How important is the ‘pump’ for muscle growth?

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Most gym goers have the same goal: muscle growth. Nevertheless, you see different ways of training: one lifts heavy weights and does few repetitions, the other trains a little lighter but does a lot of repetitions, another works up a sweat by applying short rests and techniques such as supersets. Which of these training forms leads to the most muscle growth? In other words: is hypertrophy training mainly about lifting heavy weights or is it more about creating metabolic stress (the ‘pump’)?

Key points:

1.   Traditionally, bodybuilders train mainly intuitively: they regard muscle pump and fatigue as indicators of stimulus and thus of muscle growth.

2.   However, for muscle growth, metabolic stress (which causes, among other things, the muscle pump) is secondary to mechanical stress: mechanical tension is the primary pathway to muscle growth.

3.   Mechanical tension is the creation of fatigue in the muscle by successively doing several contractions with a certain weight (the fatigue that arises during a set and then slowly disappears again). This can be a light or heavy weight: it is (mainly) about the stimulating (‘effective’) repetitions before muscle failure. Yet in practice, you get the best results when training in the 6-20 reprange, with relatively low metabolic stress.

4.   Yet, possibly creating metabolic stress (the pump) in itself is also a way to achieve a growth stimulus, although it offers much less muscle growth potential than mechanical tension. Through metabolic stress, for example by doing lots of reps, you would especially stimulate sarcoplasmic muscle growth. For that reason, it may be useful to train a small part (10-20%) of your training program in higher rep ranges, whether or not in combination with short rest periods between sets.


When it comes to muscle growth, there are multiple roads that lead to Rome (and that same Rome wasn’t built in a day). Which path you should take depends partly on your precise goal (only muscle growth or, for example, also fat loss and/or condition improvement?) and on practical factors, such as how much time you have to train.

We assume in this article that getting bigger, so muscle growth, is your most important or only goal. And that your agenda allows you to do a few training sessions of roughly an hour every week.


As bodybuilders, we’re pretty spoiled these days, with one scientific study after another, and with the worldwide web to absorb all that knowledge. Bodybuilders of the past did not have that luxury and had to train purely intuitively, by feeling.

Usually they tried to ‘burn’ their muscles as much as possible during training – chasing the ‘pump’, which causes muscles to swell. If it hurts and the muscles are already getting bigger during the workout, it must be an effective workout, right?


If we want to find out what the best way of training for muscle growth is, we must first distinguish between natural and non-natural (enhanced) bodybuilding. In our view, these are two different sports, each requiring a completely different approach.

If you use steroids, you are Superman and you can throw all researches and rules overboard. You will grow no matter what. As a natural, however, you have to deal with the natural limits of your body and it comes down to thorough recovery management.

Unfortunately, that distinction is not made (consciously or unconsciously) on many websites and in many YouTube videos, causing the theories behind those two sports to intersect. And that’s like thinking you’re reading information about running, while also containing advice for sprinters.

Because these two forms of bodybuilding are intertwined (in the gym even literally), you see many naturals train in the riotous way that steroids users can afford. As a result, many ‘natties’ are in a constant state of overreaching and are more concerned with muscle breakdown than with muscle building…

Different rules apply to naturals who strive for optimal muscle growth and we try to get them out in this article.


In 2010, scientist Brad Schoenfeld published a now classic study, in which he described three mechanisms that lead to muscle growth: muscle damage, mechanical tension and metabolic stress xxiii ] . Those are the elements that should be in your training if a muscle is to grow. According to Schoenfeld, based on his scientific findings at the time.

Nearly ten years and many studies later, insights have changed – also among Schoenfeld himself i ] – and at the same time, much remains unclear about what exactly causes muscle growth. Let’s go through the three original mechanisms.


Training causes damage in the muscle, your body repairs that damage and at the same time the muscle is made bigger and stronger. That’s the theory about muscle damage you’ve probably heard. But is muscle damage a prerequisite for muscle growth?


