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As workers at long-shuttered gyms dust the cobwebs off the barbells and polish up their spin bikes, we’re about to embark on a grand national experiment: How hard is it to regain lost fitness after a prolonged training interruption?

Think of it as the summer of “muscle memory,” the keynote topic at a recent conference hosted by York University’s Muscle Health Research Centre. University of Oslo bioscientist Kristian Gundersen, a pioneer in the field, presented an overview of the evidence that it’s easier to regain lost muscle than it is to gain it in the first place. But, as the ensuing discussion revealed, this idea and its underlying mechanisms remain hotly contested.

For decades, studies have suggested that you can gain muscle more quickly if you’ve previously had big muscles, even after a layoff measured in months or years. In 2010, Gundersen and his colleagues suggested an explanation for how this works, based on the acquisition of new nuclei, the control centres that store the genetic material and regulate the growth of cells.

Muscle cells are the biggest ones in the body, with some reaching up to 40 centimetres long. As a result, unlike most cells, they require more than one nucleus to store all the necessary DNA. When you train, each muscle fibre gets bigger and builds more nuclei to serve the additional area. Gundersen’s theory: Those new nuclei stick around even if you stop training, ready to spring into action once you start training again.

This theory had some unexpected implications – especially since Gundersen believes that the nuclei are “more or less permanent once they are formed.” One is that nuclei acquired when you’re young, when muscle is most easily gained, will stick around to help you age gracefully decades later.

And when he administered anabolic steroids to mice, they too exhibited enhanced ability to gain strength long after they’d stopped the drugs, suggesting that athletes caught for doping might still have an edge even after serving a multiyear suspension. U.S. sprinter Justin Gatlin, the five-time Olympic medallist who served a four-year ban for testosterone, didn’t appreciate the comparison: “For the few haters out there,” he complained in 2014, “seems like that’s what they want to do, discredit my name and label me with laboratory rats in Oslo.”

In the years since Gundersen’s initial work, rival theories have emerged. “I believe there are lasting effects of training,” says Charlotte Peterson, a muscle researcher at the University of Kentucky, “but I do not believe there is strong evidence that this is due to [muscle nucleus] retention.”

Instead, Peterson’s research suggests that muscle memory is a result of epigenetic changes, which alter the activity of genes responsible for muscle growth without changing the underlying DNA. Both Peterson and Gundersen have found evidence that these epigenetic switches stay flipped long after training is stopped, though Gundersen doesn’t believe it’s the main factor.

For now, scientists are split on the question. “Europeans probably favour Gundersen, whereas Peterson is very highly regarded in North America,” says David Hood, a cell physiologist at York and co-host of the recent conference.

The one thing pretty much everyone agrees on is that muscle memory is real – which is good news if you’re heading back to the gym this summer after a long layoff. You won’t pick up where you left off (and you shouldn’t try to), but progress should be quicker the second time around.

That applies to muscle and strength, but what about aerobic fitness and endurance?

“Long-standing data on fitness says no,” Hood says. “Sorry.” But even without a cellular advantage, there’s something to be said for the familiarity of a training routine you’ve mastered before. Settling back in will be like, well, riding a bike.

And there’s one final dispatch from the frontiers of exercise science to keep in mind. In June, Finnish researchers published data on the individual variations in how much muscle people gain with training and lose with sloth. Those who gained muscle most rapidly also lost it most rapidly. There is justice after all. Now hit the gym – break’s over.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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