Building muscle: what actually counts – Knowledge

1. Why this article?

There is one figure almost nobody knows and almost everybody is affected by. It is: one week.

That is how long it takes for a healthy person lying flat in bed to lose a measurable part of their leg muscle. Not a year. Not six months. One week.

The way back takes considerably longer. And in older age it may no longer be complete.

That is what this article is about. It deals with building muscle – in technical language hypertrophy, literally «excess nourishment», meaning simply: the muscle gets thicker. Not «more muscles», but bigger ones. Humans essentially do not form new muscle fibres; the existing ones grow.

We are writing this piece because building muscle is an underrated subject in physiotherapy. In the clinic we mostly talk about strength, mobility and pain. Muscle mass is treated as a concern of young people in the gym. That is a misunderstanding with consequences, because muscle mass is the reserve you draw on when something goes wrong: an operation, pneumonia, three weeks of influenza, a fall. Anyone who enters such a situation with little muscle comes out of it worse.

What you will find here:

• How much muscle a week of bed rest costs – and how long it takes to get it back.
• What actually happens inside a muscle when it grows. Including the old question of whether there are «two kinds» of muscle growth: one that makes you strong and one that only makes you big.
• Why the famous «8 to 12 repetitions» no longer works as the sole prescription – and what range applies today.
• What «repetitions in reserve» are, why this is the most useful measure in strength training, and how close to the end you have to go if your goal is muscle.
• And at length: how to choose exercises. This is the part that gets written about least and goes wrong most often in practice.

How this relates to our general article. This site already carries a detailed guide to strength training. That one covers what strength training achieves overall, how often to train, whether machines or free weights are better, what applies to children and older people, and how safe the whole thing is. This article assumes all of that and pursues a single question: how does the muscle get bigger? If a term is missing, you will find it explained in the general article – the most important ones are repeated just below.

The text is written for patients and interested readers without a background in exercise science. Technical terms do appear, because you will meet them in training plans and on the internet – but each one is explained the first time it comes up. The numbers in square brackets refer to the reference list at the very bottom.

And the usual, important note: this article does not replace an examination. What your doctor decides for your situation takes precedence.

1.1 Six words you will need

So that the rest stays readable, here are the terms that recur throughout:

• Repetition. One complete movement. Down into a squat and back up once: one repetition.
• Set. A series of repetitions with no pause in between. Ten times down and up, then stopping: one set of ten repetitions.
• Maximum load. The heaviest weight you could move once with good form in a given exercise. All percentages in this text refer to it. The technical term is «1RM», for one repetition maximum.
• Momentary muscular failure. The point in a set at which literally no further repetition succeeds. The weight stops halfway even though you are pushing with everything you have. That is something other than «it burns» or «it is unpleasant».
• Repetitions in reserve. How many repetitions you could still have completed at the end of a set. Abbreviated RIR. Two in reserve means: you could have done two more, and the third would have failed.
• Volume. The amount of training, usually counted as the number of sets per muscle group per week. Two leg press sessions a week with three sets each means six weekly sets for the front of the thigh.

2. What a week in bed costs – and how long the way back takes

2.1 The numbers from the bed-rest laboratory

How quickly muscle disappears is known surprisingly precisely, thanks to a particular kind of study: experimental bed rest. Volunteers – usually healthy people – lie in bed for a set number of days and get up only to use the toilet. Muscle mass and strength are measured before and afterwards. It sounds drastic, but it is the only way to measure the loss cleanly, without an illness distorting the results.

The most important findings, all obtained in healthy people:

• One week of strict bed rest, young men. Ten healthy young men spent seven days in bed. The result: 1.4 kilograms less fat-free mass and a 3.2 per cent reduction in the cross-sectional area of the thigh muscle [19]. Along the way, insulin sensitivity fell by 29 per cent – so sugar metabolism was thrown off within a single week.
• Five days of one immobilised leg, young men. Here only one leg was placed in a brace, the other serving as comparison. After five days the thigh muscle cross-section was 3.5 per cent smaller and strength 9 per cent lower. After fourteen days it was 8.4 per cent less muscle and 22.9 per cent less strength [20].
• Ten days of bed rest, older people. Twelve healthy, moderately active people with a mean age of 67 lost around 0.95 kilograms of fat-free mass in the legs alone over ten days [21]. For comparison: in young people this loss is several times smaller over the same period.
• The summary of all bed-rest studies in older people. One review pooled 25 investigations with 118 healthy older volunteers who spent between five and fourteen days in bed. On average around 1.5 kilograms of fat-free mass were lost. The loss of leg muscle was considerably greater in the older participants, at 0.86 kilograms, than in younger ones at 0.24 kilograms [22].

How should these numbers be read? Three qualifications are needed, otherwise they mislead.

First: «fat-free mass» is not the same as muscle. What is measured is everything in the body that is not fat – so water, connective tissue and organs as well. Part of the drop in the first days is water that the body excretes while lying down. This is why the measurements of muscle cross-section (the 3.2 and 3.5 per cent) carry more weight than the figures in kilograms.

Second: these numbers come from healthy people who ate adequately. Someone in hospital additionally has inflammation in the body, eats less and often barely moves even after discharge – there the loss is greater. What exactly happens during a hospital stay, and what helps against it, is covered in our article Leaving hospital weaker.

Third: the point is not that «bed rest is bad». Sometimes lying down is medically necessary. The point is to know the scale – and to know that every day counts on which someone gets up, sits, takes a few steps, or even just presses their legs against a resistance in bed.

2.2 Strength falls faster than mass

Compare the two pairs of numbers in the immobilised-leg study: after fourteen days the muscle was 8.4 per cent smaller – but strength was 22.9 per cent lower [20]. Strength therefore fell almost three times as far as muscle mass.

This is not a measurement error but an important finding. Strength depends not only on how much muscle is there, but also on how well the nervous system calls it up. Immobilisation costs both – and the neural side is lost faster than the mass.

