I must admit, years ago when I heard a bunch of bros talking about muscle memory in the gym I didn't believe them...
"Once you're jacked if you take time off its so easy to get it back!" and "You never actually lose the strength you build in the gym" I thought this was just an excuse to take some time off and not feel guilty about it.
As the years went by I heard more and more people discussing muscle memory in terms of not only gaining lost muscle mass but also in terms of regaining strength and aerobic endurance too.
Then I experienced it first-hand.
Not once, but twice.
Is Muscle Memory Legit?
From my own personal experience as well as the scientific research and studies of those far smarter than me, yes.
A lower back injury (L5S1 to be precise) caused by failing to warm up before jumping into heavy barbell squats had me out of the gym for 3 months.
I could walk on the treadmill and do a few supported exercises but I couldn't lift any significant amount of weight around, let alone touch a barbell or dumbbell.
I couldn't believe it.
I lost size and strength.
It took years and years to make those gains.
More recently I encountered a bad impingement in my right rotator cuff from excessive amounts of heavy pressing exercises.
Once again, i was sidelined for a couple of months from lifting with light cardio being the only training staple keeping me sane.
I can tell you from my own experience that muscle memory is real.
Hell, it might just be the 8th wonder of the world.
Upon return from both of these hefty injuries I was significantly weaker than before, but not for long.
It was like newbie gains all over again, every single workout since returning from the aforementioned injury I was stacking on muscle and regaining my lost strength.
In the space of a month and half I was back to putting up a pair of 110lb dumbbells on my incline dumbbell press.
The first workout back from injury my arms were shaking as I struggled out 10 reps with a pair of 65lb dumbbells.
I continued to eat in a caloric surplus to promote muscle growth while I continued to work my way back up to the same level of strength I once had and bingo, a month and a bit after returning to training I was back to where I was prior to the injury!
To put this into perspective it literally took me years to work my way up from 65lb incline dumbbell bench press to the 110lb dumbbells! Around 3 years to be precise.
But since I'd lifted them before for reps I regained that strength in a short amount of time! The magic of muscle memory.
How Long Does It Take To Start To Lose Size & Strength?
The first thing to know when we're discussing body composition is our body wants to remain the same.
Whatever your current size/strength/body fat is is where your body is happy to stay.
Our body is not concerned with strength or how shredded we are, it's #1 interest is survival and efficiency.
That said, after 2 - 4 weeks you without placing load (regular training) on our muscles you'll begin to see a decline in both strength and muscle mass as our body begins to deem it inefficient to be carrying around your current level of muscle mass if it isn't being used.
Keep in mind your body requires a greater caloric intake to maintain these muscles, which to your body is technically inefficient.
How Does Muscle Memory Actually Work?
Muscle memory isn't magic.
To keep things simple, when you lift weights and increase the demand for your body to pack on strength and muscle mass your muscle cells recruit more nuclei (stored within the cells).
These nuclei are permanent.
Consider these nuclei to be a 'command centre' of sorts for our muscle cells.
The more command centres you have operating and communicating with these cells the more efficiently these muscle cells can grow.
"Two independent studies -- one in rodents and the other in insects -- have demonstrated that nuclei are not lost from atrophying muscle fibers, and even remain after muscle death has been initiated."
Although you may lose muscle mass due to inactivity research now shows that these nuclei don't instantly die like we once thought, research shows they now appear to be permanent.
The more nuclei you have the quicker you'll regain that lost muscle.
When you were initially building that level of muscle you didn't have the sheer number of nuclei to assist with the repair of those muscle cells that you now do, thus explaining the rapid rate of regeneration for your muscle cells once you begin applying them under the necessary load (heavy weights!) once again.