Other muscles, which support inhalation and exhalation [inspiration and expiration] besides the diaphragm, are called accessory muscles of respiration.
Accessory muscles of respiration, which support inhalation [inspiratory muscles], lift the rib cage up. Most of them are located at the back of the ribcage.
Accessory muscles of respiration, which support exhalation [expiratory muscles], cause the rib cage to contract
Most of them are located at the front of the ribcage. The intercostal muscles, muscles between the ribs, are an exception to this rough classification; they run all the way around the rib cage. Over working one of these muscle groups has many different consequences.
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Mode and intensity of the exertion (e.g. stair climbing compared to sitting still) determine the ratio in which the diaphragm and accessory muscles of respiration divide the breathing work. Like the name accessory muscles already suggests, the accessory muscles of respiration are merely supposed to help and support breathing, if necessary.
If the lungs are "harder to move" due to narrowed airways as in the case of COPD, the accessory muscles of respiration are already to a great extent necessary for breathing at rest. In general, impeded respiration costs more energy (keyword "cachexia"). Respiratory muscles can be overstressed. Consequences are far-reaching.
