system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it easier to gain back muscle after a period of not working out versus when you first started going to the gym?

The title.
gpt
There are two mechanisms for muscle growth. Hypertrophy and hyperplasia. The former refers to protein accretion, so think of it as muscle cells gaining mass. The latter refers to a form of proliferation, where you either get more muscle cells or the cells get more nuclei. Both lead to an increase in size, but when the muscle atrophies (loses size) due to lack of use or mechanical stress, size is lost because of a reduction in size of individual cells due to loss of protein content (and others), but number of cells and number of nuclei is more resistant and stays longer.

So when you lose muscle mass and want to regain it, it is easier to do when you already have more cells and more nuclei per cell. However, hyperplasia capacity decreases over age. Because satellite cells (muscle stem cells) decrease as you age. So if you gain muscle size at a young age, lose it, and train again, it's easier to regain the size than if you started training at an older age, lost it and tried to regain it again.