The Science of Muscle Memory

I’ve heard about ‘muscle memory’ a lot over the years but what is it? Is is when your body just naturally knows how to do something because you’ve practiced it so much. That I kind of get but is it something else too?

I’ve also heard it talked about in terms of getting back into shape. In other words, if you used to run marathons, even if it was over a decade ago you will get back to that level of fitness much faster than someone who was never a runner. I’ve also heard it as an explanation as to why someone who was in shape looks much better than someone who never was with the same lifestyle.

As so often happens on here when there is a scientific thing that relates to health and fitness that I don’t understand, it’s time to hit the journals! Specifically the peer reviewed journals. So is muscle memory a thing? If it is what does it mean? Let’s find out together!

In the zeitgeist

This conversation comes up around our house from time to time. For example I will often lament that honey’s kids still have great ab definition even though some have been pretty inactive for years. I often ascribe that to a genetic win while honey will say that since they were so active as children it’s a case of muscle memory.

In 2020 and 2022 life took me largely away from running for about 6 months or more each year. I did run my first marathon in the year in between though. Upon my return I was shocked to find I could easily still run 5k or even more continuously on my very first runs back. Is this an example of muscle memory or just because we don’t lose fitness nearly as fast as we think we do?

While honey clearly thinks muscle memory is about retained muscle strength and definition I am more likely to credit muscle memory with the fact I can still navigate technical obstacles on my bike after years away from the trails. Are these both examples of muscle memory or is one something else entirely?

The actual definition

It turns out that there are two parallel and sometimes overlapping discussions of muscle memory in the literature. The first might be better called procedural memory. This is the classic example of ‘you’ll never forget how to ride a bike’.

Procedural memory is the idea that once you do the same movement over and over again it comes as second nature without really having to think about it. It might even mean that over time your body becomes well suited to this type of movement. This type of muscle memory might carry over from one activity into another closely related activity. In this post I will call this type of muscle memory procedural memory however it is not the main focus of the article.

The second type of muscle memory is often referred to as they type of muscle memory involved in body building. It applies to virtually every other sport and movement under the sun too. This has little to do with the ease of a particular movement but rather how easily or fast a muscle regrows or regains strength after a period on disuse. 

What we will just call muscle memory means that if for example you had highly developed upper body strength from several years working as a laborer even after years spent working in an office you would regain that strength faster should you return to that work. This leads to the idea that if you previously developed a certain muscle you are never again ‘starting from zero.’ This increased ability to regain muscle function will largely be the focus of this article.

So now that we have a pretty good idea of the definitions of muscle memory and procedural memory, is it even real? After long periods of inactivity are we starting from scratch or do we never really get back to our untrained state?

How do muscles get stronger?

Muscles themselves are made up of many, many muscle fibers which work together to lift or pull by getting shorter when the muscle contracts. That’s what happens when you flex a muscle. It is shortening and all that material pops so that you can see it better. When you relax there is an opposite set of muscles on the other side of the limb that contract and return the limb to a resting position. 

When we train a muscle past a certain point it will physically get bigger. That is because there is an increase in the number of muscle fibers in the muscle and physically each one of those muscle fibers gets larger. That’s why your biceps physically grows when your lift weights. When you see gains in the gym it is as a result of having more muscle fibers in that muscle group and each one of those fibers being bigger.

Muscle cells have another interesting property. They are one of the few cell types that can have more than one nucleus per cell. Interestingly when we train a muscle we see them number of nuclei (plural of nucleus) per cell increase too. The theory is that each nuclei supplies one tiny fixed little area of the long muscle fiber. As those fibers get thicker they require more nuclei to keep up as the fiber is physically a bit larger (Snijders et al., 2019).

What happens in muscles when we stop working out?

There is some debate in the literature about what actually happens to a muscle when you stop working out at a cellular level. However, there is no debating the fact that the size of the muscle returns to what it was before you started training it. So if you do a bunch of bicep curls every day for 6 months the muscle will get bigger, stop for a year and it will return to its previous size. (Rahmati et al., 2022)

When researchers look at the muscle they see that the size and number of fibers returns to a pre-trained state on a cellular level. What appears unchanged after a period of inactivity is the number of nuclei in that particular muscle. After the muscle atrophies and goes back down to its regular size most of those ‘extra’ nuclei produced in training actually stay in the muscle.

What happens when we retrain the same muscle?

Want to know something cool? Scientists can actually prove that after a period of inactivity, even with muscles that have shrunk back down to ‘normal’ size, your muscle will get back to that trained state even faster the second time. If the second training period is as long as the first the muscle will get to an even larger size the second time! (Lee et al., 2018). After a break science does prove that you are not starting from scratch!

Now is this because you go back to your sport and you are physically more efficient at it (procedural memory) or because of some built in memory within the muscle cells? Well it turns out that it is likely the latter (Bruusgaard, 2010). There is something special about muscle cells that have been trained before and we probably even know what that is!

Scientists suggest that these ‘extra’ nuclei are what is responsible for muscle memory. Since each nucleus can support a certain area of muscle cells and you have more than you need for the current size of the muscle there is more capacity already built into a previously trained muscle. This is why scientists think a retrained muscle is built faster than one that was never trained before!

Science suggests that the number of nuclei in a muscle stays elevated after you stop working out. Image from Gundersen et al., 2018

Allow me to use an analogy. Imagine that you are starting a construction company to build homes (muscle fibers). As your company grows there are more homes (fibers) to build and so you hire more workers (nuclei) to get the job done. Then winter comes (inactivity) and there is no work to do. But you have decided to keep your workers on the payroll (nuclei) so that in the spring when there is more work (training) to be done your company is able to build more homes faster than a company that has laid everyone off.

Other cool things I learned

  • As you age it becomes harder but not impossible for your muscles cells to add more nuclei. (Bruusgaard, 2010)
  • Muscles that were trained lose mass at a slower rate than those that were never trained when immobilized. 
  • Training done in adolescence provides a benefit into adulthood. Adults who train again put on more muscle mass faster than their counterparts who were inactive as juveniles (Eftestøl et al., 2022)
  • Some researchers hypothesize that once nuclei in the muscle are created they could be permanent.
  • The extra nuclei show up in a muscle before the muscles get much larger. This suggests the increased number of nuclei are needed for substantial muscle growth.


  • You aren’t starting from scratch after a break from exercise
  • It’s about twice as easy the second time around
  • Periods of training are more efficient at building muscle memory the younger you are
  • The effects of getting your kids to be active will truly last a lifetime
  • Muscle memory is a real biological thing rather than just a learned behavior

So muscle memory really is a thing. In fact it is something everybody has already built in! Of course your skill level at a certain sport and your genetics play a role in the athlete you become but previous experience is a huge part of it as well! Do you have an experience that supports the idea of muscle memory in your life? Leave it in the comments below!

Read what I read!

Bruusgaard et al., 2010. Procedings of the National Academy of Sciences. Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining.

Eftestøl et al., 2022. Acta physiologica. A juvenile climbing exercise establishes a muscle memory boosting the effects of exercise in adult rats.

Gundersen et al., 2018. The Journal of Physiology. Muscle memory: virtues of your youth?.

Lee et al., 2018. The Journal of Physiology. A cellular mechanism of muscle memory facilitates mitochondrial remodelling following resistance training.

Rahmati et al., 2022. Journal of Cachexia, Sarcopenia and Muscle. Myonuclear permanence in skeletal muscle memory: a systematic review and meta-analysis of human and animal studies.

Snijders et al., 2019. Acta physiologica. The concept of skeletal muscle memory: Evidence from animal and human studies.

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