Exercise induces the incorporation of nuclei in muscle fibers that may help the fibers regain size upon retraining after a period of atrophy brought on by muscle disuse, according to a study. 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.