There is some debate over muscle memory and whether it does exist.  
In our opinion, it does and this is due to the number of studies we have seen.
Our understanding is:
A) Muscles are overloaded and new nuclei are acquired for the first time. Through proper diet, these nuclei build new muscle proteins and so the muscle fibers grow.
B) Upon cessation of lifting, the fibers are resistant to atrophy due to the increased number of nuclei. When de-training is for long enough, protein degradation exceed protein synthesis rates and ones muscles shrink in size but the nuclei aren’t lost.
C) Some into the future, when training is restarted, muscles rapidly grow in size because the step of adding nuclei is “skipped”– as they are already there. This means they are ready to synthesize muscle proteins again, and so rapidly increasing muscle size.
And so, we have muscle memory explained.What is your take?
More reading here…
Muscle Memory –
Muscle Atrophy –