Imagine you have achieved your ideal physical condition. You're at a perfect weight, muscle tone, strength, etc. To celebrate, you decided to take it easy for 100 days. What happens? If you did nothing, your muscles would waste away, or atrophy. (see figure)
Of course, we are never completely inactive. The next figure shows that our muscles would atrophy until a certain point sustained by pushing buttons on our remote controls, getting up off the sofa, opening the refrigerator door, lifting forks to our mouths, etc.
But we miss being in shape, so every weekend we go crazy. We hike, do yard work, ride our bike, play tennis, soccer, basketball, etc. The figure below shows that we are doing better than our couch potato days, but still not to our ideal because we atrophy during the week.
This should be good enough for you, but then you watch an episode of American Ninja Warrior and hear your friends talk about the upcoming Spartan Race and Mud Run. You can do it!!! You decide to go hard core and join a local CrossFit gym. You're working out harder than you ever have before. It feels good!!! After about a month, you get hurt. You have to stop working out for a few weeks (while your muscles atrophy). Then BAM, you're back in the gym KILLING it!!! You get back to your peak and then you hurt yourself again. With atrophy, you're on your way back to couch potato. Good Job!!! (See Figure)
So what's it going to be for you?