Scientists finally understand muscle memory – here's what it means for you

Man training on the horizontal bar at the gym
We speak to two pioneering researchers about the mechanism underlying muscle memory, and how you can use it in your training (Image credit: RossHelen)

Many people who were active when they were young end up inadvertently taking a long break from sport in adulthood. Perhaps you lifted weights in your teens and 20s, then had kids and didn't get back to the gym till you were in your mid-40s. Often, you’ll discover that your original strength and capacity for training returns quite quickly.

I started to think seriously about muscle memory when I was watching the film 14 Peaks about Nims Purja, the Nepali mountaineer who summited all 14 mountains over 8,000 meters in 2019. A former Ghurka and British Special Forces troop, if anyone is going to have the physical fortitude required to pull off such a stunt, it should be him. 

However, the film begins with a surprisingly pudgy-looking Purja reluctantly starting to get in shape for his mission by slinging sandbags around after taking what appears to have been a lengthy break from any form of physical activity, judging by his ample midsection.

Many people in their late 30s and in this condition might give up altogether, but he demonstrates what seems like a highly implausible “bounce back” to complete his astonishing mission in six months and six days – including one with a hangover. Is Purja superhuman? In many ways, yes, but the ability of his body to recall its former capacity may not be as elusive to us mere mortals as it seems.

Nims Purja and a partner climbing

Nims Purja summited all 14 mountains over 8,000 meters in 2019 (Image credit: Netflix © 2021)

It’s easy to assume that Purja has an unusual psychological disposition uniquely poised to conquer the world’s tallest peaks (and doubtless, he does), but it turns out that muscle memory – the ability of your muscles to recall their former strength – is a real thing.

I recently spoke to two researchers – Professors Kristian Gundersen in Norway and Lawrence Schwartz in Massachusetts – whose groundbreaking work has revealed what might actually be happening when it feels like your muscles are remembering something you used to do. As it turns out, the old adage of ‘use it or lose it’ may not be all there is when it comes to training.

woman lifting kettle bell

It turns out that muscle memory – the ability of your muscles to recall their former strength – is a real thing (Image credit: Getty)

What is muscle memory? 

Upfront, both scientists are quick to clear up some confusion surrounding what muscle memory actually means. Many people liken the phenomenon to riding a bike, or a former gymnast performing a perfect backflip after a 10-year hiatus, but it turns out that this type of recall actually lies in your nervous system, not your muscles. 

In the case of being able to restore strength that previously seemed lost to the ravages of time and inactivity, the term ‘muscle memory’ actually describes the ability to regain muscle mass and strength in muscles that have been previously trained. Both scientists have performed lab research that may support the idea of long term structural changes in the muscle through even short term training – in moths and mice, anyway.

Schwartz’s chief area of study is not on humans – in fact, he admits to avoiding mammals altogether – but his lifelong research into cell death brought him to some interesting conclusions on muscle memory when he was studying tobacco hawk moths, a species that begins life as tiny larvae and increases in size about 10,000 fold over three weeks. After the larval stage, they stop feeding for several weeks or months and dig into the ground to complete their metamorphosis, following which they emerge as a flying moth popular amongst neurobiologists for their immense size. 

In his observations, Schwartz realized that the abdominal muscles of the moths demonstrates a bounce back that puts even Nims Purja to shame.  

“In the course of our study we found that these muscles go through a period of atrophy where they lose 40 percent of their mass and then there's this very dramatic regrowth in muscle mass and volume.”

Close up of person's feet while they use a resistance band to train

When muscles atrophy, they get smaller, but the number of nuclei within the muscle remains the same (Image credit: Getty images)

Around the same time in 2010, Gundersen undertook an experiment using mice that showed that when muscles atrophy, they get smaller, but the number of nuclei within the muscle remains the same. 

He gave mice testosterone for two weeks to increase their leg muscle mass artificially. Then he stopped for three months, which he points out is a very long time in mouse years. equivalent to a tenth of their life. If you live until you’re 90, that would be akin to you taking a nine-year break from training, and it would be no surprise at all if you were daunted by the idea of pulling on your running shoes or walking into the gym after that amount of time. 

To get back to the mice, as you’d expect, after three months, their leg muscles had atrophied and the test group was indistinguishable in size from the control group. Next, Gundersen retrained the mice by overloading their leg muscles and observed that the muscles of the mice that had been treated with testosterone grew much faster – to the tune of 36 percent.

