Biology
What's the meaning of the muscles
The muscles work like a pump. We must use them to move because they are use to bring the for arm on the arm by example.
The muscles are also a protection for the organs, and they maintain the organs at their place in the body.
How are build the muscles and how they work
The muscles are composed by fibres. When you flex a muscle, those fibres ,who are side by side, come up one on one. So the length of the muscle become is wide. And it's the reason why a muscle growth when you flex it.
To be precise, this fibres are in fact cellules. So they are organics and works like the body. And the body is like the nature, it always want to do the fewest he can.
So the volume of the muscle is always do for the work of all the day. Not too big, not too small. And if you begin to do a work (like training) who need to use more power, more muscles. Who tired the body, he'll find a solution. This solution is the make the muscles growing.
They growth till they are able to make the new work.
It's like that (with more chemicals process and biologic process) that the muscles growth.
In the opposite solution, when you stop to do a work who need a certain volume of muscle, the body react by reducing the volume of the muscle.
So when you'll stop training for a long period, the body don't need to have big muscles, so he reduce their volume.
This become faster that the growing, because to growth the body must build bigger cells. And it's longer than build normal cells.
But don't worry, the body has memory, and when you'll train back, he rebuild fast bigger cells.