The muscles will atrophy (shrink) if not used.
you excersise
They contract
you will die
you exhale (:
contrast
Im not expert, but isn't it the diaphragm and rib muscles, the muscles between the ribs, internal and external intercostals.
What happens when a muscle shortens is that it is contracting. This is when the fibers of the muscles generate tension.
muscles relaxes
Atrophy
The internal intercostal muscles relax and the external intercostal muscles contract, pulling the ribcage upwards and outwards.
The fibres are ripped apart
Movement