How does proximity to failure affect hypertrophy?

Training to failure is a contentious topic in bodybuilding. Some people have argued that it is essential for muscle growth to occur, while others claim that it is detrimental, and will ultimately lead to stagnation. But what is muscular failure really, and how does it affect hypertrophy?


What is muscular failure?

Muscular failure refers to the point during a strength training set at which we can no longer perform the lifting (concentric) phase of a given exercise with a certain load through a predetermined range of motion, without substantially altering our technique.

This point is determined by the ability of the muscle to exert a given level of force. When our muscles can no longer exert force above a certain threshold (which is slightly greater than the mass of the weight), we reach the point where we can no longer perform the lift.

Importantly, the muscle does not stop working completely. It can still produce a certain level of force, but it cannot produce the required level of force in order to perform the required lift. This is very clear when using drop sets, in which muscles perform sequential sets with lighter and lighter weights, immediately after reaching failure with the first weight.

Therefore, the point at which muscular failure is reached is arbitrarily determined by the weight that we use, and consequently the amount of muscle force that is required to lift that weight.

Our use of the term “muscular failure” is not particularly accurate, because it is not the muscle that fails, but rather our ability to perform the task. Indeed, many researchers use the term “task failure” in some of the more technical literature exploring the nature of fatigue, since this is a much better description of what is really happening.


What causes muscular failure?

Muscular failure is caused by fatigue.

Fatigue causes a reduction in the ability of muscles to produce force during exercise. Once the amount of fatigue reaches a certain level, the ability of the prime mover muscles to produce force is reduced to a point that is insufficient to lift the weight.

Although fatigue is a single concept, it describes a range of different processes that can occur in either the central nervous system (brain and spinal cord) or inside the muscle itself. Which processes contribute most to the reduction in muscle force differ, depending on the level of force being produced, and on the contraction type.

Again, this indicates that our use of the term “muscular failure” is probably not particularly helpful, because in some cases it may be the central nervous system that stops the muscle from being able to complete the task in hand, rather than any phenomenon going on inside the muscle itself. This is another reason why the term “task failure” is a better way to refer to what happens when we are no longer able to perform another repetition.

Let’s look more closely at the two types of fatigue.


#1. Central nervous system fatigue

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 feeling tired or demotivated. It is simply the extent to which we can voluntarily activate the trained muscle.

Our ability to voluntarily activate a muscle can be measured by testing our maximum static (isometric) strength in a voluntary effort, and also in an involuntary effort, by stimulating the muscle electrically. Typically, muscles produce slightly more force when they are stimulated electrically, compared to when we produce force voluntarily. We can express the force we produce voluntarily as a percentage of the force we produce by electrical stimulation, and this is our level of voluntary activation.

The level of voluntary activation reflects the number of motor units that have been recruited. When voluntary activation is very high, most (if not all) motor units are recruited. When voluntary activation is reduced, some motor units are not recruited.

When central nervous system fatigue occurs, voluntary activation is reduced, and this means that the number of motor units that are recruited decreases. Since motor units are always recruited in size order, this means that some of the highest threshold motor units (which control the largest numbers of the most highly responsive muscle fibers) are not recruited, and are therefore not stimulated to grow after strength training.


#2. Peripheral fatigue

Peripheral fatigue can be caused by many different processes that happen inside the muscle itself.

These processes can be classified into those that (1) reduce 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), and those that (2) affect the ability of individual muscle fibers to produce force, and which involve impairments in the function of actin-myosin crossbridges.

It is commonly believed that peripheral fatigue is caused by the accumulation of lactate that occurs during anaerobic glycolysis, or because of the associated release of hydrogen ions (acidosis). However, research show that some of these metabolic byproducts are not central to the process of fatigue, while other factors are probably more important.

Peripheral fatigue seems to originate from various sources, including (1) the accumulation of ions that reduce the release of calcium ions (extracellular potassium) or that impair the sensitivity of the actin-myosin myofilaments to calcium ions (reactive oxygen species), (2) the production of metabolic byproducts that interfere with actin-myosin crossbridge function (phosphate ions and adenosine diphosphate), and (3) a reduction in the availability of substrate in the various energy pathways.

