That is why you can regain lost muscle mass faster!

More experienced strength athletes and bodybuilders know that maintaining a certain level of muscle mass is much easier than building it up to that level. At the same time, many athletes know that they can rebuild lost muscle mass faster than was originally the case. But even if these observations are intuitive in nature, the mechanisms behind them are not directly obvious. To understand how these things work, we need to look at how exercise and skipping exercise affect the rate of muscle protein synthesis, the number of nuclei in muscle fibers, and the level of activation of motor units that we achieve during exercise.

What happens during training, when you skip training, when you return, and when you are maintained?

In short, we continuously gain strength and muscle mass during training. The increase in muscle volume is called ” hypertrophy “. In contrast, we lose more muscle during the phase of no training. This is called ” atrophy “. Then when we resume training, we will build up the lost strength and muscle mass that we once had.

Building muscle mass through training

In phases in which we actively train, we build muscle mass by increasing the cross-section and length of individual muscle fibers. The muscle fibers that grow are those that are regulated by motor units with a high stimulus threshold (type II), whereas muscle fibers that are regulated by motor units with a low stimulus threshold(type I) usually do not increase in volume. The total growth of all individual muscle fibers then accounts for the increase in muscle volume.

We discussed the principle of motor units with high and low stimulus thresholds in this article. 

The growth of muscle fibers occurs due to a temporary increase in the rate of muscle protein synthesis (MPS) within the fibers. The muscle fibers are in a state of change at all times, whereby their protein content is determined by the relationship between MPS and muscle protein breakdown. After a training session, the rate of MPS in the trained fibers increased for about 48 hours, whereas the rate of muscle protein breakdown did not change significantly. The result is a small increase in the protein content within each fiber being trained. If this process is repeated a few to hundreds of times, this leads to a noticeable growth of the trained fibers over time and thus to the growth of the entire muscle.

However, this is not the whole truth!

There are two different mechanisms by which the rate of MPS can be increased and protein can be added to a muscle fiber. First of all, it is possible to increase MPS by increasing the activity of existing cell nuclei. Furthermore, it can be increased by increasing the number of cell nuclei, which requires the activity of satellite cells.

Some scientists have proposed the theory that there is a threshold muscle build-up below which there is no addition of cell nuclei due to the fusion of satellite cells with the muscle fiber [1]. This value is said to be between 15 and 26%. In this model, small increases in the volume of muscle fibers are caused by the increase in MPS from existing cell nuclei, whereas larger increases are brought about by the increase in the number of cell nuclei. The point at which new cell nuclei are added is when the area that each cell nucleus has to regulate becomes too large. The process of adding new nuclei requires satellite cells that add another element to the process of muscle fiber growth. It is the cell nuclei that are able to control the reconstruction of proteins.

In addition, the ability to recruit motor units with a high stimulus threshold increases as the training experience progresses. Beginners are often unable to control a large part of their motor units with a high stimulus threshold. Therefore, they do not manage to activate the muscle fibers regulated thereby. This means that despite executing a strenuous set until muscle failure, they leave many thousands of fibers deactivated within the muscle. More advanced beginners are also unable to activate all muscle fibers, even if the proportion is lower than for bloody newbies. Trained athletes activate the majority of their motor units and can, therefore, train the muscle fibers on which they are based.

As a result, an important mechanism that leads to muscle growth over time is to improve the ability to activate additional motor units with a high stimulus threshold. With increasing strength, the number of controllable muscle fibers increases, which means that more fibers can be stimulated to grow.

Loss of muscle mass during phases of the training break

If we don’t exercise for a while, we lose muscle mass quite quickly [2]. This happens because muscle fibers need a mechanical stimulus to maintain MPS at a given rate. In fact, limb immobilization leads to an immediate and significant reduction in MPS [3]. The rate of muscle protein breakdown, however, is not affected to the same extent. As a result, the rate of breakdown exceeds the rate of synthesis in phases when we are not training, which in turn leads to rapid loss of muscle protein.

It is important to note that the mechanical stimulus that the muscle fibers experience depends on whether they are activated by the recruitment of motor units. When we stop strength training, we also stop activating motor units with a high stimulus threshold, at least as long as we don’t do hard physical work. However, we continue to recruit motor units with low and medium stimulus thresholds through our other daily activities. This means that only the fibers that are regulated by motor units with a high stimulus threshold experience a loss of the usual mechanical stress and therefore only these fibers reduce their size. As a result, we notice a significant, but not dramatic, decrease in overall muscle volume.

