Unused muscles atrophy.
The muscles will atrophy (shrink) if not used.
They contract
you excersise
well , total paralysis on one side of body is called hemiplegia...... where as weakness is called paraplegia..... these may be due to damage at the brain or spinal cord...
contrast
you will die
you exhale (:
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.
Atrophy
muscles relaxes
The internal intercostal muscles relax and the external intercostal muscles contract, pulling the ribcage upwards and outwards.