Myonuclear domain

Most cells have one nucleus. Muscle fibers, however, are different. Many small cells called myoblasts line up and fuse into one long, thick cell, leaving numerous nuclei scattered throughout a single muscle fiber.

The nucleus acts as a headquarters supplying the blueprints (mRNA) for making proteins. But because a single muscle fiber is so large, it is difficult for one nucleus to take care of the entire cell. Each nucleus is therefore described as managing a particular area around it. This territory is the myonuclear domain. An easy way to picture it is to think of each nucleus as a fire station responsible for its own neighborhood.

This raises a natural question. If there is a limit to the territory one nucleus can manage, shouldn't the number of nuclei increase as a muscle fiber gets thicker?

The nuclei inside a muscle fiber do not divide on their own. New nuclei are supplied when muscle satellite cells, located along the outside of the muscle fiber, fuse with it. Stimuli such as resistance exercise are known to activate satellite cells and cause them to multiply, with some then fusing with the muscle fiber and adding nuclei. This is why satellite cells come up so often in discussions of muscle hypertrophy. However, the extent to which an increase in nuclear number is necessary for muscle growth—in other words, whether domain size is fixed—varies with muscle fiber type, location, and stage of growth, so research and discussion are ongoing.

This concept became widely known through a mouse study published in 2010 by Norwegian researchers (Bruusgaard et al.) in the Proceedings of the National Academy of Sciences (PNAS). Imaging of muscle fibers in living mice showed that the additional myonuclei acquired through overload exercise appeared before the muscle fibers grew thicker. These nuclei then remained, rather than disappearing, even as the muscles subsequently shrank considerably.

In 2013, the same research group (Egner et al.) reported that after mice were briefly given anabolic steroids and then subjected to overload again much later, the previously exposed group showed greater growth. The interpretation is that long-lasting myonuclei may represent a kind of 'cellular memory.'

These observations fit well with the common experience that people returning to exercise after a long break regain their previous physique much faster than people exercising for the first time. As a result, they attracted considerable interest as a cellular explanation for muscle memory.

Whether the same thing happens in humans remains an active area of research. In 2019, Psilander et al. analyzed and reported changes in myonuclear number as people trained, stopped training, and then resumed training. The pattern differed from what the mouse models predicted. Further studies and commentaries have since examined whether myonuclei really decrease in number or are retained during a break from training.

There are two main points of debate.

• How should myonuclei be counted? — Results can vary depending on the criteria used to identify which nuclei in a cross-section of muscle tissue belong to the muscle fibers.
• How far can animal models be applied to humans? — Mouse experiments use conditions that cause extreme muscle shrinkage, whereas the effects of stopping training in humans are much more gradual.

The myonuclear domain is therefore one of the leading candidates for explaining muscle memory, while conclusions about humans remain for future research to establish.

Several factors work together to help the body return quickly to its previous level.

• Nervous system learning — The neural circuits that allow familiar movements to be performed remain longer than the muscles do.
• Exercise technique and habits — The body remembers which positions to use and how to apply force.
• Chemical changes within muscles — Changes in how genes are switched on and off are also being studied.

• It is okay to start again after a break. Even if you have been unable to exercise for a while, you are not starting completely from scratch. Your body retains some memory, so the feel for exercise often returns relatively quickly.
• Build up slowly when you restart. Even if you remember your previous weights or pace, your muscles and tendons need time to catch up. Working through the stages of progressive overload puts less strain on them.
• Consistency is your greatest asset. Training accumulated over a long period remains not only in muscle size but also in movement skills and neural circuits.