What determines training frequency?

Training frequency for bodybuilding or recreational strength training with the purpose of increasing muscle size is a contentious topic.

A quick look around the internet will reveal that experts hold a very wide range of opinions about the ideal training frequency to use when strength training for increased muscle size.

Some authorities recommend training a muscle very frequently, while others (especially those who are closer to mainstream bodybuilding) still suggest training each muscle only once per week. Those who follow the literature fairly dogmatically will point to recent meta-analyses that have reported an ideal training frequency of 2–3 times per week, although most bodybuilders still only train muscles 1–2 times per week.

Even so, whether to train a muscle 2 or 3 times per week is a major decision that will have a big impact on the way a strength training program is set up. Consequently, the current literature does not yet have the level of precision that we need to decide on the optimal training frequency by crunching the numbers from the available long-term studies.


What factors might affect training frequency?

Training frequency can be affected by the duration of time for which muscle protein synthesis rates are elevated after a workout, and also by the ability of the lifter to recover from the first workout, because of the residual effects of fatigue on the hypertrophic stimulus experienced in a second workout.

Let’s take a look at both of those factors.


#1. Changes in the rate of muscle protein synthesis

In a strength training workout, the fibers of a muscle experience mechanical loading. This mechanical loading is detected by mechanoreceptors, which leads to anabolic signaling. This signaling triggers an increase in the rate of muscle protein synthesis inside each loaded muscle fiber. The increase in the internal rate of muscle protein synthesis is what causes increased protein content inside the worked muscle fibers.

After the workout, the rate of muscle protein synthesis increases over several hours, reaches a peak, and declines. We can draw a curve of this change over time, and the area under this curve is the overall effect of the workout on the size of the muscle fiber. When the rate of muscle protein synthesis is increased to a greater height, or for a longer time, the muscle fiber experiences a greater increase in size.

Logically, it makes sense to wait until the rate of muscle protein synthesis returns to approximately pre-workout levels after a workout before doing the next workout, otherwise the stimulus provided by the second workout could be partly wasted. Therefore, the duration of time for which the rate of muscle protein synthesis is elevated after a workout is quite important.

However, the shape of the curve differs between trained and untrained individuals. In untrained people, the rate of muscle protein synthesis takes longer to reach a peak, takes longer to decay, and has a larger total area under the curve. Also, the shape of the curve differs depending on which proteins are measured. When we assess all of the proteins in a fiber (by a measurement called mixed muscle protein synthesis), the curve displays a different profile from when we assess only the contractile proteins (by a measurement called myofibrillar muscle protein synthesis).

We should note that both myofibrillar and mixed muscle protein synthesis rates can be elevated for the purpose of increasing muscle fiber size, and also for the purpose of repairing damage to the fiber, since this damage can occur to non-contractile and contractile proteins. Therefore, even though the rate of (myofibrillar or mixed muscle) protein synthesis is increased, this does not mean that the muscle fiber is adding new proteins to increase in size. It could simply be repairing damage that has been done.

When we consider all proteins in untrained individuals, the rate of mixed muscle protein synthesis reaches a peak after 16 hours, and still has not reached pre-workout levels at 48 hours. Looking at the shape of the curve suggests that it might reach pre-workout levels at approximately 72 hours, which would imply an ideal training frequency of once every 3 days, or twice a week. Since the curve peaks earlier and declines faster in trained individuals, the same data for all proteins in that population implies that training a muscle more frequently would be ideal. Even so, this analysis is limited because all muscle fiber proteins are being taken into account.

Ideally, we would perform a similar analysis regarding only the contractile proteins, but such data have not been published. Yet, research has shown that the elevation in the rate of myofibrillar protein synthesis over a 48-hour period is strongly associated with the hypertrophy resulting from a strength training program (when individuals are trained, but not when they are still untrained). This indicates that the majority of the elevation in myofibrillar protein synthesis rates has occurred within 48 hours in trained people, which would imply an ideal training frequency of at least once every 2 days, or three times per week.


#2. Recovery of the lifter

Immediately after a strength training workout, we experience a reduced ability to exert force. There are three factors that produce this effect: (1) peripheral (local muscular) fatigue, (2) muscle damage, and (3) central nervous system fatigue.

Importantly, the impact of each of these factors changes over time.

Peripheral (local muscular) fatigue can occur due to reductions in the activation of individual muscle fibers (either because of a decrease in the sensitivity of actin-myosin myofilaments to calcium ions, or because of a reduction in the release of calcium ions from the sarcoplasmic reticulum), or through factors affecting the ability of individual muscle fibers to produce force, which involves impairments in the function of actin-myosin crossbridges. However it occurs, the effects of peripheral fatigue are very transitory.

We are recovered from peripheral fatigue within a few hours.