‘In the past’ many scientists thought that inflicting muscle damage (as a result of strength training) wakes up muscle protein synthesis ii ] . And that muscle protein synthesis, the building of proteins in muscles, is pretty much the most important link in the process of muscle growth.

We now know that muscle breakdown and muscle building are two more or less separate processes, although both require muscle protein synthesis iii ] . We also know that muscle damage is greatest in people who have just started strength training, while with them the major muscle growth only starts after a few weeks, when the muscle damage as a result of training is no longer so great iv ] .

A round of literature over the past ten years tells us that muscle damage in itself probably does not cause muscle growth v ] . A recent meta-study confirms that vi ] , as did a recent study by Haun et al, which showed that easy and hard gainers do not differ significantly in how much muscle damage they incur from strength training xxxii ] .

In addition, we see that people gain serious mass with blood flow restriction training, where you squeeze your blood and train with baby weights. And that while there is hardly any muscle damage with this training method.


Doing a lot of muscle damage could even get in the way of your gains, because muscle protein synthesis is then mainly used for repairing muscle tissues instead of building new muscle proteins vii ] .


When training for muscle growth, it is about creating overload: that you put more stress on the muscle than before. Only if a muscle has to do heavier work than it is used to will your body see the need to let the muscle grow. In this way, it can arm itself against future heavy loads.

Overload usually also leads to some muscle damage, but that is more of a consequence, a by-product xxxiii ] . Muscle damage in itself is therefore not a goal.


So, for example, it makes no sense to do excessively slow repetitions. ‘Slow eccentrics’ (holding the eccentric phase of an exercise for a second) are often ‘prescribed’ as a way to achieve additional muscle damage and thus muscle growth. But it is now clear that this training method can even lead to less muscle growth, because overly slow reps come at the expense of the weight and/or the total number of reps you can do.

Controlled, slightly slow reps are fine, but don’t let them last longer than a few seconds.


Mechanical tension is the amount of muscle tension a muscle must deliver when it has to resist dumbells, barbells, or other heavy weights. In short, mechanical stress is the result of resistance to gravity. The right amount of mechanical tension provides a growth stimulus: it ‘triggers’ muscle protein synthesis.


Muscles have receptors called mechanosensors that detect how much force is being applied to the muscle. Once these sensors detect voltage, it can eventually lead to muscle growth through a chain of chemical reactions.

Mechanical tension is experienced by muscle fibers individually, not by the muscle as a whole. As soon as a muscle fiber detects mechanical tension, it produces the necessary force. Because the muscle as a whole is not stimulated, some parts of a muscle can increase in size more than others with muscle growth.


Long ago it was thought that muscle growth is only (optimally) possible by training with heavy weights. After all, with heavy weights, the muscle detects the greatest mechanical tension, was the thought. We now know that you can also achieve maximum muscle growth with light weights, provided that the weight is at least 30%1RM x ] [ xxxi ] .

Whether you train with heavy or light weights, it is very important that you make enough effort. This means that you train a set until (near) muscle failure. When using lighter weights, you will simply need more reps to get to that point.

The fact that effort (also called relative intensity) is so important is because you have to use as many so-called motor units in a muscle for muscle growth. Each unit is made up of nerves and all the muscle fibers it activates. The heavier the effort, the more motor units make a muscle contract. And so the more muscle fibers are activated.

It is also important to address especially large motor units. The muscle fibers in these units contract slowly and that is what produces the most muscle growth xvii ] . Slow muscle contractions are necessary as a muscle becomes more fatigued in resisting a given weight. This explains why muscle fatigue (during a set) is an important factor for muscle growth xxvii ] .


It is sometimes claimed that training with light weights leads to muscle growth because metabolic stress builds up in the muscle fibers during such a long set. Although the latter is correct (see also below), metabolic stress does not seem to be the main cause of the fatigue required for muscle growth xxvi ] , namely peripheral fatigue. Other processes in the muscle are responsible for this.