For practice this has an encouraging flip side: the part that is lost quickly also returns quickly. Neural drive recovers within weeks. That is why people often feel considerably stronger two to four weeks after an illness, even though the muscle is nowhere near its former thickness.

The discouraging side: the part that is lost slowly – the mass – also returns slowly. That is the subject of the next section.

2.3 The way back: what is possible in 1, 4, 8 and 12 weeks

A disappointment first, so that the numbers afterwards are placed correctly.

In the first week of training, nothing happens in the muscle that could be called growth. A Brazilian–Canadian research group examined this with particular care. In the same individuals they measured, at three time points – after the first training session, after three weeks and after ten weeks – how much muscle protein was being newly built and how much the muscle fibres had actually grown [16]. The result: in the first days protein synthesis shoots up, but it has nothing to do with growth – it is repairing the damage from unaccustomed loading. A measurable increase in fibre thickness appeared only after ten weeks.

This incidentally explains a familiar phenomenon: after the first few sessions the muscle feels fuller and the upper arm even measures more. That is swelling – fluid stored in the tissue. It goes away again. Anyone measuring in this phase is measuring water.

And now the realistic magnitudes:

• After 1 week: nothing measurable in the muscle. What improves is neural drive – you manage more repetitions with the same weight, and the movement feels safer. That is real progress, but it is not muscle growth.
• After 4 weeks: measurably little. As a rough guide, the pooled literature suggests an increase in muscle cross-section of about 0.1 to 0.2 per cent per training day in the early phase [24]. With three sessions a week that comes to roughly 1 to 2 per cent after four weeks – a figure nobody sees in the mirror and no tape measure reliably captures.
• After 8 to 12 weeks: this is where it becomes visible and measurable. In well-controlled studies, typical increases in thigh or upper-arm cross-section after two to three months lie in the region of 3 to 10 per cent. In a summary of 49 studies with 1328 older participants, the mean gain in whole-body fat-free mass was 1.1 kilograms, in programmes lasting on average a little over four months [25]. That figure comes with a range of 0.9 to 1.2 kilograms, which specialists call the confidence interval: the range within which the true value very probably lies on these data. The narrower it is, the more certain the figure.

Set that against the loss. One week of strict bed rest costs a young person about 3 per cent of thigh cross-section [19]. Regaining those 3 per cent with decent training takes, roughly estimated, four to eight weeks. The ratio is therefore about one to five: one day in bed costs what around five days of training bring back.

This rule of thumb is deliberately crude. It is not a formula but an order of magnitude – and it is the best reason we know not to stop entirely during a period of illness.

2.4 Why the way back is no longer complete in older age

Now the number one writes reluctantly.

A Danish research group put nine older men aged 61 to 74 and eleven young men aged 21 to 27 through the same programme: two weeks with one leg immobilised, followed by four weeks of retraining [23]. Both groups lost strength through the immobilisation. Both regained it through training.

The difference lay in the muscle itself: over the four weeks of training, the older men rebuilt considerably less thigh volume than the young ones. In them, immobilisation had hit the neural drive harder, and their capacity to bring muscle mass back was smaller.

What that means – and what it does not mean:

• It means: in older people every avoided day of bed rest is worth more than in younger ones, because the way back is more expensive. And: four weeks of rehabilitation after a two-week immobilisation is often too short in older age.
• It does not mean that older people cannot build muscle. They can – demonstrably, and to an impressive degree (see section 10). It simply takes longer and requires more patience than the brochures promise.

The message of this whole chapter fits into one sentence: losing muscle is fast and happens by itself. Building muscle is slow and happens only on purpose.

3. What «building muscle» actually means

3.1 The muscle from the inside

To follow the next sections, a brief look inside is needed. From the outside in:

• A muscle – say the large thigh muscle – consists of thousands of muscle fibres. A fibre is a single, very long cell; some run the whole length of the muscle.
• Inside every fibre, tightly packed, lie the myofibrils. These are thread-like bundles, and they are the part that generates force. Picture them as ropes running lengthwise through the cell.
• Between and around the myofibrils lies the sarcoplasm. This is the cell fluid with everything floating in it: water, stored sugar (glycogen), minerals, the cell's power plants (mitochondria) and a channel system that distributes the signal to contract. The sarcoplasm generates no force itself, but nothing works without it.
• Every fibre has several hundred to several thousand cell nuclei. That is unusual – most cells in the body have one. Each nucleus supplies one section of the fibre with building instructions. When a fibre grows substantially, additional nuclei are added; they come from reserve cells sitting on the outside of the fibre, known technically as satellite cells [15].

Building muscle therefore means: the individual fibres get thicker. In humans the number of fibres stays essentially the same – you do not get new muscles, you get bigger ones.

3.2 Myofibrillar and sarcoplasmic hypertrophy

Now to the distinction that appears in every fitness forum and that specialists have been arguing about for fifty years.

The idea is simple. A fibre can get thicker in two ways:

• Myofibrillar hypertrophy. Additional force-generating filaments are added – more myofibrils, more densely packed. The muscle gets thicker and can produce more force. This is the growth everybody pictures.
• Sarcoplasmic hypertrophy. It is not the filaments that increase but the space between them: more fluid, more glycogen, more cell components that generate no force themselves. The fibre gets thicker without force growing to the same extent.

Where does the idea come from? Originally from comparing bodybuilders with weightlifters. Bodybuilders traditionally train with moderate loads, many repetitions and short rests; weightlifters with very heavy loads, few repetitions and long rests. Tissue samples showed that bodybuilders have more connective tissue and more stored sugar in the muscle relative to their muscle mass – and that their strength is not as great as their muscle mass would suggest [11]. From this arose the story of a «second kind» of muscle growth, often dismissed as «non-functional» or «shop-window» muscle.