Muscle damage can involve a range of things. It can involve very small amounts of damage to internal structures within the muscle fiber, such as the cytoskeleton or the contractile proteins. Indeed, one of the most common signs of mild muscle damage is a derangement of the Z disks, which are lines that separate one sarcomere from the next. Muscle damage can also involve tearing of the cell membrane, and severe damage can involve complete tears of the muscle fibers themselves. All of these types of damage are repaired by carrying out additions to the structures of the existing muscle fiber. Very severe damage cannot be repaired, and this leads to fiber necrosis. When this happens, the remains of the old muscle fiber are completely degraded by proteases and a new muscle fiber is grown inside the cell membrane of the old fiber, through a process called regeneration.

Some types of strength training involve little or no muscle damage, while others involve a great deal of muscle damage. Also, muscle damage can differ between muscle groups, muscle fiber types, and individuals. Depending on the degree of muscle damage, the repair or regeneration process can last anything between no time at all, and several weeks.

Central nervous system fatigue can occur either because of a reduction in the size of the signal sent from the brain or the spinal cord, or because of an increase in afferent feedback that reduces motor neuron excitability. Central nervous system fatigue is not the same thing as how we feel about doing the next workout, which seems to be more closely related to the amount of muscle damage that we have experienced. Rather, it is simply the extent to which we can voluntarily activate the trained muscle.

Central nervous system fatigue is a lot smaller and more short-lived after strength training than most people believe, which is probably because of a confusion about the meaning of the term. We assume that because we do not feel ready for the next workout, we must be experiencing central nervous system fatigue, which is not necessarily the case. In fact, central nervous system fatigue tends to increase with increasing exercise duration rather than intensity, making it more pronounced after endurance training.

However, when the muscle damage is severe, such as after unaccustomed eccentric-only training or high volumes of conventional strength training, this can trigger extended periods of central nervous system fatigue that can last up to 2–3 days after the workout.

These three types of fatigue have different effects on the impact of subsequent workouts.

When we are still experiencing peripheral fatigue at the point of doing a subsequent workout (which would be very unusual, since that would require us to perform another workout within hours of the previous one), this does not influence the hypertrophic stimulus. The high level of peripheral fatigue leads to an increase in motor unit recruitment and a reduced fiber shortening velocity, which means that our high-threshold motor units are recruited sooner, and we perform fewer reps but still achieve the same mechanical loading on the target muscle fibers.

When we are still experiencing central nervous system fatigue at the point of doing a subsequent workout, this affects the hypertrophic stimulus. If we cannot fully activate a muscle during training, we will not stimulate its high-threshold motor units, and thereby we will fail to produce any mechanical loading on the muscle fibers controlled by those motor units, and this will reduce the amount of hypertrophy that results. In practice, central nervous system fatigue is caused either by aerobic exercise or muscle-damaging strength training performed in close proximity to the workout.

When we are still suffering from muscle damage at the point of doing a subsequent workout, this affects the hypertrophic stimulus for two reasons. Firstly, it can affect the hypertrophic stimulus to the extent that it triggers any central nervous system fatigue. Secondly, it can lead to oxidative stress that interferes with the elevation in muscle protein synthesis rates that occurs as a result of the anabolic signaling triggered by mechanical loading. Therefore, even when we are capable of fully activating a muscle, muscle damage can impede hypertrophy by interfering with the signaling processes.


What determines training frequency?

Training frequency is determined by: (1) the duration of time for which myofibrillar protein synthesis rates are elevated post-workout, and (2) the duration of time for which muscle damage interferes with the hypertrophic stimulus of a second workout, either by causing central nervous system fatigue, and thereby preventing the recruitment of high-threshold motor units, or by elevating oxidative stress.

Currently, it is unclear how long myofibrillar protein synthesis rates are elevated for post-workout, but it is likely to be less than 48 hours. How much less than 48 hours is currently unknown. If we ignore muscle damage as a factor, optimal training frequency would therefore be at least once every 2 days, or three times per week. If myofibrillar protein synthesis rates are only elevated to a meaningful extent for 24 hours post-workout (which is feasible), then optimal training frequency could be daily, but only when muscle damage is not considered as a factor.

The duration of time for which muscle damage might interfere with the hypertrophic stimulus of a second workout is also unclear. When performing conventional strength training workouts, central nervous system fatigue is small and short-lived. Yet, high volume training and unaccustomed eccentric-only training can cause impaired voluntary activation for up to 2 and 3 days, respectively, because of the greater muscle damage they produce. The duration of the effects of oxidative stress caused by muscle damage on the hypertrophic stimulus last 8–24 hours in rodents, but might be expected to last longer in humans, although this has not yet been assessed.

Therefore, it seems likely that workout volume plays a key role in determining the optimal training frequency. High volume workouts require longer recovery times that lead to an optimal training frequency of once every 2 — 3 days (depending on whether the lifter is accustomed to the exercise, and accepting that the duration of the effects of elevated oxidative stress are unknown). Moderate volume workouts may be limited only by the duration of time for which myofibrillar protein synthesis rates are elevated (again, accepting that the duration of the effects of elevated oxidative stress are unknown).


How important is muscle damage?

When it comes to bodybuilding, the subject of training volume is far less contentious than the subject of training frequency.

