What happens to your muscles when you stop working out for several years?

Question

I go through spurts of working out. I'll decide to get a gym membership, achieve decent results over six months, and then, for whatever reason at the time, I'll stop and won't exercise at all for several years before I decide to hit the gym again. Not a good plan, I know, but it is what it is.

I notice, however, that when I do go back to working out, I bulk up really fast, way faster than when I first started working out. Biceps, triceps, forearms, and chest all seem to grow at a noticeable rate and I am able to increase my weight by significant proportions in a short amount of time before I eventually steady out.

Atrophy is obviously at work here, but what, exactly, is happening to my muscles when I regress to a sedentary lifestyle that still allows them to bulk right back up at rapid rates when I go back to lifting?

Answer 1

There was a 2010 study that touched on this a bit, and suggests there's more going on than muscle memory.

Effects of previous strength training can be long-lived, even after prolonged subsequent inactivity, and retraining is facilitated by a previous training episode. Traditionally, such "muscle memory" has been attributed to neural factors in the absence of any identified local memory mechanism in the muscle tissue. We have used in vivo imaging techniques to study live myonuclei belonging to distinct muscle fibers and observe that new myonuclei are added before any major increase in size during overload. The old and newly acquired nuclei are retained during severe atrophy caused by subsequent denervation lasting for a considerable period of the animal's lifespan. The myonuclei seem to be protected from the high apoptotic activity found in inactive muscle tissue. A hypertrophy episode leading to a lasting elevated number of myonuclei retarded disuse atrophy, and the nuclei could serve as a cell biological substrate for such memory. Because the ability to create myonuclei is impaired in the elderly, individuals may benefit from strength training at an early age, and because anabolic steroids facilitate more myonuclei, nuclear permanency may also have implications for exclusion periods after a doping offense.

The diagram above is from the study, and basically shows that at a cellular level you retain a lot of your gains even going on for years.

A follow up article took it a bit further and expanded on the last sentence in the study's abstract:

[The study authors] also cite a couple studies that show that the administration of anabolic steroids increases nuclei number, suggesting performance gains from steroid use may not be transient. If these findings are replicated, even limited use of steroids may bring about long term benefits- should this bring about changes in consequences to use?