And where do things stand today? Honestly: undecided. A detailed 2020 review posed the question in its own title – whether sarcoplasmic hypertrophy is a «scientific unicorn» or a genuine adaptation to training [12]. The authors' answer is cautious:

• There is some evidence that the fluid portion of the fibre expands more than the filament portion – particularly with high-volume training, that is, many sets.
• Other investigations do not find this.
• And for many observations there are simpler explanations. One is purely a matter of measurement: when a fibre gets thicker, the concentration of filaments in a tissue section can fall even though their absolute amount has increased – the filaments are simply spread over a larger volume [13]. What looks like «dilution» can be nothing more than remodelling in progress.

A third possibility described in the same review is particularly instructive: it may be that filaments are first packed more densely without the fibre becoming measurably thicker – and that the fibre only grows afterwards [12]. Strength and thickness would then increase out of step, and the relationship would be nothing like as simple as two boxes suggest.

3.3 What this gives you in practice – answered honestly

Very little. And that is the most important statement in this chapter.

Three points:

First, there is no such thing as «sarcoplasmic training». You will find training plans online promising one or the other – a «myofibrillar phase», a «sarcoplasmic phase». There is no sound basis for this. Nobody can determine from the outside which part of a person's fibre is growing, and nobody has shown that a particular plan selectively produces one or the other.

Second, «sarcoplasmic» is not a term of abuse. More stored sugar in the muscle means more endurance for that muscle. More power plants mean better metabolism. A bigger cell is a cell with more reserve. For people getting back on their feet after an illness that is exactly right – and nobody has ever shown such a gain to be «worth less».

Third, in practice something else concerns you anyway. Not «which part of my fibre grew», but: can I carry the shopping? Can I get out of a chair without using the armrests? Can I manage the walk? No tissue sample answers those questions – a test in everyday life does.

We nevertheless treat the topic at length, because it is one of the most common questions and because a great deal of confident nonsense about it circulates online. The defensible short version is: there are indications that muscle fibres do not always grow uniformly. No training recommendation currently follows from that.

3.4 What triggers growth in the first place

That leaves the question of what the muscle actually responds to. For a long time three triggers were named: mechanical tension (the force acting on the fibre), metabolic stress (the burning sensation as by-products accumulate) and muscle damage (the small injuries that produce soreness) [11].

That threefold division has shifted. On current understanding, mechanical tension is the actual trigger [14]. Inside the muscle fibre sit proteins that respond to pull and deformation and then set off a chain of signals which ends in more protein being built. The muscle, in other words, «senses» how hard it is being pulled on.

The other two triggers have not disappeared, but they have been demoted to companions: metabolic stress probably contributes to the recruitment of the large, powerful fibres towards the end of a hard set – and those are the ones that grow most. Muscle damage, by contrast, is not a goal. Soreness is not proof of a good session; it usually only signals that something was unfamiliar. It disappears after a few weeks of the same training while growth continues.

From this follows something important that sounds like a contradiction: tension does not arise only from heavy weight. A light weight also produces high tension in individual fibres – namely once the set is taken far enough and fatigue forces the body to recruit more and more fibres at once. That is precisely why the next chapter turns out as it does.

4. Does more muscle automatically make you stronger?

This is one of the more interesting open questions in exercise science, and it matters for patients too – because it determines whether building muscle is the right goal at all.

Common sense says: obviously, a bigger muscle is a stronger muscle. The data are less settled. In 2019 two research groups published opposing positions in the same issue of the same journal:

• One group argued that changes in muscle size do not contribute to training-induced strength gains [17]. Their main argument: strength often rises markedly without the muscle growing measurably – and whoever gains the most muscle is not the one who gains the most strength.
• The other group countered that muscle growth is indeed a contributory cause of strength gain [18]. Their main argument: «contributory» does not mean «solely responsible». That strength can rise without growth does not refute that growth helps.

The second position fits practice better, but the debate has a lasting value: it cleared away the notion that strength and muscle thickness are the same thing. In the first months the largest part of the strength gain comes from better neural drive – the muscle does not get bigger, it gets called up better. Exactly how that works is described in our general article.

So why bother with muscle mass? Because it does something different from strength:

• It is the reserve. During illness, injury or surgery the body breaks down muscle protein to build something elsewhere. Those who have more last longer.
• It is the ceiling. Neural drive can only be improved so far. Beyond that, muscle mass is what still offers headroom.
• It is a metabolic organ. Muscle takes sugar out of the blood. The insulin finding from the bed-rest study – 29 per cent worse insulin sensitivity after one week [19] – shows how closely these are linked.

In short: strength is what you can do today. Muscle mass is what you are putting aside for later.

5. The repetition count: from a narrow window to a broad band

5.1 Where the «8 to 12» comes from

Anyone who has ever held a training plan knows the scheme. In technical language it is the repetition continuum, and it goes like this:

• 1 to 5 repetitions with very heavy weight → maximal strength
• 8 to 12 repetitions with moderate weight → muscle growth
• 15 or more repetitions with light weight → muscular endurance

The middle range even acquired its own name: the «hypertrophy zone». This scheme has been in textbooks for decades and stood for a long time in official recommendations as well [2].

Where did it come from? From two observations. First, bodybuilders traditionally train in this range – and they visibly succeed. Second, studies from the 1990s showed that moderate sets with short rests briefly flush more growth hormone and testosterone into the blood than other forms of training.

The second argument has largely collapsed. Those short post-exercise hormone spikes have turned out to be a poor predictor of actual muscle growth [2]. The first argument – bodybuilders do it this way – is an observation, not a proof.

5.2 What the comparison of light and heavy loads shows

The question can be tested directly: have one group train heavy and another light, both equally hard, and measure the muscle after some months.