Most experts agree with the conclusions reported in the research literature, which report a dose-response of training volume on hypertrophy from <5 sets to 5–9 sets to >10 sets per week, in both trained and untrained individuals.

Interestingly, although these studies compare training volumes by altering the number of sets performed in each workout (and keep the number of workouts the same), the dose-response effects are reported for weekly training volume. Indeed, many experts refer solely to measures of weekly training volume, rather than workout volume.

Even so, increasing weekly training volume by increasing the number of workouts that are performed does not seem to have the same dose-response effects as increasing weekly training volume by increasing the number of sets performed in each workout, at least in untrained individuals. In one study, the researchers found that doing the same workout 2, 3, or 5 times a week caused similar muscle growth. Each workout involved 3 sets of knee extensions to failure, and therefore the 3 groups did either 6, 9, or 15 sets per week. This difference in weekly training volume was expected to cause a dose-response effect on hypertrophy, but it did not. The lack of difference between the groups suggests that muscle damage produced in some of the workouts may have impaired the hypertrophic stimulus of subsequent workouts each week, either by causing reduced voluntary activation, or by elevating oxidative stress.

Clearly, this effect would be smaller in trained individuals, compared to in untrained individuals, but despite the presence of the repeated bout effect, muscle damage does still occur in trained lifters, especially in the upper body musculature.

Overall, this reveals that (once again) training volume only counts when it is stimulating. When training too often, volume can fail to be stimulating if motor unit recruitment is impaired by central nervous system fatigue secondary to muscle damage, or if muscle protein synthesis is prevented from increasing despite elevated anabolic signaling, by oxidative stress secondary to muscle damage.


Why not train less frequently?

If training too frequently can run the risk of us doing workouts that do not stimulate hypertrophy, why not train less frequently, and work muscles just once per week as many bodybuilders do?

There are two reasons why we might want to train a muscle more than once per week.

Firstly, some people find that performing all of their target weekly training volume for a single muscle group in a single workout is very challenging, because of the high levels of peripheral fatigue that result during the workout, and the high levels of muscle damage and soreness that occur afterwards. This is a matter of personal preference, but it is easy to understand.

Secondly, there is likely a non-linear dose-response of training volume for a given workout. Studies have shown that performing 10 sets of 10 reps per exercise is just as effective as doing half the number of sets. Moreover, the dose-response of training volume on increases in post-workout muscle protein synthesis rates also plateau after a certain number of sets. Once we go above a certain threshold of workout volume, splitting that workout volume out into two halves and doing each half twice per week should produce greater muscle growth than performing it all in a single workout.

Optimal training frequency is therefore dependent upon the volume performed in the workout. Each workout produces a certain level of muscle damage, which will determine how often it can be done. However, each workout will only produce increasing dose-response effects of volume on hypertrophy up to a certain point. Therefore, some training frequencies (and their associated optimal workout volumes) will necessarily produce better results than others.


What about psychological stress?

Some research has shown that when individuals are exposed to high levels of psychological stress, they take longer to recover from a workout.

Although this might seem to imply a mechanism involving the central nervous system, it is more likely that the effect is mediated by a reduced healing of muscle damage. Indeed, several studies have shown that wound healing in various contexts is slower when individuals are exposed to high levels of long-term psychological stress.

It does not take a huge leap of imagination to see how muscle damage repair might be similarly impaired under conditions of psychological stress.


What does this mean in practice?

There are several practical implications of this framework.

Firstly, it is evident that it is possible to train too frequently, most likely because of the effects of muscle damage experienced in one workout on the hypertrophic stimulus produced by subsequent workouts.

Secondly, the amount of muscle damage differs between individuals (especially because of training status, but also because of stress levels), between muscles, and between workouts (especially because of volume). Therefore, the optimal training frequency will necessarily differ between individuals, between muscles, and between workouts. Trying to find the perfect training frequency that will work for everyone, all of the time, is a fundamentally flawed enterprise.

Thirdly, we cannot calculate our weekly training volume by simply adding up all of the sets performed to failure (or to a given number of reps from failure) for the week, regardless of when they were performed. If workouts are done too soon after each other, this will lead to the stimulating effects of the subsequent workout being impaired. How soon is too soon will depend on the individual, the muscle, and the workout.

Practically, this means that we need to start new training programs with a conservative training frequency, and increase that frequency until we stop making progress from one workout to the next. In most cases, that will probably involve starting by training a muscle group 1 or 2 times per week, depending on the planned workout volume.


What is the takeaway?

Training frequency is determined by: (1) the duration of time for which myofibrillar protein synthesis rates are elevated post-workout, and (2) the duration of time for which muscle damage interferes with the hypertrophic stimulus of a second workout, either by causing central nervous system fatigue, and thereby preventing the recruitment of high-threshold motor units, or by elevating oxidative stress.

The amount of muscle damage caused can vary between individuals and between muscles, and also according to the type of workout. Higher volume workouts produce more muscle damage, and therefore require more recovery time. Therefore, the optimal training frequency necessarily differs between individuals, between muscles, and between workout types.