Managing the breath requires maintaining the efforts of expansion, largely accomplished by contracting the external intercostals
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.
When singing, one attempts to retard this motion by continuing to engage the external intercostals
It is correct that the muscles of inspiration, especially the external intercostals
, can remain contracted during the first portion of expiration, to keep the air from being expired too quickly.
Since contraction of either the diaphragm or the external intercostals
during exhalation will help minimize subglottal air pressure, use of either muscle achieves the same goal.