That is what a 2017 meta-analysis summarised – a meta-analysis being a calculation that combines the results of many individual studies into one overall result. It included 21 studies in which people trained to momentary muscular failure with light loads (at most 60 per cent of maximum) and with heavy loads (more than 60 per cent) [3]. The result:

• For muscle growth: no difference. The gap between the groups amounted to 0.03 on a scale specialists call the effect size. That measure compares differences across studies regardless of whether they were recorded in centimetres or kilograms; about 0.2 counts as small, about 0.8 as large. So 0.03 is practically zero. It comes with a narrow confidence interval from −0.16 to 0.22 [2]. This is therefore not merely «no difference found» but «on these data a larger difference is unlikely».
• For maximal strength: a clear advantage for heavy. If you want to be able to move more weight, you have to move more weight [3].

A later analysis comparing three loading zones at once – heavy, moderate and light, 28 studies with 747 participants in total – reaches the same conclusion: for muscle growth the size of the load played no role; for maximal strength, heavy and moderate loads beat light ones [4].

And this does not only apply to beginners. In a study of resistance-trained men, biceps, triceps and thighs grew equally over eight weeks whether training used a weight allowing about ten or about thirty repetitions [2]. A twelve-week study comparing 8 to 12 with 20 to 25 repetitions found the same [2].

The current position stand of the American College of Sports Medicine, which evaluates 137 reviews, likewise records: for muscle growth it made no difference whether training used 30 or 100 per cent of the maximum load [1].

That disposes of the narrow window. In its place comes a band running roughly from five to thirty repetitions.

5.3 The condition without which none of this holds

Now the sentence that online summaries regularly leave out and without which the statement above is simply false:

The broad band applies only if the set is taken far enough.

In every study named, training went to momentary muscular failure or very close to it. That is exactly the explanation: with a light weight, only the smaller, enduring fibres work at first. Only when these fatigue does the body bring in the large, powerful fibres – and those are the ones that grow most. With a heavy weight those fibres are involved from the first repetition.

From this follows the practical rule: the lighter the weight, the further you must take the set.

• With a weight that allows only eight repetitions, you can stop at six and still have achieved a great deal.
• With a weight that allows thirty repetitions, twelve achieve almost nothing. You have to go into the region of twenty-five and beyond.

This is why light training is so often experienced as useless. The weight is not the problem; the problem is that the set ends too early. Pulling the yellow resistance band twelve times and then stopping when thirty would have been possible has not loaded the muscle – it has warmed it up.

5.4 Why percentages mislead

Up to here we have repeatedly worked with percentages of the maximum load. Now the reason you may nonetheless forget them in daily practice.

A large 2024 analysis looked at how many repetitions people actually manage at a given percentage of their maximum. It evaluated 952 tests by 7289 people from 269 studies [5]. It is the largest compilation on the question, and it clears away the old conversion table found in textbooks.

Two findings are decisive for practice:

First, the repetition count depends heavily on the exercise. At 70 per cent of maximum, people manage on average:

• about 14 repetitions on the bench press,
• but about 19 repetitions on the leg press [5].

The old table gives eleven for both. It therefore substantially underestimates leg exercises in particular.

Second, the differences between individuals are large – and the lighter the load, the larger. At 80 per cent of maximum, the usual spread is about 2.5 repetitions around the mean. At 60 per cent it is already 4.4 repetitions [5]. Two people with an identical maximum can therefore easily manage twelve and twenty-five repetitions respectively at 60 per cent.

The consequence is unavoidable: a percentage does not tell you how hard a set will be for you. It is an average across thousands of people and fits you only by chance. What you need is a measure that describes your effort in the moment – and that is the subject of the next chapter.

5.5 Where the band ends

So that «it hardly matters» does not become «it does not matter at all», here are the edges:

• Below about five repetitions things become unfavourable for muscle growth. Not because the stimulus is missing, but because the total amount per set is small and joints, tendons and the nervous system are heavily taxed. For maximal strength this range is right; for muscle growth it is cumbersome.
• Above about thirty repetitions it becomes impractical. The set takes a long time, the burning is for most people more unpleasant than the effort at a heavier load, and the circulation often fatigues before the muscle does.
• The most comfortable range for most people lies between 6 and 20 repetitions. Not because it is more effective, but because it is practical: short enough to see through, long enough to keep the movement clean.

Why this is good news for physiotherapy. A great many people cannot or will not lift heavy: because of a painful shoulder, raised blood pressure, a recent operation, fear of injury, or simply because there are no heavy weights at home. For all of them: lighter works too. The set just takes longer.

6. How close to the end? Repetitions in reserve

6.1 What is meant

Instead of calculating in advance how heavy the weight should be, ask yourself a single question at the end of each set:

«How many repetitions could I still have managed with good form?»

Say you have done twelve repetitions on the leg press. Your sense is: a thirteenth and a fourteenth would have gone, at the fifteenth I would not have got it up. Then you have two repetitions in reserve. Abbreviated: 2 RIR.

Zero repetitions in reserve means nothing more was possible – that is momentary muscular failure. Four in reserve means it was noticeably hard but clearly short of the end.

The advantage over percentages is obvious. The reserve measures what actually counts – your effort today, on this machine, on this amount of sleep. It does not go out of date as you get stronger. And you need no maximum-strength test, which is unsuitable for many people anyway. How reliably people can judge their reserve is covered at length in the general article; in short: surprisingly well, with a typical error of under one repetition, and the first set of a session is almost always chosen too light.

6.2 How close to the end – when the goal is muscle?

And here it gets interesting, because the answer differs for muscle growth and for strength. In order.

The analysis that examined the relationship directly. In 2024 a group from the USA, Australia and New Zealand pooled all studies in which it was known how many repetitions participants had left in reserve at the end of a set – 55 studies in all. Instead of comparing only two groups, they fitted a curve across the whole range [6]. The result in two sentences:

• For muscle growth the effect gets larger the closer sets are taken to failure. No threshold, but a steady increase: a set with one repetition in reserve did more than one with four.
• For maximal strength it was different. There, across a wide range of reserve, no relationship could be shown – you get similarly strong whether you go near the end or stop well short of it.

