If you've let your workout routine slide, the first time back in the gym can be a humbling experience. Your shoulders quake and quads tremble at what were once easy lifts.
Luckily, research suggests that your hard-won gains of the past can still pay dividends when you restart strength training, thanks to a phenomenon known as muscle memory.
You might associate the term with the idea that you can jump back on a bike after years of not riding one, or undertake other previously-learned activities that involve motor skills and almost instinctively remember how.
This brain-muscle connection is undoubtedly part of the story, but increasingly exercise scientists have realized muscle memory is more than neuromuscular conditioning. Changes deep inside our thread-like muscle cells may also explain why previously trained muscles grow back more quickly the second time around.
"It's like a cellular memory in your muscles that remembers your past — I'd love to use the word — glory," says Kristoffer Toldnes Cumming, an exercise physiologist at Østfold University College in Norway.
In fact, a study published this fall suggests this phenomenon has real staying power: People can tap into this "memory" and readily make up lost ground even if they haven't picked up a weight for more than two months.
A bodybuilder tests a hypothesis
Generally speaking, a few weeks off from lifting weights doesn't appear to have a big effect on your muscle strength and size (although the effect can be larger in older adults). However, as that "detraining" period stretches out to a month or longer, it starts to take more of a toll. Without a stimulus, invariably your muscles start to shrink.
Even a competitive bodybuilder like Eeli Halonen remembers all the muscle mass he lost after a months-long hiatus from training.
What surprised him, though, was just how rapidly he bounced back.
"It was pretty fascinating to see this phenomenon in myself," says Halonen, a doctoral student in exercise physiology at University of Jyväskylä in Finland.
Naturally, this fed his curiosity.
So, Halonen and his team ran a controlled trial: They recruited more than 40 untrained people and put them on a 20-week workout regimen, involving standard exercises like biceps curls, bench press and seated rows.
About half of the participants did the workout straight for 20 weeks; the others split it up, lifting for 10 weeks, taking a 10-week pause and then returning for the final 10 weeks to finish up the study.
Sure enough, they had "significant" decreases in muscle size and, to a lesser extent, strength during their time away from the gym. But it only took five weeks of retraining for them to get back to where they had been previously.
"That's when the magic happened," says Halonen.
And ultimately there was no difference in the progress made between the two groups by the end of the study, which was published last month in the Scandinavian Journal of Medicine & Science in Sports.
These results are largely consistent with previous human studies that looked at shorter detraining periods, says Kevin Murach, a professor of exercise science at the University of Arkansas who was not involved in the study.
"It's a positive finding for those that need to take time off for whatever reason," he says. "You can rest assured that your muscles will readapt quite readily."
Rapid adaptation in the muscles' control centers
The study didn't delve into why participants built back their muscles relatively quickly after time off. Halonen expects they will have some clues once they've analyzed muscle biopsies collected from participants.
As it stands, the underlying mechanisms for muscle memory are hotly contested, though several lines of thinking have emerged.
"It's still a bit like a black box," says Toldnes Cumming, whose lab also published on muscle memory.
The link between your nervous system and muscles likely contributes to the ability to regain strength, especially with more complex movements like squats.
But more recently, scientists have turned their attention to the inner workings of skeletal muscle cells — with one theory centering on the nucleus.
Unlike most cells in our body, skeletal muscle cells, called myocytes, can have hundreds of nuclei. As your muscles expand, you add on more to support the growth. Once you stop lifting, the muscle fibers will get smaller, but some studies show they retain those nuclei, which may set you up for faster gains when you finally hit the gym again.
"The thought is you have more of these control centers and they can basically cause more rapid adaptation the second time around," says Murach. "There's evidence for that and against it, so it's still quite contentious."
There's another possibility: That training essentially rewires DNA in your muscles at the epigenetic level, so that certain genes get turned on or off more readily when you begin lifting again, ultimately sparking faster muscle growth.
Murach tends to put more weight in this second theory, and his lab has produced some evidence in support, but it's possible both have a part to play.
'A little bit of exercise can go a long way'
The human trials that could help hash this out are tough to do — they involve dozens of participants over many weeks. And as with this latest research from Finland, there's only so much you can extrapolate from a single study.
One remaining question: Would you see the same results in regular gym-goers? What if they took time off because of an injury?
"I wish I had a satisfying answer," says Murach. "Those studies are few and far between."
Typically, it's easier to put on muscle mass initially if you haven't lifted much or at all, compared to those already in the habit of resistance training, who have to continually do more to make gains.
That said, Toldnes Cumming and Murach both suspect that people who had previously lifted would still benefit from muscle memory in the same way.
And for many people, the disruption in their workout may not totally mirror the Finnish study, which required participants to take a complete break from lifting weights. Instead, the problem may be more about consistency, not making it to the gym quite as often.
The good news here? Murach says people tend to overestimate how much they need to do to maintain their muscle mass.
"If you need to scale back your training for whatever reason, it's surprising how much you can hang on to," he says. "A little bit of exercise can go a pretty long way in maintaining function and size."
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