People talk about Muscle Memory in the body and being that once you have muscle and lose it, you can always get it back. That being said means that if you lay off the weights for a period of time and you start working out again, the muscle memory takes over and regains it's size. But there are a lot factors to this.
Much of it depends on your training and diet and if you are lifting heavy or light. Muscle cells grow as you work them and then they split and grow again, much like fat cells. When you place a demand on them such as weight resistance they have to grow to meet the demand.
What happens when you stop lifting weights the cells shrink and drop to a smaller size but they don't go away. When you resume workouts they enlarge again. This is only part of it, the other part is to feed them with protein because this is what activates it.
If you've been off a long time then it takes a bit longer to have them come back and also your age plays a role, the longer you've been off and the older you are, the longer it takes. You have to know just how much to train them without burning out and this is trial and error. Everyone responds different and rest in between is also key.
I was sick in the hospital recently with pneumonia and congestive heart failure. I thought it was over for me and lost 30 lbs. within a couple of weeks. It was mostly water but I was eating very little as well. Of course I was worried about staying alive but as I started to feel better I saw my body shrink as if I never worked out the past 50 years. That can be pretty depressing but instead of getting depressed, I worked out a plan to gain the muscle back. I didn't have anything else to do in the hospital except think about it. I have a lot of followers out there who look up to me to motivate and inspire them so I'll show them that I can come back.
Now this picture in the hospital actually looks better than I was. I was weak, pale and skinny.
When I came home I was on oxygen 24/7 and could barely walk room to room. I got my appetite back for good food maintaining the 30 lb loss at 199 lbs. but began to feel stronger every day.
The following week I took myself to Gold's gym with my oxygen every day and did 20 minutes on the bike. I was pretty tired after that. The second week I started using some machines for a workout with very light weights. It was like starting over again 50 years ago. But rather than get defeated, it only inspired me to do more. I added more protein, drank gallons of water even though I was on 3 diuretics a day but each day felt more alive. I couldn't wait till the next day to improve even more.
The third week I was back to full body part workouts, granted I was weaker but I could see myself filling out on a day to day basis like magic. My strength was improving every day a little and my appetite through the roof. I could eat anything and everything and not get fat or gain weight however I was gaining muscle back quickly. I got off the oxygen and all medications the week before and this past week, my end of 3rd week I saw my cardiologist, pulmonary doctor and both were shocked at my recovery. All my vitals were normal, lungs clear, heart strong, kidneys clear and blood work clear. My testosterone was low but I'm 72 so it would be. They put me on 200 ml per week injections which really boosted my energy and strength.
Now going into my 4th week, I have full workouts and the muscle memory has taken effect and doing it's job. I proved my point that this could be done and if I can do it after an illness like this, anyone can after a lay off. Take it slow, don't give up and eat right. My motto is:
The Mind is Your Power, The Gym is Your Medicine