Peripheral fatigue is the fatigue in the muscle. This enables us to use more motor units and thereby train more muscle fibers. In addition, you also have central fatigue, the fatigue that arises in the central nervous system. This plays an increasingly important role the longer your training lasts. Also during long sets there is more and more central fatigue. From a certain point, this fatigue takes over from peripheral fatigue. That is also the reason why training with weights lower than 30%1RM is no longer optimal: the point of muscle failure is mainly caused by central and not peripheral fatigue xxviii ] .

In short, mechanical tension is something that a muscle has to ‘experience’ as a result of especially the peripheral fatigue caused by training to (near) muscle failure with a certain weight. In the case of mechanical tension for muscle growth, therefore, it is not so much the absolute weight (absolute intensity) that is important, but the extent to which you fatigue a muscle with a certain weight during a set (relative intensity).


The foregoing also explains why powerlifters are usually less muscular than bodybuilders. After all, they focus in their training on absolute weight and not on muscle fatigue. At the same time, they are usually a lot stronger than bodybuilders.

That seems contradictory, because bigger muscles are associated with more strength. However, you can also become stronger through adjustments in the central nervous system (neural adjustments) and through reinforcements in the body other than muscle growth, such as increasing tendon stiffness. These ways of getting stronger play a much bigger role in powerlifting than in bodybuilding.


If you can achieve maximum muscle growth with both light and heavy weights, in principle it does not matter in which rep range you train. After all, it concerns the repetitions that are heavy, the so-called stimulating, or effective repetitions. Although the repetitions before that often also give some growth stimulus, especially if you do compound exercises.

For muscle growth, it’s mainly about the repetitions that feel the heaviest, regardless of the weight and number of repetitions you use in a set. (Source: YouTube / Radu Antoniu )

And although the rep range you use does not matter in theory , for practical reasons it is best to train in the range of 6 to 20 repetitions. After all, with very long sets, an undesirable amount of central fatigue arises. And in the range of 1-6 repetitions, you train with such heavy weights (> 85%1RM) that it can hinder correct execution and promote injuries. In addition, because of the short sets you may ‘miss’ effective repetitions, which means that you would have to do extra sets.


Muscles are mainly stimulated by the tiring repetitions in a set, the effective repetitions. In order to grow, they need to be “administered” enough of those reps each week. And since a set, if trained to (near) muscle failure, provides an average of no more than five stimulating repetitions, you should do several sets for each muscle group every week.

The number of sets per muscle group per week is also known as the training volume. Training volume is the most important training variable next to training intensity (load and effort).

An average bodybuilder needs between 10 and 20 sets per muscle group per week, with these sets spread over at least two weekly workouts. Training frequency therefore also plays a role, but in particular to ensure that you do not do too many sets during one session. You can do a maximum of about 5-10 sets of productive volume per muscle group.

All this is summarized below: mechanical tension causes muscle growth and the amount of mechanical tension is determined by the effort (relative intensity) and by the training volume. So you can turn two knobs.

Mechanical tension is the main mechanism of muscle growth, and both volume and relative intensity can increase that tension. (Source: Abel Csabai podcast)


Many bodybuilders are instinctively inclined to open both knobs: they train a lot and often (volume) and hard (effort) — the old ‘bro style training’. Unfortunately, that method only works for enhanced bodybuilders.

The fact is that as a natural you will have to adjust your training volume to your intensity. If you train all your sets to absolute muscle failure, you can do significantly fewer sets than if you always keep a few repetitions in the tank. As a result, you create less effective repetitions on balance and you will therefore grow less quickly.


Metabolic stress is the accumulation of metabolites, including lactate (the acid residue of lactic acid), as a result of repetitive muscle contractions.

Metabolic stress is often associated with ‘the pump’, the phenomenon whereby more and more blood, oxygen and nutrients are sent to the trained area. Muscles swell as a result, making you immediately look a lot fuller. The more reps and sets you do, the more the pump will be felt and visible. Short rest periods and special techniques such as supersets and dropsets can also contribute to the pump.

