Exercise enthusiasts know all too well that strength training of muscles leads to an increase in muscle size that is lost when the training is discontinued; the muscle fibers atrophy because of inactivity. But the mechanism by which previous episodes of training help atrophied fibers regain size relatively soon after retraining has long remained a mystery.

Professor Kristian Gundersen and colleagues at the Department of Molecular Biosciences, University of Oslo conducted imaging experiments on rodent muscles to find the cellular substrate for such ‘muscle memory.’

After an episode of overload exercise that resembled strength training, new nuclei were added to muscle fibers before the fibers grew in size, the authors found. The fibers retained the nuclei for a considerable time of the mouse life span after the overload was discontinued, the authors report. In addition, the nuclei helped delay muscle atrophy. Because the ability to create new muscle nuclei wanes with age, people may benefit from strength training at an early age, the authors suggest.

Further, the relatively long-lasting muscle memory, likely encoded by the nuclei, implies that the length of time for which athletes are banned for a doping offense may need to be reevaluated, according to the authors.

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