Muscle fiber increase in myonuclei during training is not lost during succeeding detraining and may explain the quick increase in muscle mass during retraining, termed "muscle memory".
Detraining is the phase that occurs after one stops training. Of course, when you stop working out for an extended period of time you lose fitness. However, you may be surprised at how quickly you regain that lost fitness when you resume working out months, or even years, later. Studies have shown this phenomenon is not just in your head, it actually exists. In fact, one study found that elderly participants in a two year resistance training program retained significant fitness after three years of detraining (Two years of resistance training in older men and women: the effects of three years of detraining on the retention of dynamic strength, 2003. Smith, et al.). In addition, studies have found that you regain fitness faster during a retraining regimen following a detraining phase than the initial training regimen. One study showed that women who trained for twenty weeks regained muscle mass faster following 32 weeks of detraining (about 7.5 months) than women who did not participate in the initial twenty week training program (Strength and skeletal muscle adaptations in heavy-resistance-trained women after detraining and retraining, 1991. Staron, et al.)
The ability to quickly regain muscle mass lost during detraining is termed "muscle memory." (This term is not to be confused with muscle memory in regards to performing a specific task such as swinging a baseball bat or typing, which stems from development in the motor cortex region of the brain.) According to research published several years ago, this muscle memory may stem from increased muscle fiber (myocyte) nuclei (Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining, 2010. Bruusgaard, et al.).
Muscle cells are one of the few multi-nucleated cells in the mammalian body. In response to training, the muscle fiber increases in size. This increase in size is matched by an increase in myonuclei. Each new nucleus in the myocyte is incorporated from the fusion of a neighboring satellite cell. The figure below shows data from the Bruusgaard experiment demonstrating an increase in myonuclei following muscle overload.
The researchers tagged the new nuclei produced in the muscle overload. Surprisingly, these new nuclei did not die during muscle denervation (disconnection of muscle from nerve, rendering the muscle useless) following the overload. During retraining the muscle fiber increases in size; however, new nuclei are not necessary for the increase in muscle fiber volume. This suggests that muscle nuclei may serve as the mechanism of muscle memory. During retraining the nuclei are already present so hypertrophy (an increase in muscle mass) occurs quickly. The proposed "muscle memory" mechanism is shown in the figure below.
This is a novel explanation to why we can regain muscle so quickly. Muscle fiber nuclei produced in training are not lost during extended periods of rest allowing the individual to quickly regain lost muscle mass during retraining. This must provide comfort to athletes forced to miss extended periods of training due to injury.