Even if you don’t emphatically chase that pump, there will always be some metabolic stress during bodybuilding training. That’s because you usually do sets of at least six repetitions. Most metabolic stress occurs with sets that last between 20 seconds and 2 minutes.


Due to the presence of metabolic stress in hypertrophy training, it is often assumed that it contributes to muscle growth. So that metabolic stress, like mechanical stress, is a mechanism that triggers muscle growth, as posited ten years ago by Brad Schoenfeld.

The view has now changed. More and more coaches think that muscle growth is mainly about mechanical tension and that metabolic stress is rather a by-product of strength training, just like muscle damage.

In fact, metabolic stress comes at the expense of mechanical stress, so it’s best to keep that stress as limited as possible. At least that seems to be the conclusion if we look at studies into rest times between sets, for example. Short rest periods cause more metabolic stress. But according to several studies, for optimal muscle growth, you should rest a little longer between sets – two to three minutes instead of just one xiii ] . If you rest a little longer, you can use more reps and/or more weight in your next set, which is apparently more important for muscle growth than the extra pump you create with short breaks.


Metabolic stress is also often claimed to increase the acute hormonal response, i.e. the increase in testosterone levels during and immediately after exercise. While this is true to some extent, there is no scientific evidence to date that this also leads to increased muscle growth xiv ] .


Yet we should not write off metabolic stress as a mechanism for muscle growth. Although Brad Schoenfeld has now also acknowledged that the role of metabolic stress in hypertrophy training has probably been thought to be smaller, that stress does seem to trigger ‘something’. In an October 2018 review article, Schoenfeld writes:

Interventions that combine blood flow restriction and especially low load resistance exercise suggest that resistance exercise-regulated metabolites could be hypertrophy stimuli but this is based on indirect evidence and metabolite candidates are poorly characterized. i ]

Six months later, that ‘something’ still seems to be taking shape. This is because metabolic stress could affect a specific form of muscle growth. There are two types of hypertrophy: myofibrillar and sarcoplasmic hypertrophy. And it is possible that metabolic stress plays a major(er) role in the second type.

In myofibrillar hypertrophy, the muscle grows because the number of myofibrils increases. Myofibrils are supposedly the building blocks of muscle fibers. An increase in the number of myofibrils therefore causes a muscle fiber to grow larger. Myofibrils, in turn, are made up of contractile elements (sarcomeres), which allow the muscle to contract. An increase in the number of contractile elements is mainly associated with an increase in maximal strength and training intensity.

In sarcoplasmic hypertrophy, muscle growth occurs due to an increase in the sarcoplasm, the sheath of the muscle fibers. This form of hypertrophy is mainly associated with an increase in strength endurance and training volume. The sarcoplasm contains energy stores, in the form of glycogen, which increase with sarcoplasmic hypertrophy. The sarcoplasm contains so-called non-contractile proteins, which means that they do not directly contribute to force production. So sarcoplasmic hypertrophy doesn’t make you stronger in terms of absolute weight.

Sarcoplasmic and myofibrillar hypertrophy. The third, most common possibility is missing from this image, namely the growth of both the sarcoplasm and the number of myofibrils. (Source: Zatsiorky/Science and Practice of Strength training via Lyle McDonald)

There is still a lot of uncertainty and therefore controversy about these two forms of muscle growth. It is often thought that they occur simultaneously and that you cannot train specifically for one type of muscle growth. The sarcoplasmic hypertrophy would then be mainly intended to create space for larger muscle fibers, as a result of the myofibrillar hypertrophy. Others claim that you can train specifically for a certain type of muscle growth. Myofibrillar hypertrophy would mainly occur with a lot of mechanical stress, sarcoplasmic hypertrophy especially with a lot of metabolic stress.

