Why regaining muscle is faster than building it the first time
Muscle memory in the context of size refers to how previously trained muscle can be rebuilt faster after a layoff than it was originally gained. A leading explanation involves myonuclei, the control centers that direct protein production within a muscle fiber. Training adds myonuclei, often by recruiting satellite cells, and research suggests at least some of these nuclei may be retained even when a fiber shrinks during detraining.
If the extra nuclei persist, a returning muscle starts with more synthetic capacity than an untrained one, allowing it to scale back up quickly when training resumes.
How strong the evidence is
The myonuclear retention hypothesis is well supported in some animal models, while the extent and duration of retention in humans is still being studied and debated. Neural relearning of movement patterns and faster re-adaptation of supporting tissues likely also contribute to how quickly trained individuals bounce back.
This is general educational information about a proposed mechanism rather than a settled certainty, and it is not medical advice. The practical takeaway is that prior training is rarely fully lost during a temporary break.