How does taking a break from strength training affect hypertrophy?

Many lifters are concerned that if they stop training for a vacation or take too much time away from the gym for any reason, they could lose a large amount of muscle mass.

But is this a valid concern?


What happens when we stop lifting weights?

When we stop lifting weights (which researchers call detraining), we do experience losses in both maximum strength and muscle size.

However, the speed at which we lose maximum strength changes over time.

In the first few days after our final strength training workout, we can often experience a small increase in maximum strength, because muscle damage is repaired and any associated central nervous system fatigue dissipates. This is the main reason why tapers are effective for strength athletes. In the few days after this, there can be a small drop in maximum strength, which may be due to reductions in coordination, and perhaps also motor unit recruitment. Over the next four weeks, progressive reductions in maximum strength occur mainly due to losses in muscle fiber size (both in length and diameter), although reductions in motor unit recruitment do also occur. Thereafter, muscle mass stabilizes, and reductions in maximum strength are likely attributable to further, steady decreases in motor unit recruitment.

In contrast to maximum strength, muscle size seems to reduce most in the first month after stopping training, and quite little thereafter. Indeed, there are very noticeable losses in muscle fiber size in the first 4 weeks after stopping training, but reductions slow markedly after this point.

Why does this happen?


Why does muscle size reduce after we stop lifting weights?

Muscles contain many tens of thousands of muscle fibers, grouped into a few hundred motor units. Motor units are recruited by the central nervous system to perform tasks in strict order of recruitment threshold, with low-threshold motor units always being recruited before high-threshold motor units. Moreover, low-threshold motor units remain recruited when high-threshold motor units are recruited.

Importantly, the number of muscle fibers controlled by each muscle fiber increases exponentially with increasing motor unit number. Low-threshold motor units each control a few dozen muscle fibers, while each high-threshold motor unit controls tens of thousands.

Motor units are recruited in response to the amount of effort we put into a movement. When we exert maximum effort, whether to lift an extremely heavy weight, or to throw an object as fast as possible, or to perform the final few reps of a very fatiguing set of an exercise, we recruit our high-threshold motor units. If a movement also involves a slow muscle fiber contraction velocity, then the muscle fibers that are controlled by high-threshold motor units will experience high levels of mechanical loading due to the force-velocity relationship. This mechanical loading stimulus triggers an increase in the rate of muscle protein synthesis for a period of approximately 48 hours.

Consequently, when we lift weights regularly (either using heavy loads or training to failure with light or moderate loads), we provide a mechanical loading stimulus to the muscle fibers of the high-threshold motor units every week. When we stop lifting weights, these muscle fibers stop receiving the regular stimulus that they need, and they begin to atrophy.

All muscle fibers atrophy when they stop being regularly exposed to a sufficiently high level of mechanical loading. However, the muscle fibers of low-threshold motor units experience sufficient mechanical loading when they move at slow speeds in response to submaximal efforts during activities of daily life, and also simply to keep us upright in response to the force due to gravity. Going about daily life on Earth is actually enough for the muscle fibers of low-threshold motor units to remain a constant size, but it is not enough for the muscle fibers of high-threshold motor units.

The need of muscle fibers for constant mechanical loading is why muscular atrophy is such a big problem for those astronauts who visit the International Space Station. Since this large satellite is accelerating constantly towards the Earth in a rotational orbit in response to the force of gravity, the astronauts inside do not need to produce a force with their muscles against any of the surfaces inside it. Rather, they float around, and the muscle fibers of their motor units (both low-threshold and high-threshold) do not experience any mechanical loading unless they deliberately exert force to accelerate the mass of another object, or to push themselves off from the outer walls to travel from one part of the satellite to another. Consequently, this absence of mechanical loading causes rapid and severe muscular atrophy, unless it is counteracted by regular strength training.

Ultimately, this explains why muscle size decreases rapidly in the first few weeks after ceasing a strength training program, but stabilizes afterwards. The large and numerous muscle fibers that are controlled by high-threshold motor units lose their regular stimulus, and atrophy rapidly, whereas muscle fibers that are still stimulated by the activities of daily life are unaffected. The number of muscle fibers that are unaffected will depend on the type of recreational activities that the individual carries out, in addition to the strength training program. A person with an active occupation will experience less overall muscle atrophy than someone with a sedentary occupation, and even less than someone who undergoes bed rest or limb immobilization after hospitalization.


What happens when we take breaks from strength training, then start lifting again?

