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We all have that annoying friend who used to be a jock and now seems kind of lazy, and all she has to do is exercise for a month to be ready for a triathlon. Well, new research says there's a reason for that.

It's because muscles actually have a memory of their former strength level that may last indefinitely. That's according to a team of researchers led by Kristian Gundersen. He's a physiologist at the University of Oslo in Norway, and he's with me now.

Dr. Gundersen, welcome to the program.

KRISTIAN GUNDERSEN: Good to be here.

CORNISH: So this idea of muscles having a kind of memory is pretty fascinating. But before you get into the findings, tell me a little bit about what used to be the thinking about what happened to muscles over time when you weren't using them.

GUNDERSEN: Well, I guess the major idea was that you could build them up with strength training. But then when you stopped training, you would lose your strength, and you would get back to where you started.

Our findings suggest that there are permanent structural changes in the muscle - or at least very long-lasting; we don't know if they're really permanent. But they're very long-lasting in animals, at least, that are maintained in the muscle even when you stop training.

So you would lose your muscle mass, but you will retain the muscle nuclei, which are the small factories that will produce new muscle that can give you back your strength.

CORNISH: So Dr. Gundersen, talk a little bit about how you came to these findings. How did you actually figure it out?

GUNDERSEN: Yeah. So our studies have been performed in mice, and basically what we do is, we use an artificial way of strength training the muscles, and these are muscles in the hind limb of mice. With specialized microscopes, specialized cameras, we have been able to observe these muscle nuclei directly.

CORNISH: Give me a sense of where you think these findings could be used and how they could apply to people, too.

GUNDERSEN: Well, we know that to get this new nuclei, it's much easier when you are young. To me, that suggests that you should then train while you are still young, and you might benefit from that when you get older.

CORNISH: What about the issue of drug use in athletics, where people are gaining a lot of muscle through the use of drugs? I guess the thought originally is that over time, the effects of this kind of wear off when you stop using the drug. I mean, are your findings indicating that that is not necessarily the case?

GUNDERSEN: I am afraid so. So that's, I guess, the dark side of these findings. But it is known that steroids will also give you more muscle nuclei. And if those are permanent - and I think there are reasons to believe that they will be - then the benefits you have gotten from your cheating might also be permanent. And then, I guess, it's reasonable to suggest that exclusion time after a doping offense should be forever, perhaps.

CORNISH: And selfishly, I want to ask about myself here because I used to be a jock. I played many sports in high school.

GUNDERSEN: Okay. But you stopped training, or...

CORNISH: I - needless to say, I have stopped training. Is there any hope for me? Can I be back on the basketball court anytime soon?

GUNDERSEN: Well, I haven't seen you, ma'am. What is your age, may I ask?

CORNISH: Ah, let us say that I am in my 30s.

GUNDERSEN: I think there is still time, although I don't know when the sort of critical period is. But I think you are still good.

CORNISH: So I've got plenty of time...

(SOUNDBITE OF LAUGHTER)

GUNDERSEN: Well, I am just past 50, you see. So maybe I should really start worrying.

CORNISH: Kristian Gundersen is a physiologist at the University of Oslo. His findings on muscles and their cellular memory was published in the Proceedings of the National Academy of Sciences.

Dr. Gundersen, thanks so much.

GUNDERSEN: It's been a pleasure.

CORNISH: And to see pictures of Dr. Gundersen's glowing cellular nuclei, head to npr.org.

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