Peripheral fatigue occurs through different mechanisms depending on the load that is used. When performing conventional strength training with light loads, peripheral fatigue is closely linked to processes that are connected to metabolite accumulation. When performing eccentric contractions, peripheral fatigue occurs without any metabolite accumulation and is probably determined by impaired calcium ion release. Similarly, lifting heavy loads does not involve very much metabolite accumulation, and the fatiguing mechanisms are likely different from those involved in lifting light loads.

However, when the muscle experiences peripheral fatigue (through whatever mechanism), this reduces the amount of force that each muscle fiber can produce, and so the central nervous system increases the level of motor unit recruitment to compensate. When peripheral fatigue is very high, the central nervous system recruits all available motor units, which activates the majority of the muscle fibers inside the muscle.


What stimulates hypertrophy?

Hypertrophy is mainly the result of single muscle fibers inside a muscle increasing in volume. Single muscle fibers grow once they are subjected to a sufficiently high mechanical loading stimulus. This mechanical loading stimulus is the force exerted by the muscle fiber itself.

To achieve a sufficiently high force during conventional strength training, fibers need to contract actively at a slow speed. The shortening speed of a fiber is the main determinant of the force it produces, because of the force-velocity relationship. Slow shortening speeds allow greater forces since they involve more simultaneously attached actin-myosin crossbridges, and it is the attached actin-myosin crossbridges that produce force.

Muscles contain many thousands of fibers, which are organized into groups of motor units. There are hundreds of motor units in each muscle, and they are recruited in order of size, from small, low-threshold motor units to large, high-threshold motor units.

Low-threshold motor units govern small numbers (dozens) of comparatively unresponsive muscle fibers, which do not grow very much after being subjected to a mechanical loading stimulus. High-threshold motor units govern large numbers (thousands) of highly responsive muscle fibers, which grow substantially after being subjected to a mechanical loading stimulus. Such motor units might control both slow twitch and fast twitch fibers, or solely fast twitch fibers, depending on the fiber proportions of the muscle.

Only those contractions that involve the recruitment of high-threshold motor units while muscle fibers are shortening slowly will stimulate meaningful amounts of hypertrophy. The recruitment of low-threshold motor units does not stimulate very much muscle growth, because such motor units govern only a small number of relatively unresponsive muscle fibers.


How does muscular failure affect the hypertrophic stimulus? (part I)

As we fatigue, the shortening speed of the muscle fibers reduces and motor unit recruitment increases. These are the key factors that are required for stimulating hypertrophy, because they allow high-threshold motor units to experience sufficiently high levels of mechanical loading. However, the type of fatigue we experience affects how much motor unit recruitment we can achieve at the point of muscular failure.

When muscular failure is caused entirely by peripheral fatigue, all of the motor units in the prime mover muscle are recruited, including the high-threshold motor units that control the majority of the large, highly-responsive muscle fibers that grow after strength training.

Many commentators assume that muscular failure only involves peripheral fatigue during strength training. This then leads them to the conclusion that training to muscular failure always leads to full motor unit recruitment. For much of the time, this is a valid model of strength training (although there are important exceptions, as we will see shortly). Indeed, when we compare the effects of strength training with heavy and light loads, the model appears to hold up reasonably well, albeit not perfectly.


What does training to muscular failure achieve, when training with heavy or light loads?

#1. Heavy loads

When we lift a heavy weight (5RM, or 85–90% of 1RM), all of our motor units are recruited (to the extent that we are capable of voluntarily activating them in that particular muscle). Also, when we lift a heavy weight, we cannot move quickly. Therefore, muscle fiber shortening speed is slow. Therefore, lifting heavy weights stimulates hypertrophy to occur even without experiencing any peripheral fatigue (or training to failure).

