One of the most frustrating things about exercise is how easily progress can be undone – "use it or lose it," as the old saying goes. If you don't keep up the workout routine, your muscles slowly shrink and you'll end up back at square one before you realize it. But maybe that's not quite the case after all. Putting a new meaning to the phrase "muscle memory," researchers have now found that muscles keep their nuclei from past growth, which makes it easier to rebuild later after a period of inactivity.
Muscles are made of a strange type of tissue called syncytium, where the cells fuse together and essentially behave like one giant cell. As muscle grows and more cells are added, they bring with them their nuclei, which stores the DNA and acts like a control center for the cell. Likewise, it was thought that the opposite was true.
"Muscle growth is accompanied by the addition of new nuclei from stem cells to help meet the enhanced synthetic demands of larger muscle cells," says Lawrence Schwartz, author of the study. "This led to the assumption that a given nucleus controls a defined volume of cytoplasm – so that when a muscle shrinks or 'atrophies' due to disuse or disease, the number of myonuclei decreases."
But that might not be how muscles work after all. The new study reviewed the literature around the subject – known as the Myonuclear Domain Hypothesis – and found that in two separate experiments, cell nuclei gained through exercise seemed to stick around after muscles were left to atrophy.
In the first study, researchers tested rodent muscles in the lab that had been dyed to make the nuclei stand out. After exercising the muscles over two weeks, the team observed a large increase in nuclei, as expected. Next atrophy was induced, and although muscle fiber volume dropped by about half, only about 0.002 percent of the nuclei were lost.
The second study examined the muscles of the tobacco hawkmoth, and found similar results. Schwartz says that although the results of the review run counter to previous evidence, they aren't entirely surprising.
"Muscles get damaged during extreme exercise, and often have to weather changes in food availability and other environmental factors that lead to atrophy," says Schwartz. "They wouldn't last very long giving up their nuclei in response to every one of these insults. It is well documented in the field of exercise physiology that it is far easier to reacquire a certain level of muscle fitness through exercise than it was to achieve it the first place, even if there has been a long intervening period of detraining."
Of course, it's worth keeping in mind that these studies were both performed in animals, so the results might not perfectly carry across to humans. That said, another recent study did find that human skeletal muscle appears to have an epigenetic memory that makes it easier to regrow previous muscle mass after a period of inactivity.
Along with improving our understanding of muscle biology, the new findings could help inform public health policy. For instance, it highlights the extra importance of exercising earlier in life in order to prevent frailty in old age. On the other hand, it also suggests that athletes who have used steroids could continue to benefit in muscle building long after they've stopped taking the drugs, which may require a rethink on punishments for proven cheaters.
The review study was published in the journal Frontiers in Physiology.
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