It gave off the small branches which arose on average 18 mm from the origin point, supplied the first external intercostal muscle
When the external intercostal muscles
contract, they lift the ribs up and out, expanding the lungs and causing inhalation.
For our thoracic cavity to expand, we must employ specific muscles; namely, the diaphragm and the external intercostal muscles (muscles lying between our ribs).
Thus, when people speak of "using the diaphragm" during playing, they are misinterpreting what actually happens; the diaphragm undergoes controlled relaxation during expiration and, together with relaxation of the external intercostal muscles, can only help to control the rate of emptying rather than the extent.
The external intercostal muscles which contracted concentrically to raise the rib cage during inhalation act eccentrically during exhalation as they relax and lengthen to lower the rib cage back down under control.
Both the external intercostal muscles
and the intercondral elevate the ribs, thus increasing the width of the thoracic cavity, while the diaphragm contracts to increase the vertical dimensions of the thoracic cavity, and also aids in the elevation of the lower ribs.
The main muscles of inspiration are the diaphragm, external intercostal muscles, and accessory muscles of the shoulder and the upper back.
Thus, also stimulating external intercostal muscles or abdominal muscles is not necessary to maintain adequate alveolar ventilation in a nonexercising, paralyzed individual .
Contraction of the external intercostal muscles pulls the ribs outward, thereby preventing the inward movement of the upper rib cage; the net effect is to augment the volume of inspired air (Figure 3).
She reminded me that the "checking action" during expiration, which keeps the elastic recoil forces of expiration from expelling all the inhaled air immediately, is provided by the external intercostal muscles
Some singers might be able to regulate pulmonary pressure for breath support through contraction of the diaphragm during exhalation (muscular antagonism); many more believe they are using their diaphragms for this purpose, but are actually engaging the external intercostal muscles