On the other hand, if we stop doing any kind of physical activity and keep absolute bed rest (or become an astronaut), then we stop recruiting more than just the high-threshold motor units. It follows that we also experience the loss of mass in the fibers that are activated by motor units with low and medium stimulus thresholds. This leads to a very dramatic reduction in total muscle mass and is very likely to affect our ability to perform the tasks of our daily life.

Increase in muscle mass after re-entry

If we build muscle mass and strength after returning to training, this is typically faster than when we originally built it up [4, 5]. This happens for two reasons.

Firstly, the reduction in muscle fiber volume does not affect the number of cell nuclei within the muscle fibers [6]. Therefore, atrophy due to lack of strength training does not affect our maximum capacity to achieve a given rate of MPS. It only changes our current rate. As a result, if we expose a fiber to a mechanical load in the future, it can reach the same MPS rate that it had before, and therefore quickly rebuild the lost mass.

As a beginner, muscle building is slow because new satellite cells have to fuse to increase the number of cell nuclei in each muscle fiber. During a training break, however, the number of cell nuclei remains at the same level, only the protein mass of the fiber decreases. When training is resumed, muscle protein synthesis can run at full speed due to the high number of cell nuclei and therefore you gain muscle mass more quickly.

Furthermore, we lose our ability to recruit high-threshold motor units only very slowly compared to the rate at which we lose our muscle mass and other peripheral adjustments such as tendon stiffness [2]. As long as we don’t keep the time we don’t train very long, we can usually achieve a comparable level of motor unit recruitment. This means that we can activate all the muscle fibers that we originally trained and we do not have to learn again to control these motor units with a high stimulus threshold.

Maintaining muscle mass through training

Each training week contains its own micro-cycle of training, its own absence of training and its own resumption of training. This will continue to differ a bit depending on the muscle fiber, depending on which motor unit it regulates.

For muscle fibers of the motor units with a high stimulus threshold, each workout and the subsequent 48 hours is a phase of training in which the rate of muscle protein synthesis increases beyond the rate of muscle protein breakdown. The period between these 48 hours and the next training session is a phase of the training break in which the rate of muscle protein breakdown exceeds the MPS. This phase exists because the fibers generally do not experience activation or mechanical stress and normal physical activity does not lead to recruitment from motor units with a high stimulus threshold. The subsequent workout and the 48 hours afterward are therefore a phase of the training break and training.

For the muscle fibers of the motor units with a low threshold, each workout and the 48 hours afterward produce only a minimal stimulus, since these fibers are stimulated to the same extent during everyday activities. The workout is barely noticeable to them, as they experience the same mechanical load almost every hour of the day by simply walking around and picking up and carrying things. This is also the reason why slowly twitching muscle fibers (type I), which are regulated by motor units with a low stimulus threshold, generally hardly respond to strength training [7, 8].

During a maintenance phase, we do not need to do anything with the muscle fibers of the motor units with a low threshold and we do not have to raise the rate of MPS above the rate of muscle breakdown for muscle fibers under motor units with a high threshold. All we have to do in the maintenance phase is to balance the rate of MPS with the rate of muscle protein breakdown. It is not necessary to increase the MPS so much that muscle protein is built up. Translated, this means that we don’t have to do as much volume if the goal is only to maintain muscle mass. It is different when we try to build muscle mass because we know that a higher training volume leads to an increased rate of MPS after training [9].

In addition, we need to improve the recruitment of motor units and also increase the number of muscle cell nuclei if the goal of strength training is to build muscle. In contrast, we do not have to provoke either in a maintenance phase, which makes the challenge less demanding, both in terms of mental effort and physical expenditure of energy.

conclusion

We can rebuild lost muscle mass faster than making new gains because the original training process requires us to both improve our motor unit recruitment to activate new groups of muscle fibers and increase the number of cell nuclei within a muscle fiber, whereas that’s not the case when we try to rebuild lost muscle. 

It is easier to maintain an existing level of muscle mass than to build up that muscle mass because workouts designed to build muscle mass need to stimulate a higher rate of muscle protein synthesis, which in turn requires a higher volume of training. In addition, training phases in which muscles are to be built must improve the recruitment of motor units and increase the number of cell nuclei within each muscle fiber in order to ensure further progress.