Agonist intercostals in the parasternal region and lower external intercostals also have a large inspiratory mechanical advantage [3-4], and they contract in a predictable manner at rest [1,5].
Of the intercostal segments, the midthoracic external intercostals are best understood, and they have been found to atrophy  and hypertrophy  in clinical diseases and conditions that often occur in conjunction with chronic changes in inspiratory motor activity.
Example: The internal and external intercostals
are located between the ribs and have the duties of stabilizing the framework of the rib cage and assisting with respiration.
Exhalation for singing is more counterintuitive than inhalation because it requires unfamiliar controls by muscles that are relatively unused in daily life, particularly the external intercostal
muscles of the ribs.
When the external intercostal
muscles contract, they lift the ribs up and out, expanding the lungs and causing inhalation.
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
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