[ I used to go gym a couple of years back, now it's on and off and haven't been in months but if I start gym again within a week I gain the majority of my muscles back, why is this the case? ](https://www.quora.com/I-used-to-go-gym-a-couple-of-years-back-now-its-on-and-off-and-havent-been-in-months-but-if-I-start-gym-again-within-a-week-I-gain-the-majority-of-my-muscles-back-why-is-this-the-case) Originally Answered: I used to go gym a couple of years back, now it's on and off and haven't been in months but if start gym again within a week I gain the majority of my muscles back, why is this the case? Muscle memory! Yep, that saviour of ‘I used to lift but haven’t for ages’ guys everywhere, it’s an amazing phenomenon where if you grow your muscles to a to a certain size, your body ‘remembers’ how to do it and makes it much much easier to return to that size after a period of atrophy. The science behind this is that when you work out, before your muscles grow, the first step is for them to produce more nuclei. Nuclei facilitate the transition of protein into muscle, the bigger the muscle, the more nuclei need to be produced to grow it. Once you have produced nuclei in your muscles, they stay there even if you stop working out. This means, when you pick up the dumbbells again, your nuclei are already there just waiting to be reactivated and so it’s relatively easy to grow muscles back to their previous size compared to the effort of building them first time. If you try to go past that size, your body needs to produce more nuclei to allow it and that’s why many people find it easy to get back to a previous size but then ‘plateau’ there. An example of this in action is Drew Manning: ![](https://qph.fs.quoracdn.net/main-qimg-93f2d606b24a924e62074bc85213e2bb-lq) Drew is a personal trainer who purposely stopped exercising and gained 70lbs in an effort to better understand his overweight clients. It took him just six months training to get back to full fitness. I love muscle memory, it’s what allows a lazy bugger like me to take 3 months off lifting and then go back for two weeks and get all of it back.