Moreover, neither experiencing peripheral fatigue nor training to failure should make much difference to the amount of hypertrophy that occurs when lifting heavy weights (which is exactly what the research reports).

Lifting five reps with a 5RM should therefore produce *largely* the same amount of hypertrophy whether it is done as five singles with rest periods in between, or as a 5RM. It is true that performing a 5RM might lead to slightly greater muscle growth than five singles due to (1) a longer stimulating time under tension, and (2) higher levels of mechanical loading, because of the slower bar speeds in the final reps, but this will likely have only a tiny effect.


#2. Moderate and light loads

When we lift a moderate or light weight (<5RM, or <85% of 1RM), not all of our motor units are recruited until we experience enough peripheral fatigue. Similarly, when we lift moderate or light weights, we can move very quickly. Therefore, muscle fiber shortening speed is fast, and mechanical loading on each muscle fiber will be small (we could reduce the speed we move by voluntarily using a slow tempo, but this would simultaneously reduce motor unit recruitment, so is of no practical use). This means that lifting moderate or light weights does not stimulate hypertrophy to occur unless we experience enough peripheral fatigue.

But how much peripheral fatigue do we need when lifting moderate or light weights? Do we actually need to train to failure?

Research has shown that motor unit recruitment increases progressively throughout a set with light loads (which makes sense) and may reach full motor unit recruitment at the point of muscular failure (although some research suggests that full motor unit recruitment is not reached). Similarly, bar speed (and therefore muscle fiber shortening velocity) also reduces progressively over a set with light loads, and bar speed is the same as the speed used in a one repetition maximum (1RM) in the final rep.

This suggests that when using light and moderate loads, hypertrophy will be stimulated in several reps *before* reaching muscular failure, as well as on the final rep itself, because those reps also involve high levels of motor unit recruitment and slow muscle fiber shortening velocities.

Practically, since bar speed likely reduces progressively towards the speed used in a 1RM on the final rep regardless of the weight used, and motor unit recruitment is full when using a 5RM load, it is probably the final five reps of any set performed to failure that stimulate hypertrophy. Each of those final five reps probably contributes approximately the same amount to the overall hypertrophic stimulus, which is why training to failure and stopping one rep short of failure with moderate and light loads produces largely the same effects, as there is only one stimulating rep difference between the programs. Clearly, stopping three or four reps short of failure would lead to a bigger difference in the number of stimulating reps per set, compared with training to failure, and this would probably lead to less hypertrophy. Stopping more than five reps short of failure should produce no hypertrophy at all.

Ultimately, this means that sets with light and moderate loads can probably be terminated a couple of reps before muscular failure (1–2 reps in reserve) and still produce meaningful amounts of muscle growth. For some trainees, this may be helpful, since training with a closer proximity to muscular failure increases muscle damage, making it harder to train more frequently.


How does muscular failure affect the hypertrophic stimulus? (part II)

As we fatigue, the shortening speed of the muscle fibers reduces and motor unit recruitment increases. These are the key factors that are required for stimulating hypertrophy, because they allow high-threshold motor units to experience sufficiently high levels of mechanical loading. However, the type of fatigue we experience affects how much motor unit recruitment we can achieve at the point of muscular failure.

When muscular failure is caused by peripheral fatigue, all of the motor units in the prime mover muscle are recruited. In contrast, when muscular failure is (partly) caused by central nervous system fatigue, our ability to produce a given level of force is reduced without all of our motor units being recruited.

Therefore, although fatigue generally always causes a reduction in muscle fiber shortening speed, it does not always lead to full motor unit recruitment. Only when muscular failure is caused solely by peripheral fatigue (and no central fatigue is present) does full motor unit recruitment occur. There are situations in which central fatigue does seem to be increased during strength training, which is very important for hypertrophy.


When does central nervous system fatigue occur?

During strength training, peripheral fatigue and central nervous system fatigue both occur, and both contribute to us reaching muscular failure in a set. Moreover, there are certain factors that seem to increase the proportional amount of central fatigue that is present within the workout.

