That's called muscle memory.
For example...You have never trained before. You sit at 200 lbs and 20% bf. You train for 4 years and get to 225 lbs and 10% bf. Then you get injured and have to take 6 months off. You end up right back at 200 lbs and 20% bf....right back to where you started years ago. You are finally cleared to train by your doctor and you get back to it. Within 9 months you're back at 225 lbs and 10% bf. Why were you able to get back to that point so quickly? Because of muscle memory.
Here's how it works...
When you train your muscle cells grow by taking in nutrients and converting them to energy, protein, & amino acids to fuel and repair the muscle fibers. What also happens during training is that muscle cells create more nuclei and mitochondria.
-Nucleus - The brain of the cell that communicates with the other organelles (i.e. mitochondria) and tells them when and what to do.
-Mitochondria - The powerhouse of the cell. Converts nutrients to energy/regulates protein synthesis in muscle cells.
As you train your muscle cells create more and more nuclei and more and more mitochondria. Now your muscle cells have more brains telling it what to do, and more powerhouses performing the work. All of these nuclei and mitochondria cross-communicate with each other to efficiently keep the muscles energized and repaired after training. (see diagram below)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6138296/
Where does muscle memory come into play?
All these new nuclei and mitochondria that you have created by training your muscles REMAIN even after extended periods of non-training. The size of your muscle cells may get smaller, but the number of nuclei and mitochondria are still the same and much greater than they were before you started training. Now, when you get back into training your nuclei and mitochondria start communicating with each other with the same efficiency as when you were in peak shape. They go into overload to bring your muscles to the point they last "remembered" them.
See diagram below. Scientists were able to dye the nuclei of muscle cells in 4 different subjects so you can clearly see the increase compared to non-trained subjects.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6138296/
And that's pretty much the simple version of it folks.
