During the first three to four weeks, much of what you measure may be swelling. According to a researchers' model, real muscle growth starts after about eighteen sessions. After a few months, controlled studies land on average at a little under a kilo of extra lean mass, with huge differences between people.
Muscles grow faster than you think and slower than you hope. During the first three to four weeks, much of what you see in the mirror or on a scan may be fluid: swelling in a muscle that still has to get used to the load. According to a Brazilian research team, real muscle growth starts after about eighteen sessions.
After a few months, controlled studies land on average at a little under a kilo of extra lean mass. On average, because the spread between people is huge.
That is the short answer. Below you will find what was measured week by week, why you get stronger before you get bigger and why that very gap is the moment many people give up.
In 2011 an American team scanned the thigh of 25 sedentary men every week. After just two sessions that thigh was 3.5 per cent thicker. That sounds like a muscle miracle. The researchers themselves noted that oedema, fluid in the muscle, could have influenced that early figure.
A team in São Paulo set out to find what lay behind that early gain. In ten untrained young men the muscle was 2.7 per cent thicker in the third week. At the same time damage markers in the blood were raised and the ultrasound showed the typical picture of fluid in the tissue. After ten weeks the muscle was 10.4 per cent thicker and the measurement, corrected for the larger muscle, no longer pointed to swelling.
Their conclusion: what you measure in the first weeks may largely be swelling caused by muscle damage. What looks like growth is then mostly a muscle recovering.
Source: DeFreitas et al.1 2011, European Journal of Applied Physiology · Damas et al.2 2016, European Journal of Applied Physiology
Whether your muscles grow also depends on what happens outside the gym: your sleep, your nutrition and whether your sessions actually happen. The free health check shows you in five minutes where your biggest leak is. Take the free health check
The same Brazilian researchers pulled their findings together in a 2018 review. They propose three phases. Up to the fourth session the increase is mostly swelling. Around the tenth session modest growth begins. Only after about eighteen sessions do they speak of real hypertrophy.
That is a model and not a measurement from a single study, so read those numbers as reference points. The American team, which did not measure fluid and only assumed it, concluded that growth already starts around week three to four. The Brazilian team did measure the swelling and is more cautious.
Translated to your diary, eighteen sessions is six weeks at three a week or nine weeks at two a week. That assumes sessions rather than weeks are the clock, which the model assumes without testing it. Anyone looking in the mirror in disappointment after four weeks is measuring at a moment when growth is modest at best.
Source: Damas et al.2 2018, European Journal of Applied Physiology · DeFreitas et al.1 2011, European Journal of Applied Physiology
Strength leads the way. In a Finnish dataset of 287 untrained men and women aged 19 to 78, who trained twice a week for 20 to 24 weeks, strength rose by 21 per cent on average. Muscle size, measured as cross-sectional area, muscle thickness or leg lean mass, grew by 4.8 per cent over the same period.
An important part of that early strength gain comes from your nervous system. A beginner first learns to drive a muscle harder and with better coordination. Measurements of the electrical activity in muscles suggest that this drive increases in the first phase of a programme, often without noticeable growth of the muscle itself.
For anyone who wants to see progress, this is the most useful sentence in the article: your training log shows progress weeks before the mirror does. Anyone who only looks in the mirror is measuring the wrong thing at the wrong moment.
Source: Ahtiainen et al.4 2016, AGE · Gabriel et al.5 2006, Sports Medicine
For a figure in kilos there are two large analyses. The first pooled 111 studies with 1927 healthy men with an average age of 23.5, from untrained to experienced. After programmes of 4 to 24 weeks, about ten weeks on average, they had 1.53 kilos more lean or muscle mass. The confidence interval ran from 1.30 to 1.76 kilos.
Two caveats. That analysis compared before and after within the training groups, without a control group. Lean mass is also more than muscle alone: water and glycogen count too.
The second analysis is more rigorous: 114 randomised trials with 4184 participants with overweight, from children to older adults. Those who only did strength training gained 0.8 kilos more lean mass than those who did not train, with an interval of 0.6 to 1.0 kilos. The trials ran from 4 to 96 weeks. Combining strength training with eating less preserved lean mass while an average of 5.3 kilos of fat disappeared.
Roughly a kilo over the first months, then, as a rough guide. Spread across your whole body, that is invisible from week to week.
Source: Benito et al.6 2020, International Journal of Environmental Research and Public Health · Lopez et al.7 2022, Obesity Reviews
The largest experiment on this question had 585 men and women at eight sites train one arm for twelve weeks. The researchers then measured the biceps by MRI. Growth ranged from minus 2 to plus 59 per cent. The gain in one-repetition maximum strength ranged from zero to plus 250 per cent.
