Passage A

Muscle memory is a puzzling phenomenon. Most bodybuilders have experienced this phenomenon, yet virtually no discussions of it have appeared in scientific publications. Bodybuilders who start training again after a period of inactivity find that gaining muscle size seems easier the second time around—even if starting from the same place. With so many athletes observing muscle memory, some plausible explanation must exist.

One potential explanation of muscle memory involves the neurons (nerve cells) that stimulate your muscles, telling the muscle fibers to contract. It is well established that during weight lifting, only a small percentage of neurons for the working muscles are recruited. The more weight you lift, the more neurons are involved and the more muscle fibers are stimulated. But even when attempting your maximum weight, you don't recruit all the fibers in your working muscles. Now it could be that one way your body adapts to the demands of consistent training is by gradually increasing the percentage of muscle fibers that are stimulated by neurons during maximal lifts. When you're making a comeback, this ability to recruit more muscle fibers may remain intact. If so, your muscles would start with a greater capacity to develop force. Although you may think you're starting from the same place, this greater strength would enable faster progress.

Then again, it's also possible that the ease of retraining has nothing to do with your muscles: it could all be in your head. The first time you trained, you didn't know how much you could lift. So you increased weight cautiously. When retraining, you already know you can handle increasing weight because you've done it before. So you are likely to add weight more rapidly. These more rapid weight increases produce quicker gains in strength and size.

Passage B

Pumping up is easier for people who have been buff before, and now scientists think they know why—muscles retain one aspect of their former fitness even as they wither from lack of use.

Because muscle cells are huge, more than one nucleus is needed for making the large amounts of the proteins that give muscles their strength. Previous research has demonstrated that with exercise, muscle cells get even bigger by merging with stem cells that are nested between them. The muscle cells incorporate the nuclei that previously belonged to the stem cells. Researchers had thought that when muscles atrophy, the extra cell nuclei are killed by a cell death program called apoptosis.

In a recent study, researchers regularly stimulated the leg muscles of mice over a two-week period, during which time the muscle cells gained nuclei and increased in size. The researchers then let the muscles rest. As the muscles atrophied, the cells deflated to about 40 percent of their bulked-up size, but the number of nuclei in the cells did not change. Since the extra nuclei don't die, they could be poised to make muscle proteins again, providing a type of muscle memory at the cellular level.