Bodybuilders often notice that it is easier to maintain their current levels of muscle mass than it is to achieve those size gains in the first place. Similarly, most lifters know that they can regain lost muscle mass more easily than they can make completely new gains.
Even so, while these observations are intuitive, the underlying mechanisms are not immediately obvious.
To understand why these things happen, we need to consider how training and detraining each affect the rates of muscle protein synthesis, the number of myonuclei inside a muscle fiber, and the level of motor unit recruitment that we can attain during a workout.
What happens during training, detraining, retraining, and maintenance?
In brief, during training, we progressively gain strength and muscle size. The increase in muscle size is known as “hypertrophy.” Conversely, during a period of detraining, we progressively lose strength and muscle size. This decrease in muscle size is known as “atrophy.” During retraining, we regain strength and muscle size that we lost during a detraining period after a previous training period. In a maintenance period, we do not change strength or muscle size.
#1. Gains in muscle size through training
During training, we gain muscle size due to increases in the diameter and length of individual muscle fibers. The muscle fibers that grow are those that are controlled by the high-threshold motor units, while the muscle fibers controlled by low-threshold motor units do not generally increase in size. The sum total of the growth of all of the individual muscle fibers is what causes overall muscle growth.
Muscle fiber growth occurs due to a temporary increase in the rate of muscle protein synthesis inside the fiber. At all times, muscle fibers are in a state of flux, with their overall protein content being determined by the net of their rates of muscle protein synthesis and muscle protein breakdown. After a workout, the rate of muscle protein synthesis is increased for approximately 48 hours in the trained fibers, while the rate of muscle protein breakdown is not substantially altered. The result is a small increase in the protein content of the trained fibers. Repeating this process many hundreds of times causes a noticeable increase in the size of the trained fibers, and therefore of the whole muscle.
However, this is not the entire picture.
There are two different mechanisms through which the rate of muscle protein synthesis is increased, and protein added to muscle fibers. Firstly, the rate of muscle protein synthesis can be increased through increases in the activity of existing myonuclei. Secondly, it can be increased by an increase in the number of myonuclei, which requires the activity of satellite cells.
Some researchers have proposed that there is a threshold amount of muscle growth (between 15–26%) below which myonuclear addition does not occur, and that it is the proximity to this myonuclear domain ceiling that drives the addition of myonuclei. In this model, small increases in muscle fiber size are produced by an increase in muscle protein synthesis rates through increases in the activity of existing nuclei, while larger increases in fiber size are caused by an increase in the number of nuclei. The point at which new myonuclei are added is when the domain governed by each of the existing myonuclei becomes too large. The process of myonuclear addition requires satellite cells, which introduces an extra element into the process of muscle fiber growth.
In addition, the ability to recruit high-threshold motor units increases with training. Beginners are often unable to recruit a large proportion of their high-threshold motor units, and are therefore unable to activate the muscle fibers controlled by those motor units. This means that despite performing a hard set to muscular failure, they leave many thousands of muscle fibers inside the muscle completely unstimulated. Intermediate lifters are similarly unable to recruit a proportion of their high-threshold motor units, although that number is much smaller than it is in beginners. Advanced lifters will usually be able to recruit the large majority of their motor units, and can thereby train the muscle fibers associated with these motor units.
Consequently, one key mechanism by which muscle growth occurs over long periods of time is an increase in the ability to recruit additional high-threshold motor units. As the lifter gains in strength, they increase the number of motor units they can recruit, and this opens up an additional group of muscle fibers that can now be trained. This group of muscle fibers then contributes to further increases in muscle size.
#2. Losses in muscle size through detraining
During periods of detraining, we lose muscle size very quickly.
This happens because muscle fibers require a mechanical stimulus in order to continue carrying out muscle protein synthesis at a given rate. Indeed, the immobilization of a limb causes immediate (and very substantial) reductions in the rate of muscle protein synthesis. However, the rates of muscle protein breakdown are not similarly affected. Consequently, the net effect is for muscle protein breakdown to exceed muscle protein synthesis during periods of detraining, and this leads to rapid losses in muscle fiber protein.
Importantly, the mechanical stimulus that muscle fibers experience depends upon whether they are activated through motor unit recruitment.
When we stop strength training, we stop recruiting our high-threshold motor units, unless we have a very physical occupation. However, we continue recruiting low- and medium-threshold motor units as a result of our activities of daily life. This means that only the fibers controlled by high-threshold motor units experience a loss in habitual mechanical loading, and therefore only these fibers reduce in size. As a result, we notice a meaningful (but not a dramatic) reduction in overall muscle size.