When we stop lifting weights for short periods of time (which researchers call detraining, and which strength coaches call deloading), we lose muscle size due to the removal of the regular stimulus that the muscle fibers of high-threshold motor units need to remain a given size.

Most of the losses occur in the first four weeks, and overall muscle size then stabilizes thereafter.

Taking a break from strength training of more than one week therefore involves allowing the muscle fibers of high-threshold motor units to atrophy, which reduces overall muscle size.

However, when we start lifting again, hypertrophy occurs more quickly than it did the first time we trained the muscle.

This probably happens for two reasons.

Firstly, muscle fibers appear to have an epigenetic memory, which includes information about their previous maximum size. This memory may be related to an elevated number of myonuclei, which are donated by satellite cells in order to permit an increased resting rate of muscle protein synthesis, since these myonuclei are not lost in the detraining period despite losses in muscle size. Consequently, it is much easier for muscles to return to a size that they have previously attained, than for them to attain a new, larger size.

Secondly, motor unit recruitment decreases more slowly during than muscle fiber size. Therefore, when we return to our strength training program after several weeks of detraining, we are able to recruit more motor units than we could the first time we did the program. This means that we are able to load more muscle fibers, and can increase the size of the muscle more quickly the second time around.


Can we enhance hypertrophy by deliberately taking short breaks from strength training?

Some commentators have proposed that it might be possible to accelerate muscle growth by deliberately taking short breaks from strength training programs.

This is unlikely, because muscle growth occurs more quickly during a period of training after a period of detraining compared to after an initial period of training due to adaptations that are related to the initial period of training, including increased (1) myonuclear number, and (2) motor unit recruitment. So while the rate of muscle growth will be faster in the period of training after a period of detraining up to the point where the lost muscle is regained, the rate of growth will slow to the same rate once this point is reached.

Just to be clear, this means we cannot artificially bring about a greater increase in myonuclear number (and thereby increase hypertrophy) by deliberately taking short periods of time off training.

While it is true that taking time off training can lead to more muscle damage in the first workout back after training (and this unaccustomed, muscle-damaging workout does stimulate greater satellite cell activity), it does not then lead to increased myonuclear number. In fact, satellite cell activity often seems to be used solely to repair the muscle damage that unaccustomed workouts have caused. This is in line with research showing that increases in muscle size are not very closely linked to increases in satellite cell activity, and that satellite cell activity is a general response to all types of exercise.

Consequently, while producing muscle damage by taking time off training (or by doing very hard workouts) will probably trigger a large satellite cell response, this does not indicate that myonuclear addition is occurring, rather that the satellite cells are working to help repair the damage, along with a large proportion of increase in muscle protein synthesis rates as well.


Are there any downsides with taking time off strength training?

Sometimes, it may be necessary to take time off strength training in order to allow central nervous system fatigue to dissipate (although a very reduced volume training program is better, as it allows central nervous system fatigue to dissipate without losses in muscle mass occurring).

Until recently, it was assumed that there were no downsides of taking time off strength training, especially since it seems fairly easy to catch up to where we left off training. However, recent research suggests that repeated periods of loading and unloading may lead to an accumulation of collagen inside the muscles, which may make them stiffer and more prone to injury.

Consequently, where the choices available are to engage in complete detraining or to take part in a much reduced volume strength training program, it seems that the reduced volume strength training program is likely to be a much better long-term option.


What does this mean in practice?

In practice, muscles grow in the 48 hours after they are stimulated by a strength training workout (except when central nervous system fatigue impairs the stimulating effect, since this probably prevents full motor unit recruitment from being achieved).

Consequently, deliberately stopping training for more than a week cannot help enhance muscle growth (although if high levels of central nervous system fatigue are present due to a block of very muscle-damaging workouts, then it may be necessary for progress to occur).

Muscle size reduces rapidly over a period of approximately four weeks after stopping strength training, likely until it reaches an equilibrium produced by the customary levels of mechanical loading experienced by the lifter during their daily life.

When starting strength training again after a period of detraining, muscle size increases far more quickly than it did the first time, which makes taking a vacation or time off the gym much less of a problem than it first appears. We can get back to where we left off relatively quickly.


What is the takeaway?

Muscle mass does decrease relatively quickly when we stop strength training, and most of the losses occur in the first four weeks. Thereafter, the losses are much smaller, depending on our level of activity. Fortunately, when we start strength training again after a period of time off, muscle size increases far more quickly than it did the first time, which means that we can get back to where we left off relatively quickly.