And now the counter-voices, because the matter is not settled:

• A 2023 meta-analysis of 15 studies found only a trivial advantage for training to failure over training just short of it – and no advantage for going to the absolute end over stopping one repetition earlier [7]. The authors note that the relationship is probably not a straight line.
• An eight-week study in resistance-trained people compared training to momentary muscular failure with training at one to two repetitions in reserve. Muscle growth was the same [8].
• An older Spanish study had 42 men train for sixteen weeks either to failure or short of it. Both groups became equally strong; the group that did not go to failure even did better on power measures [9].
• And the ACSM position stand, evaluating 137 reviews, concludes that training to momentary muscular failure shows no consistent advantage for strength, muscle mass or muscle power – and explicitly advises older people against it [1].

How does that fit together? Quite well, if read carefully. The curve analysis [6] does not say «you must go to failure». It says the difference between «four left» and «one left» is real. The difference between «one left» and «none left» is small – and that is exactly where the studies finding no difference sit.

The authors of that analysis themselves write that it is exploratory, that is, hypothesis-generating rather than conclusive [6]. One reason is concrete: in most studies the reserve was estimated, not measured. The curve therefore rests in part on self-assessment.

6.3 What we make of this in practice

We recommend different target ranges depending on who is training and for what:

• If muscle growth is the goal and nothing argues against it: 0 to 2 repetitions in reserve. So close to the end, but not necessarily to the last tremor. This is the range in which the effect is currently largest without fatigue taking over.
• If maximal strength is the goal: 2 to 4 repetitions in reserve, with heavier loads. Going closer to the end demonstrably adds nothing here [6] but costs recovery.
• For older people and in rehabilitation: 2 to 3 repetitions in reserve. This is a deliberate departure from the calculated optimum, and it has reasons: at maximal effort blood pressure rises steeply, and someone who is completely exhausted no longer performs the movement cleanly [1]. The small loss of effect is worth the safety margin.
• On the last set of an exercise you may go closer to the end than on the first – there is nothing left afterwards that fatigue could spoil.

One point that often gets lost: going to momentary muscular failure has a price that does not appear in the growth calculation. It fatigues more, makes the following sets worse, lengthens recovery before the next session, and is unpleasant. If the same effect is available with one repetition to spare, the spare repetition is the better choice.

And quite practically: anyone who cannot judge where their own end lies should try it once – supervised, and on a safe exercise such as the leg press. That single calibration makes every subsequent estimate usable.

7. How much in total: the number of sets

After effort, the second big lever is amount. It is counted as sets per muscle group per week – across all sessions and all exercises that load the same muscle.

The most thorough analysis to date appeared in 2026. It pools 67 studies with 2058 participants and calculates not just «much versus little» but a curve across the whole range [10]. Two findings:

• More sets lead to more muscle growth – over a surprisingly wide range. The curve flattens, but it does not turn downwards.
• How you distribute the sets across the week hardly matters for muscle growth. Whether you do twelve weekly sets on one day or spread them over three makes no demonstrable difference to growth. For strength, by contrast, frequency did matter [10].

One methodological refinement of that work is practically important: the authors distinguished sets according to whether an exercise loads the measured muscle directly or only incidentally. A set of biceps curls counts fully for the biceps; a set of pull-ups counts only partly, because the back does most of the work. This distinction clearly improved prediction [10] – and it explains why «twenty sets a week» can mean entirely different things in different plans.

What does that mean for you?

• Effective lower limit: about 4 sets per muscle group per week. For example two sessions with two sets of leg press each. That maintains what you have and builds slowly.
• Sensible range for growth: about 10 to 20 sets per muscle group per week. For most people in physiotherapy, ten is already a lot – and more than enough to make progress.
• More is possible but expensive. Every additional set does less than the one before it while costing the same time and the same recovery. Somewhere the arithmetic turns bad – but that point is individual and cannot be calculated.
• Increase the amount slowly. Jumping from four to twenty sets means three days of soreness and quitting after two weeks. Two extra sets every two to three weeks is a pace that lasts.

8. Choosing the exercises

This is the longest section of the article, for one reason: much is written about sets, repetitions and loads, little about the choice of exercises – yet that is what you decide in every single session.

8.1 What actually defines an exercise

An exercise is more than a name. Four properties decide what it does to the muscle:

• Which muscles work at all. A squat loads the front of the thigh, the glutes and the back extensors; a leg extension only the front of the thigh.
• At what length the muscle works. A muscle can be loaded in a lengthened or a shortened state – and, as will become clear, that makes a large difference.
• Where in the movement it is hardest. In some exercises the beginning is the hardest part, in others the end. Specialists call this the resistance profile.
• What fatigues first. In a heavy barbell squat, the trunk and breathing often give out before the thighs are finished. The target muscle then does not get the stimulus intended for it.

That last point is the most frequent reason to change an exercise in physiotherapy. Not because it is «wrong», but because the muscle in question never reaches its limit.

8.2 Muscle length – the single most important finding

If you remember one thing from this whole section, make it this: a muscle grows more when it is loaded in a lengthened state.

First, what «lengthened» means here. A muscle spans a joint and is drawn long at certain joint positions and pushed together at others. The biceps is long when the arm is extended and taken behind the body; it is short when the hand is at the shoulder. The same exercise performed in a different body position therefore loads the same muscle at a different length.

Several research groups have exploited exactly this and compared the same movement in two body positions. Three particularly clear examples:

• Back of the thigh. A Japanese group had people train leg curls for twelve weeks – one group seated, the other lying prone. The difference: seated, the hip is flexed, which draws the hamstrings long from the start. Result: the seated version produced considerably more muscle growth [27]. Same movement, same machine, different seating position.
• Back of the upper arm. The same group compared elbow extension overhead with elbow extension beside the body. Overhead, the long head of the triceps is lengthened. Result: considerably more growth in the overhead version – even though less weight could be moved in that position [28].
• Calf. Calf raises standing versus calf raises seated. Standing, the knee is extended, which lengthens the large two-headed calf muscle; seated, it is slack. Result: standing, that muscle grew considerably more [29].