The latter theory is gradually receiving scientific support, such as in a (not yet officially published) study by Haun et al. from April 2019 xiv ]. The scientists examined the effects on muscle growth of a six-week training program with a relatively high volume and low intensity on young, experienced strength athletes. To be precise: many repetitions with fairly low weights (60%1RM) and a fairly low relative intensity (average 4 RIR). After the six weeks of training, a muscle biopsy was taken to determine exactly how the muscle growth had taken place. What turned out? The size of the muscles had increased, but not due to the growth of myofibrils. Thus, the training regimen did not result in myofibrillar, but only sarcoplasmic hypertrophy. Due to the low intensity, the myofibrils were apparently not stimulated enough to allow them to grow, but the great metabolic stress still resulted in muscle growth due to the increase in the sarcoplasm.

A quite revolutionary discovery, Brad Schoenfeld also thought, who in the aforementioned review article spoke of a lack of direct evidence for the effects of metabolic stress on muscle growth. Haun et al.’s research appears to be the first convincing direct evidence. And also a first proof that sarcoplasmic hypertrophy can also occur without myofibrillar hypertrophy taking place. In response to the investigation, Schoenfeld tweeted:

If you had asked me several months ago, I would have dismissed that “sarcoplasmic hypertrophy” occurred to any meaningful extent in the absence of contractile growth. This study has me reconsidering my opinion. xxv ]

Whether training with very heavy weights and few repetitions, in other words, powerlifting style, then stimulates ‘extra’ myofibrillar hypertrophy, is as yet unclear.

The study was also fodder for author Lyle McDonald, who devoted a hefty piece on his site. His tentative conclusion is that for maximum muscle growth, bodybuilders should also do some training in a more metabolic style, in addition to training in the traditional, more mechanically oriented style. In short, go for that pump every now and then. This can be done by combining these styles in one training program, or by alternating them in blocks (meso cycles).

Coach Mike Israetel also sees metabolic strength training as something you could add to your ‘regular’ strength training (mechanical strength training, so to speak) in order to achieve extra muscle growth in a different way xv ] . Think of doing supersets, circuits and sets with an absurd amount of repetitions. Because of the great acute fatigue that these types of exercises cause, they are best done at the end of your workout. Why people also speak of metabolic finishers xvi ] .


While the importance of metabolic strength training for muscle growth seems relatively limited for the time being, this form of strength training is deadly effective for another aspect of body composition: fat burning. For that reason, bodybuilders sometimes add metabolic finishers to their training during the cut, instead of cardiovascular training. The advantage is the rapid fat burning – comparable to HIIT –, the disadvantage is the high fatigue factor, which could hinder recovery from regular strength training in the event of a calorie deficit. As a bodybuilder, it is therefore better to limit your metabolic strength training.

Other gym target groups often benefit from predominantly metabolic strength training. For example, many women pursue goals such as losing fat while “tightening” at the same time. For them, metabolic group training forms such as body pump are suitable: vigorous semi-cardiovascular workouts in which a lot of fat is burned and at the same time the muscles are trained in a metabolic way, making them bigger and stronger.


Muscle growth is a complex process about which we still know relatively little. We do know that there are multiple mechanisms and signaling pathways on the training side, so there are probably multiple ways to create growth stimuli.


Of the three putative mechanisms for muscle growth (mechanical stress, muscle damage and metabolic stress), mechanical stress appears to be by far the most important. Mechanical tension is not so much about the absolute weight, but about the muscle fatigue (peripheral fatigue) that you create with that weight during a set.

Your muscle experiences this fatigue especially during the ‘effective repetitions’, the roughly five repetitions before muscle failure, regardless of how many repetitions the total set has. You can therefore also achieve optimal mechanical tension with lighter weights, provided you train a set until (near) muscle failure and provided the weight is not lighter than 30%1RM. After all, when weights are too light, the central fatigue takes over from the peripheral. With very heavy weights (> 85%1RM) you can do too few repetitions and therefore also too few simulating repetitions. Hence, for muscle growth it is best to train with medium heavy weights (i.e. 60-85%1RM, 6-15 reps).

