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The muscle memory question unraveled

Skeletal muscle is the largest tissue in the body. It is also the most adaptable to its environment and can undergo dramatic changes in mass and strength. In response to resistance exercise and/or anabolic steroids, muscles undergo hypertrophy (increase in mass, cross-sectional area and strength). The opposite reaction where muscles weaken, known as atrophy, can be caused by factors such as starvation, physical inactivity or health problems like diabetes, renal failure, cardiac or pulmonary problems and cancer, often associated with ageing. But what is worse, muscle weakness itself increases the risks for all causes of death.

Many older people suffer from sarcopenia, which is a muscle wasting process that begins after the age of 50. On average 1 % of the muscle mass is lost every year. This muscle degeneration results in frailty, which is the main cause of older people ending up in nursing homes. The initial muscle mass, before degeneration begins, is a key factor that determines the rate of the degeneration and life quality as we grow older.

Therefore, it is in everybody´s interest to stimulate muscle gain at young age and keep doing so for as long as we live.

To understand how muscles grow, we need to look at some biological aspects. All tissues in our body are made of cells, and the main type of cells that form muscles are the muscle fibers.A normal cell contains one nucleus, which contains the instructions to make proteins, and the actual manufacturing of the proteins occurs in the surrounding cytoplasm. A muscle fiber is unique in the fact that it is a huge cell that harbors a large number of nuclei, all together in one long cell. The combined action of these nuclei gives the muscle cell an enormous capacity to make proteins. When muscles grow, the cell becomes larger, and in order to sustain the increased protein production, the number of nuclei in the cell also increases.

These new nuclei are derived from “satellite” cells, which are a type of stem cells that are located in the vicinity of the muscle fibers. They are normally dormant, but when they are stimulated by steroid hormones or through muscle injury (like after intense exercise), they start dividing and some of the daughter satellite cells fuse with the muscle fibers to donate their nucleus. Scientific experiments show that the first thing that happens is the entering of new nuclei, and after that the synthesis of protein increases. Therefore, muscle growth does not occur immediately after exercise but takes some time.

With muscle atrophy, scientists used to believe that the number of nuclei decreases and that the nuclei are lost. Now new research has found that this is not correct. When the stimulus to grow muscles disappears (decrease in steroids or lack of exercise), the muscle cell becomes smaller because protein synthesis stops and the cytoplasm condenses, but the number of nuclei remains the same. In fact, the retention of “extra” nuclei now explains what is referred to as the “muscle memory”. It gives the muscle the ability to respond more quickly to stimuli for hypertrophy in the future.  This phenomenon is clearly noticeable as an ease to reacquire previously lost muscle mass. Current research has found  that the number of nuclei in the muscle remains stable for at least 15 years, and possibly permanent. So, even if a long time has passed without exercise, less effort is required to recover the muscle than what it took to gain the muscle the first time.

Along the human evolution, this mechanism that gives the muscles the capacity to rapidly change is considered a huge advantage for adaptation. When food is scarce, muscles quickly decrease so that the body can survive on limited resources.

When food becomes available again, the rapid increase in muscle mass brings the body back to previous shape, strong enough to fight or flight and thus increase chances for survival.  Taking this into consideration, it is not difficult to understand that muscle can not grow without adjusting food intake. Not only the quantity, but the quality of the diet is extremely important. For more information, read “The Key To Healthy Ageing: Exercise And Proteins”.

The persistence of the muscle nuclei teaches us that it is extremely important to stimulate muscle growth whilst we are young. The capacity of the satellite cell to proliferate and regenerate muscles declines with age. Since hormone production also declines with age, the highest gathering of muscle nuclei is achieved before the age of 25. People that engage in intense exercise at young age and build up a high “reserve” of nuclei in their muscle fibres are far more likely to age without problems. This is because they can draw from the reserves to keep fit with relatively small effort. But in order to boost the nuclei reserves,a sufficient level of steroid hormones and good quality food that provides all essential nutrients is crucial.


Exercise, take your proteins, vitamins and minerals, regularly check your steroid hormone levels and start doing this as young as possible. This is a golden rule to live a long and healthy life.


Dr. Sonia Van Kerckhoven

PhD in Cellular and Molecular Biology

Expert in Medical Genetics




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Bruusgaardet al., 2012. No change in myonuclear number duringmuscle unloading and reloading. J. Appl. Physiol. 113, 290–296.

Bruusgaard et al., 2010. Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining. Proc. Natl. Acad. Sci. U.S.A. 107, 15111–15116