the thorax is pulled up you have paradoxical inward abdominal wall movement in respiratory distress, you can try it yourself; breath deeply using your chest muscles only and you'll see that your tummy is sucked up and contrast that with your quiet breathing where your tummy actually rise in inspiration)
Expiration is effected by abdominal muscles pushing the diaphragm up and by internal intercostals pulling the ribs and down and inward.

External Intercostals = Pulling the ribs externally (outward) so inspiration
Internal Intercostals = Pulling the ribs internally (inward) so expiration
 
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