To grow, you need to do enough effective reps each week. Therefore, you should not only exert enough effort during your sets, but also do several sets per muscle group. An average bodybuilder needs between 10 and 20 sets per muscle group per week, with these sets spread over at least two weekly workouts. This is based on sets that are trained to near muscle failure, but not to complete muscle failure.


Metabolic stress appears to be much less important for muscle growth than mechanical stress. Doing several repetitions in a row automatically creates some metabolic stress, but it is the mechanical tension that seems to be the deciding factor. And too much metabolic stress hinders the creation of mechanical stress, which can even be at the expense of muscle growth. That is why you should not rest too short between sets, for example.

However, metabolic stress in itself may also cause muscle growth, especially sarcoplasmic muscle growth. That is why it is recommended to do some metabolic strength training in addition to the regular, mechanically oriented strength training, for example by means of supersets or sets with a lot of repetitions. The relative intensity is less important here. Only do metabolic exercises at the end of your workout or during a separate workout.


Muscle damage, after all, is often a consequence, a by-product, of training for muscle growth. But it doesn’t seem to be a prerequisite for muscle growth and so isn’t actually a mechanism of muscle growth.


We distil the most important practical guidelines from this load of theory.

  • When training for muscle growth, the weight and rep range you use in an exercise are not that important in theory, but for practical reasons it is best to use medium to heavy weights (60-85%1RM) for most of your training and so you train in the range of 6 to 15 repetitions.
  • For muscle growth it is important that you put enough effort into a set and train it to near muscle failure or possibly to complete muscle failure. The roughly five reps before muscle failure are the toughest: they create the mechanical stress necessary for muscle growth. Training for muscle growth is therefore mainly about the ‘effective repetitions’, regardless of the total number of repetitions you do.
  • To create enough effective reps for muscle growth, you need to do several sets per muscle group. An average bodybuilder needs 10-20 sets per muscle group per week, with a maximum of 5-10 sets per muscle group per workout.
  • To keep growing, you need to gradually and consistently increase the training load: progressive overload. You achieve this by doing more effective repetitions and/or by making those repetitions heavier (by means of a higher weight).
  • When training for muscle growth, some metabolic stress is always released, but in principle you have to limit it: metabolic stress comes at the expense of mechanical tension. For example, do not rest too short between sets. Use 1-2 minutes for isolation exercises, 2-3 minutes for compound exercises.
  • Possibly creating metabolic stress (the pump) in itself is also a way to achieve a growth stimulus, although it offers much less muscle growth potential than mechanical tension. Through metabolic stress you would especially stimulate sarcoplasmic muscle growth. For that reason, it may make sense to train a small part (10-20%) of your training program in higher rep ranges, ie with sets of 15-30 repetitions. This may be combined with shorter rest times and/or intensity techniques such as drop sets, super sets and pre-fatigue.


Arnold Schwarzenegger once said that you can compare the feeling of ‘the pump’ to that of an orgasm. While we don’t want to deprive you of that feeling, muscle pump isn’t something you should initially pursue during your workout. The point is that you sufficiently challenge a muscle during a set with a certain weight, regardless of whether it requires many or few repetitions. What does not mean is that metabolic strength training, ‘pump training’ so to speak, can be a nice extra in your training – as a final chord, or, for example, as one separate training in the contraction.

Also keep in mind that the whole wall of text above is irrelevant if you’re not making progress in your training. Sounds logical, but why do we rarely see people writing something down on a piece of paper in our gym? Or are we old-fashioned and do they do that on their phones? The fact is that you have to know your hard numbers to make progress. Mike Israel on this:

Nothing beats numbers. Whatever you program, it should be making you stronger in all the rep ranges you train over time. If you’re getting stronger, fundamentally, you are succeeding. xix ]

So, speaking of good resolutions:

This year: write shit down when you train. It works! xix ]

Random training by feel only works with absolute beginners and with bodybuilders on gear. Natural bodybuilding requires a rather pragmatic (and for some perhaps even a bit boring) approach.


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