In contrast, if we stop all types of physical activity and engage in total bed rest (or if we become an astronaut), then we stop recruiting more than just the high-threshold motor units. Consequently, we experience a loss in the size of the muscle fibers that are controlled by low-, medium-, and high-threshold motor units. This causes a very dramatic reduction in overall muscle size that will likely impair our ability to perform functions of daily life once we start doing them again.
#3. Gains in muscle size through retraining
This happens for two reasons.
Firstly, a reduction in muscle fiber size does not affect the number of myonuclei inside the muscle fibers. Therefore, when we experience atrophy due to a cessation of habitual mechanical loading, this does not affect our *maximum capacity* for achieving a given rate of muscle protein synthesis, it only alters our current rate. Consequently, once we cause a muscle fiber to experience mechanical loading again in the future, it can immediately increase its rate of muscle protein synthesis to its former maximum rate, and thereby regain all of its lost size very quickly.
Secondly, we lose our ability to recruit high-threshold motor units very slowly in comparison with the speed at which we lose muscle size and other peripheral adaptations, such as tendon stiffness. Therefore, so long as we do not leave it a very long time between stopping training and starting again, we can typically achieve a similar level of motor unit recruitment at the start of the retraining period as at the end of the original training period. This means that we can activate all of the muscle fibers that we originally trained, since we do not need to re-learn how to recruit our high-threshold motor units.
#4. Maintenance in muscle size through training
Each training week (whether training, retraining, or maintenance) actually involves its own micocycle of training, detraining, and retraining. Even so, the experience will differ slightly for each muscle fiber, depending on which motor unit controls it.
For those muscle fibers of high-threshold motor units, each workout and the 48 hours afterwards are a period of training, in which the rate of muscle protein synthesis is elevated above the rate of muscle protein breakdown. The duration of time after this 48 hours until the next workout is a period of detraining, in which the rate of muscle protein breakdown is elevated above the rate of muscle protein synthesis. This period of detraining occurs because the muscle fibers do not typically experience any activation or mechanical loading, since habitual physical activity does not involve a high level of motor unit recruitment. The subsequent workout and the 48 hours afterwards are therefore partly a period of retraining, and partly a period of training.
For muscle fibers of low-threshold motor units, each workout and the 48 hours afterwards produce minimal stimulus, because these muscle fibers are constantly being loaded to exactly the same extent during activities of daily life. The workout is barely noticeable to these muscle fibers, since they experience an identical level of mechanical loading almost every hour of every day, simply by us walking around and lifting and carrying things. This is most likely the reason why the slow twitch muscle fibers of low-threshold motor units do not usually respond to strength training workouts.
During a maintenance period, we do not need to do anything about the muscle fibers of low-threshold motor units, and we do not need to increase the rate of muscle protein synthesis over the rate of muscle protein breakdown for the muscle fibers of the high-threshold motor units over the course of the week. For the muscle fibers of high-threshold motor units, we just need to balance the decrease in the rate of muscle protein synthesis that occurs between workouts with an increase during and after a workout. We do not need to elevate the rate of muscle protein synthesis sufficiently to cause a net increase in muscle protein addition. As a result, we do not need to do as much training volume in a maintenance period compared to in a training period, because greater training volumes lead to greater increases in the rate of muscle protein synthesis after a workout.
In addition, when we engage in strength training that leads to new gains in strength and muscle size, we must periodically bring about increases in both motor unit recruitment (to access additional groups of muscle fibers to train) and in the number of myonuclei inside each working muscle fiber. In contrast, during periods of maintenance, we do not need to do either of these things, which makes the challenge less demanding both in terms of mental effort and energy demands.
What is the takeaway?
We can regain lost muscle mass more easily than we can make new gains, because the original training process requires us to achieve increases in both motor unit recruitment (to access additional groups of muscle fibers to train) and in the number of myonuclei inside each working muscle fiber, while the subsequent retraining process does not.
It is easier to maintain our current levels of muscle mass than to achieve those size gains in the first place, because workouts that are intended to increase muscle size must achieve larger transitory increases in the rate of muscle protein synthesis, which requires a larger training volume. In addition, periods of training that are intended to increase muscle size must increase motor unit recruitment levels and the number of myonuclei inside each muscle fiber to make continual progress.