Something that is very often discussed in fitness is whether there really is something that can be called MUSCLE MEMORY. By muscle memory, we mean that if you have been well-trained before, you will soon be strong and well-trained again after a longer break compared to someone who has never trained before.
A well-trained nervous system usually remains very well functioning people who have been very strong continue to be very strong even as older. Just as an old football player can often stand and do drills with a ball at a higher age, an old weightlifter can thus coordinate and maximize the activation of his muscle mass in a very good way. The problem in this situation is that the muscles themselves are not prepared for the strong activation. It is more a rule than exceptions that old elite athletes take on an injury when they try to do their sport with full force after a longer break. The nervous system can send a very powerful signal but the muscles are not ready for it as they have not worked hard for a very long time.
If you’ve been training hard before, you know what you need to do now.
Another factor that is probably an important part of explaining why previously well-trained people can quickly become well-trained again is the past experience. If you once trained a lot, you already know what it takes. You also know what works well for you personally when it comes to habits. Everything from the number of training days, what and when to eat, what exercises you have easier to use than others and when it is time to rest to not get an injury.
Muscle memory is something that really exists.
It is easier to build muscle the second time compared to the first. Your muscles are better built to grow fast. There are of course other factors that will come into play here. If you are 60 years old, you should not expect to be as strong and muscular as when you were 20 if you really trained hard at that time. There are many other factors that have changed over these 40 years that are likely to limit your results. But you will have better conditions to build muscle compared to your other 60-year-old friends that never used to training hard as a youngster.