Three different muscles, three different research groups, the same direction. For exercise science that is an unusually consistent picture.

What you can do with it, concretely:

• Hamstrings: prefer seated leg curls to lying ones.
• Triceps: include overhead exercises, not only pushdowns.
• Calf: standing, not seated.
• Front of the thigh: go deeper into the squat or leg press rather than using only the top portion – provided the knee tolerates it without pain.
• Chest: exercises in which the arms travel well back, rather than only the front part of the movement.

And the qualifications that belong with it:

• These studies were done in healthy, mostly young and trained people. For someone with an irritated joint, the lengthened position may be precisely the painful one. Then the order is: pain-free first, optimal second.
• The lengthened position produces more soreness, especially at the start. Build up slowly.
• This is about loaded length, not about stretching. Static stretching without resistance builds no muscle.

8.3 Full movement or partial?

Out of the finding above a training fashion has grown: lengthened partial repetitions. The idea is to perform only the lower, lengthened half of a movement – for a biceps exercise, for instance, only from a fully extended arm to halfway, over and over, never coming all the way up.

The idea sounds logical. What do the data say?

• A study in 30 trained people compared both within the same person – one arm trained through the full range, the other only the lengthened portion, for eight weeks. Result: muscle thickness and strength increased equally. The statistical analysis supported equivalence, not merely «no difference found» [30].
• A large trial in 297 participants across fifteen sites over twelve weeks reached the same conclusion: the differences between the two forms were so small that the authors classify them as practically equivalent [31].

That is a pleasingly clear answer to a much-debated question: partial repetitions in the lengthened range are the equal of the full movement – but they are not superior to it either.

For practice this means:

• Normally perform the full movement. It is easier to explain, easier to supervise, and for maximal strength there are indications that it is better.
• Partial movements are a legitimate tool when the full movement is not available – for instance when the last part of knee extension hurts or a shoulder will not go fully overhead. Then the lengthened part is the better half to keep.
• What you should not do: shorten the movement because the weight is too heavy. That is not a chosen partial but a failed full repetition.

8.4 A muscle does not grow evenly everywhere

A finding that is less secure but explains exercise selection: a muscle does not grow uniformly along its length. Specialists speak of regional hypertrophy.

In a study of trained people, two exercises for the front of the thigh – squat and leg extension – were compared and the muscle then measured at several points. Result: the two exercises grew different sections of the same muscle [32]. Neither one alone covered the whole muscle.

A qualification, and an important one. These studies are small, the measurements demanding and the results not always consistent. For bodybuilders, to whom the last percent of shape matters, this may be decisive. For everyone else the message is plainer and still useful:

Two different exercises per muscle group are better than the same one twice – but five different ones are not better than two.

8.5 Compound and accessory exercises

Two terms you will find in every plan:

• Multi-joint exercises (also «compound» exercises) move several joints at once and involve several muscle groups. Examples: squat, leg press, row, bench press, press-up, pull-up.
• Single-joint exercises («isolation» exercises) move one joint and target one muscle. Examples: leg extension, leg curl, biceps curl, calf raise, lateral raise.

The obvious question: do you need both? One study compared untrained men over 10 weeks – one group did only multi-joint exercises, the other added single-joint ones. Both groups gained muscle thickness and strength, and to the same extent. The additional single-joint exercises brought no further gain [33].

That applies to beginners. For more advanced trainees and in special situations it looks different:

• When a particular muscle has fallen behind – the thigh after knee surgery, say – a single-joint exercise reaches it more reliably. On the leg press the healthy leg can help push; on a single-leg extension it cannot.
• When a muscle never reaches its limit in the compound exercises because something else fatigues first, it needs its own exercise. The hamstrings are the classic case: they work during squats but never reach their limit there.
• When a compound exercise is not possible – painful shoulder, recent surgery, balance problems – several single-joint exercises replace it perfectly serviceably.

Our rule: build the programme around two to four multi-joint exercises and add specifically where something is missing. Not the other way round.

8.6 How much variety?

«The muscle has to be surprised again and again» is one of the most durable half-truths in training.

A review of eight studies examined this [34]. Its result can be summarised in three points:

• Some variety helps – mainly because different exercises reach different sections of a muscle (see section 8.4).
• Too much variety harms. If you constantly swap exercises you get better at none of them – and because you read progress off the exercise, you also lose the yardstick by which you could increase.
• Exercises that do the same thing are not variety. Three different chest pressing exercises are not «varied»; they are the same thing three times under different names.

Practically: keep an exercise for at least eight to twelve weeks. That is how long it takes to genuinely get better at it. Then change deliberately – out of boredom, because of pain, because of a new goal – but not on principle.

8.7 Machines, free weights, bands, body weight

This question is covered at length in the general article, so here only what matters for muscle growth.

For growth, the choice of tool is surprisingly secondary. The ACSM paper records that elastic bands, circuit training and training at home also demonstrably increase strength and muscle mass [1]. What counts is the effort in the individual set – not where the resistance comes from.

Two differences are nonetheless practically relevant for growth:

• Bands are hardest at the end. A band is tightest where it is stretched furthest – which is usually when the muscle is already shortened. That is precisely the opposite of what section 8.2 suggests. Bands are effective, but for the lengthened range weights, machines or body weight are the better choice.
• Bands are hard to dose upwards. Beyond a certain strength level the strongest available band no longer gets you into the range of a few repetitions in reserve. Anyone building over months eventually needs something heavier.

8.8 When an exercise hurts

In physiotherapy this is the commonest reason to change an exercise – and two mistakes are made regularly: dropping the exercise with no replacement, or carrying on unchanged with gritted teeth.