“It was a rather strong effect. They harbored some kind of muscle memory that made it easier for us to retrain them.”

Gundersen explains that his findings are surprising because, unlike cells in other parts of your body such as your gut, muscle tissue doesn’t regenerate, so an obvious conclusion is that once it degrades, it stays that way. 

woman doing exercise single leg glute bridge at home

Unlike cells in other parts of your body such as your gut, muscle tissue doesn’t regenerate (Image credit: undrey)

Nuclei are “like a factory” in your cells

The working explanation for this phenomenon comes down to the cell nuclei, which serve as the storehouse of genetic information and act as the cell's central command. When you exercise, stem cells in the tissue increase the number of nuclei to help synthesize proteins. 

“It was previously believed that when you stop training, you lost these nuclei in kind of a nuclear death called apoptosis, but that doesn't seem to be the case,” says Gundersen, likening the whole process to a factory inside your cells.

“If you have gotten all these new nuclei by exercising or doping, they remain there and they would then be like a closed factory that could quickly open up again and produce protein.”

When you exercise for the first time, you need to produce all these new nuclei in the muscle fiber, which is a bit like building a factory, hiring staff, starting production and stocking inventory all at once – in other words, it takes more effort and time. Shut the factory doors and send all the employees home and you’ll soon lose any visible gains, but when you want to get back into shape, it’s as easy as unlocking the factory doors and powering up the production line again.

At least, that’s the case for mice. 

How and whether this all applies to humans remains to be seen, and Gundersen is clear that a 15-year study is warranted in order to ascertain whether these findings can be related to humans – a project he and his team have requested funding for – and whether it’s applicable to endurance training as well as strength training.

“It’s still early days, so how important this is in humans I don’t think we really know yet, but there are indications that this also applies to humans.”

three people doing planks in a gym

When you exercise for the first time, you need to produce all these new nuclei in the muscle fiber (Image credit: Getty)

“Bank those nuclei and draw on them later”

The old joke among lab researchers goes that humans are a very poor model for mice, but if these findings do apply to humans, then the earlier you get into strength training, the better. 

“Train when you are young and get the benefit when you age,” advises Gundersen.

Though it’s not impossible to train and build strength as you age, it becomes more difficult as the years go by and Schwartz proposes that these findings really need to help shape the curriculum at the public school level for them to have the most benefit.

“You can bank those nuclei now and draw on them later. Muscle strength is the number one factor in reducing the incidence of falls and falls are what drives people into nursing homes.”

man exercising with an elastic band

Muscle strength is the number one factor in reducing the incidence of falls (Image credit: juanma hache)

“Even if you stop training for a while you can still benefit”

An oft-repeated mantra in fitness circles is “use it or lose it,” as well as the popular “consistency is key” messaging, but if these findings prove correct, you may actually be able to take long breaks from training and still be able to restore some or all of your previous conditioning.

“The idea has been that you need to exercise all the time and this memory might suggest that you don’t need to do that. Even if you stop training for a while you can still benefit,” says Gundersen.

No one would argue that a sedentary lifestyle ever trumps an active one, but it seems that if you are strong to begin with, you can afford to lose strength occasionally – say when you have children, get sick or are injured. This knowledge may serve to reduce the anxiety around taking breaks from training if it turns out that your muscles remember what you’ve done before. 

Of course, there are implications for those athletes who prefer to cheat their way to the podium too. 

“You can bulk up and you can go off the steroids so it's no longer detectable in your blood, but you've already acquired all those nuclei. You get to keep them for what appears to be your whole life and therefore when you go back and train again you have an advantage over your competitors,” theorizes Schwartz.

Currently, the World Anti Doping Agency enforces a maximum exclusion time of four years for doping offenders, but if Gundersen shows his work is relevant to humans, athletes that get caught doping may eventually face much longer exclusions – terms that may completely discount them from competition forever.

Julia Clarke

Julia Clarke is a staff writer for and the author of the book Restorative Yoga for Beginners. She loves to explore mountains on foot, bike, skis and belay and then recover on the the yoga mat. Julia graduated with a degree in journalism in 2004 and spent eight years working as a radio presenter in Kansas City, Vermont, Boston and New York City before discovering the joys of the Rocky Mountains. She then detoured west to Colorado and enjoyed 11 years teaching yoga in Vail before returning to her hometown of Glasgow, Scotland in 2020 to focus on family and writing.