You get sore muscles when you train, because you muscle strings rip and when they heal, your muscles grow. As far as I know. So why and how do they shrink?
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Humans are adaptable. If they have a job that forces them to lift 200 lbs, they can do that. If they have a period of starvation, they can adapt to that as well. Not needing their large muscles and starving causes them to shrink so as to require less food.
Thats basically the why.
Maintaining muscle mass is expensive so if you don’t use your muscles the body recycles them and uses the resources elsewhere while cutting the cost of maintenance.
Your body doesn’t actually like having that much muscle; muscle is expensive to maintain, both in terms of energy, and nutrients. It’s very antisurvival to have more muscle than necessary.
Because of that, if you don’t use muscle for a while, your body won’t find it necessary to repair them; they’re dead weight so to speak. It’ll just break the muscle down, and reabsorb whatever’s left as salvage.
The reason we aren’t all shriveled boney husks is because exercise simulates protein synthesis. You use your muscles, your body reinforces them because it’s in your best interest.
Tldr: Your muscles are always breaking down; it’s just that exercise simulates muscle creation to counteract it.
These answers are mostly inaccurate. There is a difference between hypertrophy and hyperplasia. Most people who experience growing of their muscles are seeing the effects of hypertrophy, which is an increase in the SIZE of the muscle cells, NOT in the AMOUNT of muscle cells. Basically, the more you use a muscle, the more water/nutrients the muscle decides to absorb so that it has more resources at its disposal. When you stop using the muscle, it realizes it doesn’t need as much water/nutrients anymore (huge oversimplification) and it let’s it go, thus shrinking.
People are talking about hyperplasia, which is an actual increase in the NUMBER of muscle cells, and usually only comes with dedicated weight training after a long time. Even after you stop lifting though, the number of cells remains the same, they just shrink because of above mentioned “deflation”. This means that once you’ve built up a higher number of muscle cells through hyperplasia, it will be easier to regain size in your muscles if you’ve lost it. Because more cells growing=faster growth.
To hit on the “why” portion:
Humans had to deal with periods of feast or famine in the past (moreso than other species) and keeping bulky muscles is expensive. So if we don’t use them, we have the neat evolutionary trait of losing our unneeded muscles so we don’t waste calories maintaining them. Which is perfectly fine in the sort of hunter-gatherer scenario we were evolved for and is a real kick in the gut for sedentary office workers.
Humans have a survival trait that let’s our muscle mass adjust to stave off hunger. We will digest our muscles to keep our organs going. With most predators it is the opposite. They keep a set amount of muscle mass to hunt and will starve much sooner without eating and resting. We are the extreme survivors of the animal kingdom.
Our body is constantly assessing what is needed and what’s not. The things you don’t use on the regular are deemed as not relevant and thus reduced.
The classic “use it or lose it”. It’s a matter of energy preservation. Especially muscles burn a lot of energy, even if not in use.
As covered by others, your body is always trying to maximize efficiency and rid itself of unused excesses. So if you build muscle mass through hard work but then back off on the hard work your body thinks it doesn’t need the muscle and it starts to degrade.
Inactivity is the devil when it comes to building and maintaining muscle mass. It can also happen if you aren’t eating enough calories regularly to maintain your weight.
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