[kridsdale1]

Fortunately for you, regaining a strength level that one had in the past is vastly lower effort than what was required to get to that level in the first place. There are many fascinating cellular and molecular causes for the macro-phenomenon that we call "muscle memory" but it is a real thing.

TLDR, your muscles, unlike most tissues, have more than one nucleus per cell. Your "meat", which are protein fibers, can be atrophied through catabolism during diet or dis-use, and this is a form of protein recycling. But the multiplication of nuclei which occurred during your original training does not revert.

So a formerly strong person who let it go is sitting around with a high nuclei/cell ratio, and low muscle tissue. If he resumes his training and calorie surplus, his nuclei will act like parallel processors outputting the commands to manufacture new muscle fiber much faster.