The order we work in:

1. Reduce the weight and do more repetitions. Because load is almost arbitrary for muscle growth (section 5.2), this costs nothing. A set of twenty light repetitions can do as much for the muscle as one of eight heavy ones – and it is considerably friendlier to an irritated joint.
2. Adjust the range of movement. Often only one part of the movement hurts. Leaving that part out and using the rest fully is better than stopping.
3. Change the position. The same muscle group can almost always be loaded differently – seated instead of standing, on a machine instead of free, one leg instead of two.
4. Only then replace the exercise. And with one that hits the same muscle – not with «something else».

A word on the pain threshold: in persistent complaints, mild and unchanging pain during the exercise is often acceptable, as long as it settles within a day and the complaints do not increase over the weeks. For fresh injuries, after operations, and where the cause is unclear this does not apply – there the limit is set, not tested.

8.9 A framework that suits most people

To make this concrete – twice a week, about thirty to forty minutes each time, with two to four sets per exercise and 1 to 3 repetitions in reserve:

• One pushing exercise for the legs: leg press, squat, or standing up from a chair with added weight. As deep as is pain-free.
• One exercise for the back of the thigh and the glutes: seated leg curl, stiff-legged deadlift or hip thrust. These muscles come along in the first exercise but never reach their limit there.
• One pulling exercise for the upper body: row or lat pulldown. Trains the back and the elbow flexors at once.
• One pushing exercise for the upper body: bench press, chest press, press-up (against a wall or on the knees is fine) or shoulder press.
• One exercise for the calves, standing.
• Optionally one targeted addition where something is missing: the thigh after knee surgery, shoulder external rotation after a tendon problem, grip strength in sarcopenia.

That is five or six exercises and, over two to four sessions a week, four to eight sets per muscle group. It is deliberately modest – because a programme somebody keeps up is worth more than one that looks perfect and is abandoned in three weeks.

And if you have even less time: keep the first four exercises and do two sets of each. That takes twenty minutes and covers the essentials.

9. Tempo, rests and technique – in brief

These three points are often thought more important than they are.

Tempo. A meta-analysis of eight studies found that for muscle growth it made no difference whether a single repetition lasted half a second or eight seconds [35]. Only at very slow lifting of more than ten seconds per repetition does it become unfavourable – and for an indirect reason: only something that is light anyway can be moved that slowly. The practical rule: lower under control, push or pull briskly, do not use momentum. Number sequences such as «3-1-2-0» in training plans may be ignored.

Rests. For muscle growth the data do not support a clear statement [1]. What practice suggests: very short rests under a minute mean you manage considerably less in the next set – the fatigue carries over. Two minutes is a good default, three for heavy leg exercises.

Technique. For muscle growth, good technique does not mean «pretty» but: the load lands where it is meant to, and the target muscle reaches the end of the set. Swinging the whole trunk during a row shifts the work to the lower back. Lowering the leg press only a third of the way avoids exactly the range in which the muscle gains most.

10. Building muscle in older age

This is the place for the most encouraging study in the whole literature.

In 1994 the New England Journal of Medicine published a study of 100 frail nursing home residents. Their average age was 87. Over ten weeks some of them trained on machines at high effort; the comparisons were nutritional supplementation, both together, and neither [26]. The results:

• Muscle strength: plus 113 per cent in those who trained, against plus 3 per cent in those who did not. Strength therefore more than doubled.
• Walking speed: plus 11.8 per cent, while it declined slightly in the comparison group.
• Stair-climbing power: plus 28.4 per cent.
• Thigh muscle area: plus 2.7 per cent, while it fell by 1.8 per cent in the comparison group.
• Supplementation alone achieved nothing.

Note the ratio of the numbers: 113 per cent more strength for 2.7 per cent more muscle. Most of the effect runs through neural drive rather than muscle growth – but it also shows that even at 87 and in a frail state, muscle is built, and not merely lost more slowly.

The large summary of 49 studies with 1328 older participants puts the gain in fat-free mass at 1.1 kilograms. Two additional findings from it are practical [25]: programmes with more sets achieved more, and the older the participants, the smaller the gain. Both argue for planning more volume in older age and thinking in months rather than weeks.

And how should older people train?

• The load may be light. There are even indications that lighter loads are more favourable for the growth of fast muscle fibres in older people than heavy ones [2] – possibly simply because heavy loads are harder to manage with painful joints.
• Not to momentary muscular failure [1]. Two to three repetitions in reserve.
• Brisk training builds as much muscle as slow training. A meta-analysis of so-called power training in older people – moderate loads moved briskly – found a growth effect of 0.31 against doing nothing, and no difference against classical slow strength training [36]. So training briskly for the sake of everyday function costs nothing in muscle growth.
• Think in months. The impressive numbers above come from ten weeks with near-daily supervision – not from four weeks with a handout.

What muscle weakness in older age actually is, when it counts as a disease and how it is identified, is covered in our article Muscle weakness in older age; what dwindling reserves mean more broadly is covered in Frailty.

11. Protein and supplements

Muscle is made of protein. So building muscle needs protein – the question is only how much and from where.

The definitive analysis pools 49 studies with 1863 participants and compares «training plus a protein supplement» with «training alone» [37]. The results:

• The supplement produced on average 0.3 kilograms more fat-free mass and 2.5 kilograms more maximal strength.
• Above about 1.6 grams of protein per kilogram of body weight per day, more protein brought no further gain. For a person weighing 70 kilograms that is around 112 grams a day.
• The effect was smaller in older people and larger in resistance-trained ones.

What that means in practice. 0.3 kilograms across a whole training programme is little, measured against what the training itself achieves. If you eat a balanced diet and reach 1.6 grams anyway, you need no powder. For comparison: 1.6 grams per kilogram is attainable for most people on an ordinary mixed diet if every main meal includes a protein source – dairy, eggs, meat, fish, pulses, tofu.

Conversely: for anyone who eats too little – common in older age, with loss of appetite and after illness – protein intake is not a side issue but the precondition for training to work at all. That belongs in a conversation with your family doctor, and the solution is food first and a supplement only afterwards.

