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