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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Muscle naturally degrades, but when you work out or use your muscles regularly, you are promoting protein synthesis. If you work out enough you are synthesizing protein at a faster rate than it’s degrading . But when you are sedentary they degrade faster than you can synthesize them. *why* this happens is not entirely understood and there are many contributing factors
Complicated things take more energy to maintain than simple things do. If you’re building an elaborate sand castle on the beach, it will probably not fare as well when a wave hits it as a mound of sand would.
Muscle is more complicated than fat because it has to constantly do things: contract, relax, heal itself, etc. Fat basically just sits there being fat until your body tells it to either add more because you have a surplus of energy or release some because you have a deficit of energy. So having one pound of muscle on your body takes more energy to maintain than having one pound of fat.
In prehistoric times, humans often would go weeks or months without sufficient food. If you had a lot of muscle, you would find yourself starving much sooner than someone without a lot of muscle. So we ended up evolving so that our muscles degrade pretty quickly when they are not being used. If there is a famine, you can just sit around, your muscles will atrophy, and you will need less food to survive until times of plenty again.
Your body is constantly trying to economize and manage what it considers to be limited resources. Muscle is expensive to maintain, so, if you don’t keep making sufficient demands on your body to build and use that muscle, your body will consider it an unneeded expense and allow it to degrade.
The fact that most of us have enough food available to maintain a lot of muscle is a very new concept to evolution, which is geared to prevent us from starving to death.
Muscle mass uses a LOT of energy for your body to maintain. If you aren’t regularly using it, your body will gradually break that muscle down to save energy. But some remnants of that original bulk remain, so it’s easier to rebuild lost muscle than to gain it in the first place.
Other people have mentioned the muscles degrading. With that said, the most noticeable shrinking from someone who quit working out isn’t from a loss of muscle. The most noticeable shrinking is losing glucose storage in your muscles, resulting in less water storage in the muscles.
Also, if you are actively working out, your muscles are in a prepared state to lift heavier objects. Your muscle fibers are slightly firing in their preparedness state; this makes your muscles appear larger. After a short time of quitting exercise, they will shrink because they are no longer in that state.
The body can break down muscles for energy, and if you aren’t using them, your muscles are probably consuming more resources than they are worth (biologically speaking in the context of our path of evolution). Muscles are expensive tissue to maintain, and before the modern world, food scarcity was pretty much guaranteed sometimes. The body has to pick what of that limited energy goes where.
The human body runs on a use it or lose it principle. Having large muscles to maintain if you are not constantly using them is detrimental to survival in the long run. Especially if you are not consuming large amounts of protein. If you have all this protein need and not enough incoming protein to maintain it your body says “it’s free real estate” and consumes any extraneous protein in muscle tissue reducing its mass and making it easier to maintain.
Large muscles do not actually offer any benefits to survival, so they have to be actively maintained. You will never see massive slabs of muscle outside of the first world where there are abundant protein sources available. Generally, really strong tribal people are specimens of efficiency, not massive muscles.
Your body has a need and by eating muscle it can fill it. Your body picks the muscles it thinks you don’t need and eats them.
Lots of people explaining why they shrink (to save energy when in disuse) but not the biological mechanics of how.
Obviously the body doesn’t magically know when to get rid of muscle, so what’s going on at a molecular level?
Your body is like your mom. It doesn’t like anything going to waste.
If something takes energy and your body decides it doesn’t need it, your body shuts it down. This happens in your brain in a process called synaptic pruning and in your muscles in a process called muscle protein breakdown.
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