12. A special case: muscle loss on weight-loss injections

This section is here because the question has become common in the clinic.

The new weight-loss medications – GLP-1 receptor agonists, colloquially «weight-loss injections» – produce substantial weight loss. Part of that loss, however, is not fat but fat-free mass, which includes muscle. A 2026 review summarises what is known [38]: the proportion of fat-free mass in the total weight lost lies, across the studies, in a range that warrants attention – particularly in people who have little muscle mass to begin with.

Two qualifications belong with this. First, some loss of fat-free mass is normal in any weight reduction, including dieting without medication. Second, the question of how far this loss actually impairs function is not yet settled.

What follows is nonetheless unambiguous: anyone losing weight on such a medication should be doing strength training at the same time and paying attention to sufficient protein. This is not an optional extra for well-being; it is the part of the treatment that ensures the weight lost is predominantly fat. Raise it with the doctor prescribing the medication.

13. What you can measure – and what you cannot

Muscle growth is the training effect that is hardest to monitor yourself. So here is what works and what does not.

What does not work:

• The scales. They include water, gut contents and fat. One kilogram of muscle gained over three months disappears entirely into the daily noise of one to two kilograms.
• The mirror. It depends on lighting, daily form and expectation – and you see yourself every day, which makes any change invisible.
• Body-fat scales and handheld devices using electrical current. Their readings fluctuate with how much you have drunk and with the time of day more than the effect you are trying to measure.

What does work:

• The training log. Note three things after every set: weight, repetitions, estimated reserve. If in March you manage twelve repetitions with 40 kilograms at two in reserve and in June twelve with 50 at two in reserve, that is the best evidence of progress obtainable at home.
• The tape measure, used properly. Always at the same marked spot, always at the same time of day, always relaxed, and not more often than every four weeks. Expect small numbers: one centimetre of thigh circumference in three months is a good result.
• Everyday tests. How many times can you stand up from a chair without using your hands in thirty seconds? How long can you hold a single-leg stance? These values are not muscle mass – but they are what you are training for.
• Patience. Measure meaningfully at eight weeks at the earliest, better at twelve.

14. Seven misunderstandings

• «For muscle you have to do 8 to 12 repetitions.» No. The muscle grows equally well from about five to thirty repetitions – as long as the set is taken far enough [3][4].
• «Light weights only build endurance, not muscle.» No – but light weights put down too early really do build nothing. The difference lies not in the weight but in how many repetitions you actually do (section 5.3).
• «Without soreness the session was worthless.» No. Soreness indicates that something was unfamiliar – not that it was effective. It disappears after a few weeks of the same training while growth continues.
• «There is training for real muscle and training for show muscle.» This distinction cannot be supported with the available data. There are indications that muscle fibres can grow in different ways – no training recommendation follows from that (section 3.2).
• «The muscle has to be constantly surprised.» No. Changing too often stops you getting better at an exercise – and with it, your ability to measure and to increase [34].
• «After four weeks you should see something.» No. Before eight to ten weeks, hardly anything is measurable in the muscle itself [16]. What you notice before that is better neural drive – and that is just as valuable.
• «In old age building muscle is pointless.» No. In 87-year-olds in a nursing home, thigh muscle grew measurably in ten weeks and strength doubled [26]. Growth is slower than in youth, but it happens.

15. When you should get in touch

Not everybody needs a professional for strength training. In the following situations supervision is sensible or necessary:

• After an operation or an injury, as long as loading limits apply.
• When a muscle is clearly weaker than on the other side – the thigh after knee surgery, for instance. Two-sided exercises hide this, and the weak side falls further behind.
• With pain that occurs in every variant of the exercise and does not lessen over weeks.
• With unintended loss of weight or strength for no apparent reason – that needs medical assessment first.
• If you see no progress after three months of regular training. Usually it is the effort in the individual set, or the fact that nothing was ever increased – both can be sorted out in an hour.
• With untreated severely raised blood pressure, certain heart conditions, recent fractures or advanced osteoporosis – here the rule is: get it assessed, then adapt, rather than wait.

16. In summary

• Muscle is lost quickly. One week of strict bed rest costs healthy young men around 3 per cent of thigh cross-section and 1.4 kilograms of fat-free mass [19]; in older people the loss in the legs is about three and a half times that in younger ones [22].
• The way back takes several times as long. Before eight to ten weeks of training, hardly anything is measurable in the muscle [16], and older people do not regain everything after immobilisation [23].
• Building muscle means the existing fibres get thicker. Whether it is mainly the force-generating filaments or also the fluid portion that increases is scientifically unresolved – and has no consequences for planning training [12].
• The narrow «hypertrophy window» of 8 to 12 repetitions is out of date. From about 5 to 30 repetitions the muscle grows equally well, provided the set is taken far enough [3][4].
• Percentages of the maximum are no use for steering training. At 70 per cent people manage on average 14 repetitions on the bench press and 19 on the leg press, with large differences between individuals [5].
• Steer by the reserve instead. For muscle growth 0 to 2 repetitions in reserve, for strength 2 to 4, for older people and in rehabilitation 2 to 3.
• Amount matters. Four sets per muscle group per week is the effective lower limit, ten to twenty a good range; how you spread the sets across the week is secondary for growth [10].
• In choosing exercises, muscle length is the biggest lever. Curl seated rather than lying, extend overhead rather than beside the body, raise the calves standing rather than seated – the same finding three times over [27][28][29].
• Two to four multi-joint exercises form the framework, with accessory exercises added deliberately [33]. Keep exercises for at least eight to twelve weeks [34].
• The gain is greatest in old age. In 87-year-olds, strength doubled within ten weeks and the thigh muscle grew [26].

The last word belongs to the ratio of effort to return. Muscle growth is the slowest adaptation the body makes to training. It is invisible over weeks, measurable over months and decisive over years. Knowing that, you do not stop disappointed after six weeks – and in ten years you have a reserve that carries you through an illness.

References

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