When 
you think back to the first few months you ever spent in the weight room, you probably remember adding more and more muscle with seemingly every session. What’s more, this was despite not having half the knowledge that you may have now with respect to training or nutrition. But still, in that first year of training your weight shot up by 15 pounds and you also looked significantly leaner!
Similarly, in the past, you may have suffered a serious injury or taken a significant period of time out of the gym for one reason or another and noticed that as soon as you got back into a regular intense training program, you quickly regained all the muscle you had seemingly lost during the lay-off!
So why does this happen? What causes progress to slow after that initial year in the gym and what causes you to quickly regain muscle after a lay-off? Well, both of these questions can be answered with a basic understanding of muscle tissue myonuclear limitations and muscle satellite cells.
Unlike other cells in the body, an individual muscle cell is multi-nucleated and these multiple nuclei are able to support and control a certain amount of contractile elements within the cell. Muscle cells also do not go through mitosis in the way other animal cells will and are unable to reproduce copies of themselves. They can however grow in size by the process of protein synthesis in which the number of contractile proteins within the individual cell increases and thus the cell increases in size.
When you first enter the gym, the nuclei of your existing muscle cells are likely to be able to support considerably more muscle tissue than they currently do. Hence the stimulus you provide to trigger the synthesis of new contractile proteins results in rapid gains providing that stimulus is frequent enough and is supported by other elements outside of the gym, such as nutrition.
At first this process is only limited by the rate your body can produce these new contractile proteins and the result can be the addition of 15-20lb of muscle within your first year of training! However, there will come a point where the existing muscle cell myonuclei will be unable to support any additional muscle tissue and once this point is reached, growth can only continue via the addition of more muscle cell myonuclei.
Muscle satellite cells are effectively muscle stem cells which lie in close proximity to existing muscle cells. When the need for additional myonuclei is reached in order for additional muscle tissue to be supported, signals are sent to the satellite cells which activate them and cause them to start reproducing their DNA to produce daughter cell copies of themselves.
This process can continue only if there is a continued need for additional muscle fibers. This need is the result of continued stimulus for growth (i.e. intense weight training) which results in more signals being received by the satellite cells that cause them to stop reproducing and to create a new baby muscle fiber. The ability of the satellite cells to reproduce themselves means that they will never become fully depleted in number, although with age, their capabilities of reproduction will become reduced.
Once the satellite cell has become a new baby muscle fiber, with its own set of myonuclei, it fuses with an existing muscle fiber and this fusion, due to the added myonuclei from the satellite cell, increases the total number of myonuclei in the existing muscle fiber. This muscle cell is now capable of supporting an additional load of contractile proteins and therefore, can once again increase in size by the addition of new muscle tissue through protein synthesis.
This change is irreversible and hence even if the stimulus to maintain/increase muscle tissue is removed the new larger myonuclear domain will remain. What we term muscle memory occurs as a result of this; although decreases in muscle size will be experienced if you stop training, the additional myonuclei created to support the additional tissue remains and therefore, when training is resumed, you are able to quickly regain the muscle tissue that had been lost.
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