Additionally, when we experience muscle damage during a workout, this can lead to central fatigue occurring that can last for a couple of days. And obviously, some workouts cause a lot more muscle damage (and therefore more central fatigue) than others.


#1. During the workout

During any workout, there will be some central fatigue.

The amount of central fatigue is greater during aerobic exercise, while the amount of peripheral fatigue seems to be greater after anaerobic exercise. Also, endurance training bouts of longer durations seem to produce more central fatigue than bouts of shorter durations.

In general, it seems likely that central fatigue accumulates to a greater extent when exercise involves longer durations of muscular contractions at lower levels of force. Therefore, central fatigue is probably increased towards the end of a workout, which could help explain why higher volumes do not cause increasingly greater amounts of hypertrophy, and could also explain why exercise order affects muscle growth.

Also, the amount of central fatigue reduces quite quickly at the end of each set of muscular contractions. It is very high at the end of a set, and reduces quickly (likely in an exponential way) thereafter. Therefore, central fatigue is likely increased at the start of subsequent set when rest periods are short, which could help explain why short rest periods lead to less muscle growth.

Interestingly, exercises that involve smaller amounts of working muscle mass (such as single-joint or single-limb exercises) lead to less central fatigue and therefore allow more peripheral fatigue to be achieved, compared with multi-joint and two-limb exercises. This is perhaps because they involve greater aerobic demand. This is an argument in favor of making extensive use of single-joint exercises for bodybuilding, as well as for placing multi-joint exercises (such as lat pull-downs) first in a workout, progressing to single-joint exercises (such as biceps curls), and finishing with single-limb, single-joint exercises (such as concentration curls).

Finally, some research indicates that central fatigue during a set is higher when using light loads, compared with moderate or heavy loads, due to a higher level of afferent feedback caused by the greater accumulation of metabolites. Consequently, the level of motor unit recruitment that can be reached with moderate or heavy loads may be slightly greater than that achievable with light loads. This may mean that strength-trained individuals could attain greater muscle growth over a long-term program. However, given that most research to date shows similar gains in muscle size after training with moderate and light loads (even in moderately well-trained individuals), the difference in the amount of central fatigue is probably quite small.


#2. After the workout

Workouts that involve large amounts of muscle damage cause central fatigue to develop in the days after exercise. This means that the subsequent workout can be affected by what we do in the previous one.

Various factors influence the amount of muscle damage that is caused in a workout, although the most important one in practice is training volume. Higher volumes cause more muscle damage, and therefore require longer to recover from. In addition, using eccentric contractions can increase muscle damage, as can using unfamiliar exercises. It is also noteworthy that training to failure increases the amount of muscle damage that occurs as a result of a workout, most likely due to the greater peripheral fatigue that it involves.

Therefore, we need to be aware that when we increase workout volume, use eccentric contractions, or train to failure, we may need to reduce training frequency, otherwise we could be performing workouts in which we do not achieve full motor unit recruitment, which will be suboptimal.


What is the takeaway?

Muscular failure refers to a point during strength training at which we can no longer perform the lifting phase of an exercise through the required range of motion, without altering our technique. This point is determined by the ability of the muscles to produce force. The amount of force that muscles can produce is affected by the level of fatigue that they are experiencing.

Fatigue reduces muscle fiber shortening speed, and peripheral fatigue increases motor unit recruitment increases, but central fatigue prevents full motor unit recruitment from being reached. Increasing peripheral fatigue while minimizing central nervous system fatigue is therefore necessary to stimulate maximum hypertrophy.

When lifting heavy loads, muscle fiber shortening speed is slow and motor unit recruitment is high regardless of the amount of peripheral fatigue that is present. Therefore, training to failure is unnecessary. When lifting moderate or light loads, a certain level of peripheral fatigue is required to achieve sufficient levels of motor unit recruitment and a sufficiently slow muscle fiber shortening speed, but this can be achieved within a couple of reps of muscular failure, and it is not actually necessary to reach failure in order to stimulate muscle growth.