Everyone wants clear cut black and white solutions to everything, sadly this is never the case in science and as such there is a lot of room for debate. The belief in muscle memory is a fantastic example of this.
For years bodybuilders would talk about muscle memory, and how it worked. Their theory was that if you exercised regularly the muscles would learn how to perform the exercise and to make it easier.
The idea was that the muscles would remember how to perform the exercise even if you had not exercised for a very long time.
The expression “It's like riding a bicycle” seems particularly apt here.
Researchers disagreed that it was the muscles themselves that were memorising the movements, and attributed it to the central nervous system.
When you learn to kick a ball it is the result of your central nervous system coordinating the body through a process known as Motor learning.
Regain Lost Muscle Quickly
In recent times, researchers have found some evidence to support the belief that it is the muscles that are responsible for the fast response to training after a long lay-off.
A study by Bruusgaard et al (2009) found that when a muscle is subject to overload (lifting a weight you have never lifted before) new myonuclei are added to the muscle fibres, just before they increase in size (myonuclei are nuclei that are found in muscle fibres) .
So when exercising you are increasing the number of myonuclei within the muscle fibres, but after this has happened let's imagine that you stop training for a year. In this time your muscles will atrophy (become smaller) and your strength will lower.
In theory you are now as weak as you were before you started training in the first place.
Logically this should mean that it would take you as long to build your strength as it did when you first started. But this is not the case, instead what people find is that their strength gains are much faster.
Even though the muscles atrophied, the myonuclei number stayed the same throughout. The belief is that the myonuclei contain the mechanism for muscle memory, making it easier to increase fibre size and to perform the exercise correctly.
The more myonuclei there are the higher the capacity they have for muscle protein synthesis, so even an atrophied muscle can regain size at a very fast rate.
One problem that has been identified is that as we get older the number of myonuclei in our cells diminishes meaning that not only does hypertrophy become harder so does muscle memory. Which would explain why elderly people find it difficult to build muscle.
Muscle Memory and Steroid Use
Another interesting discovery about muscle memory is the effect that anabolic-androgenic steroids have on it.
Steroids have been shown to increase the number of myonuclei in muscle, which leads to increased capacity for muscle protein synthesis, and increased hypertrophy as a result.
A 2013 study using mice found that subjecting them to anabolic steroids (and exercise) increased myonuclei and increased muscular hypertrophy.
After the steroids were taken away, and exercise ceased the muscles hypertrophied back to regular size. Then after 3 months of non-training exercising was resumed, the mice that had taken the steroids produced huge gains in hypertrophy over the next 6 weeks, whilst the group who had not taken steroids saw much less significant gains .
So now we have an idea why bodybuilders (who are the most likely to be experimenting with steroids) were the first to report muscle memory.
We also have a possible explanation as to why it has taken so long for research to find any evidence, if they were using non-steroid taking subjects there might not have been any.
As mentioned at the beginning of the article there is a debate as to whether muscle memory exists, whilst there have been some well-received studies that support the theory (the two mentioned in this article) there have been other studies that could not find any evidence of myonuclei surviving the non-training atrophy of muscle.
What cannot be denied is that training after a long layoff can produce results a lot faster than training for the first time.
Whether this is due to the Central Nervous System or whether it is to do with muscle memory is not important to you. So just enjoy the perks!
 Bruusgaard, J., Johansen, B., Egner, I., Rana, A., Gundersen, K. 2009. Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining. Proceedings of the National Academy of Sciences of the United States of America 107(34): 15111-15116
 Egner, I., Bruusgaard, J., Eftestol, E., Gundersen, K. 2013. A cellular memory mechanism aids overload hypertrophy in muscle long after an episodic exposure to anabolic steroids. Journal of Physiology 591(24): 6221-30