you excersise
Your muscles are like rubber. At a young age they are stretchy but when you get older your muscles begin to harden. When this happens it is important that you stretch them before you do any kind of sports. If you don't stretch your muscles you could really damage your body.
grow muscles when your older
The muscles will atrophy (shrink) if not used.
They contract
When you get older you become more stubborn that what happens when you get older
you will die
you exhale (:
contrast
fibre is used for connecting muscles as you get older, as they get bigger.
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.