The internal intercostal muscles are a group of skeletal muscles located between the ribs on the inside of the rib cage.
They originate on ribs 2–12.
Insertions on ribs 1–11.
In the respiratory system, the internal intercostals is an expiratory muscle.
Most often the respiratory system, more specifically, the inspiratory and expiratory muscles, are not thought to be trainable except through aerobic type activities.
This, however, is only partially true. The respiratory system muscles can also be strengthened with specific strength training exercises.
Volume 12 - The Respiratory System of our Kinesiology of Exercise Ebooks is a first for a book on strength training for the respiratory system.