why do muscles shrink if we stop using them?
In: Biology
Nearly all parts of your body, muscles included, require a lot of energy to maintain. If you are not using the muscles it makes more sense for your body to use those nutrients elsewhere (like you brain or your heart) so the muscles start to shrink.
After a surgery or illness that requires you to have complete bedrest, if you don’t have enough nutrition, the body starts by breaking down fat for energy. After there’s no fat left, it starts breaking down muscle. Even proteins can be used for energy.
If you work out your muscles and later you stop and you also stop eating as much protein as the muscles require, it’ll again break down like in case of low nutrition.
That’s why when people work out, they need to eat alot of protein.
From an evolutionary standpoint, as others have pointed out, we simply evolved to be energy efficient. Muscles are expensive to maintain biologically speaking. So if you don’t need them, you lose the excess.
This is a common theme for everything in the body. Including the heart, which has a growth-shrinkage window of 150% depending on the requirements (can shrink down to 50% in astronauts if they don’t take care to maintain it and grows up to 150% under heavy exercise for example). Also in the brain, for things like memories and general cognitive capacity, although that’s not just a simple size growth or shrinking, but the idea holds.
The way this happens is simple. Your body is in a quasi steady state, which means countless chemical reactions and microprocesses are happening all the time, and both directions (forward and reverse) are active. Although at the microscale it seems like chaos, at the macroscale these processes balance out statistically to produce a very stable result, homeostasis, and its “quasi” because it’s not perfect, it can deviate from stability stochastically at times and the total state changes with age and disease, and of course because the body can change to adapt. In your example, the macroprocess is muscular atrophy (shrinking) and hypertrophy (growth) which is underlain by many microprocesses like protein catabolism (breakdown) and anabolism (building), which are underlain by even smaller scale things like specific cellular enzymatic and signaling pathways regarding metabolism and such. If these processes are all always happening, what determines the end result are the stimuli that push favor one direction of a macroprocess over another. Such a stimulus is mechanical tension for example, a sustained load on a muscle evokes the activation of biochemical pathways that upregulate some genes and activate some enzymes in the anabolic processes leading to building more than breaking down, and so you get a net increase in muscle mass due to protein accretion. If you stop the load, the signal stops and the system reverts back to the way it was in terms of how the two processes are balanced, and the excess cellular structures (sarcomeres for example) added are removed.
We can get into more detail in terms of the exact known pathways involved but this is already well beyond ELI5.
Edit: clarity
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