Same programme and same duration, yet a spread from no strength gain at all to more than three times the starting strength. In the Finnish dataset the muscle of 14 per cent of participants grew more than one standard deviation above the average. In 29 per cent growth stayed within what the control group without training also showed.
As a historian I also look at who paid for the research. The American arm study ran on money from the National Institutes of Health. The Finnish dataset relied mainly on public funds and foundations, with one contribution from a Finnish heart-rate monitor maker. For the meta-analysis on men and women below, two of the three authors work for a commercial training company, so I read that one more cautiously.
Anyone who sees little in the first months belongs to a large group. For that group, adjusting and continuing to measure is the logical next step.
Source: Hubal et al.8 2005, Medicine & Science in Sports & Exercise · Ahtiainen et al.4 2016, AGE · Roberts et al.9 2020, Journal of Strength and Conditioning Research
In relative terms, yes. A meta-analysis comparing men and women on the same programme found no difference in relative muscle growth. Women gained relatively more upper-body strength. In the lower body there was no difference.
The large arm study saw 2.5 per cent more relative growth in men, a small difference. Women gained relatively more strength there. In the Finnish dataset neither sex nor age affected the response, across a range of 19 to 78.
This is always growth relative to your own starting point. For the pace of your body, your sex matters surprisingly little.
Source: Roberts et al.9 2020, Journal of Strength and Conditioning Research · Hubal et al.8 2005, Medicine & Science in Sports & Exercise · Ahtiainen et al.4 2016, AGE
Beginners have a small head start. In a network meta-analysis of 28 studies with 747 adults, untrained participants grew somewhat more than recreationally trained people. Those with experience needed more sessions for the same effect.
Then muscle memory. A Norwegian and Swedish team had twelve people train one arm for ten weeks, rest for sixteen weeks and then train both arms for ten weeks. The extra muscle nuclei from the first period were retained during the break. After retraining the fast muscle fibres in the previously trained arm were larger. During that retraining, though, they did not grow demonstrably faster than in the arm training for the first time.
An accelerating effect of muscle memory has therefore not been clearly demonstrated in humans. After a long break, do not count on regaining everything twice as fast.
Source: Lopez et al.7 2021, Medicine & Science in Sports & Exercise · Cumming et al.11 2024, The Journal of Physiology
After four weeks the mirror mostly shows fluid and a better-drilled nervous system. Concluding that it does not work means stopping just before real growth gets going.
In practice three things tell you more than the mirror. Log your weights and reps, because strength moves first. Measure your arm or thigh circumference no more than every four weeks, at the same time of day. And count sessions instead of weeks: two missed weeks simply push your timeline back.
How often to plan those sessions is covered in how often to do strength training per week. What to eat to fuel that growth is in how much protein you need per day.
Source: Damas et al.2 2018, European Journal of Applied Physiology · Ahtiainen et al.4 2016, AGE
| Moment | What was measured | Where it comes from |
|---|---|---|
| After 2 sessions | thigh 3.5 % thicker, possibly fluid according to the researchers | 25 sedentary men, weekly scans |
| Week 3 | muscle 2.7 % thicker, with clear signs of swelling | 10 untrained men, ultrasound and blood markers |
| After about 18 sessions | start of real muscle growth | model from a review, not a measurement |
| Week 10 | muscle 10.4 % thicker, no raised swelling after correcting for muscle size | the same 10 men |
| Programmes of 4 to 24 weeks | on average 1.53 kg more lean or muscle mass, before versus after | 111 studies, men only, average age 23.5 |
| Trials of 4 to 96 weeks | 0.8 kg more lean mass than no training | 114 randomised trials, overweight, children to older adults |
| 20 to 24 weeks | muscle size +4.8 %, strength +21 % | 287 men and women aged 19 to 78 |
During the first three to four weeks, much of the increase may be swelling. According to the model of a Brazilian research team, real growth starts after about eighteen sessions. At two sessions a week that is around week nine, at three a week around week six.
Only partly. In a study with weekly scans the thigh was 3.5 per cent thicker after two sessions. The researchers themselves pointed to fluid as a possible explanation. Another study saw clear signs of swelling and raised damage markers in the blood in week three.
A little under a kilo, as a rough guide. An analysis of 114 randomised trials in people with overweight found 0.8 kilos more lean mass than in people who did not train. An analysis of 111 studies in men arrived at 1.53 kilos, though without a control group.
Because your nervous system learns faster than your muscle grows. In a dataset of 287 beginners strength rose by 21 per cent and muscle size by 4.8 per cent. A large part of the early strength gain probably comes from better control of the muscle.
Relative to each person's starting point, barely. A meta-analysis found no difference in relative muscle growth between men and women on the same programme. Women gained relatively more upper-body strength.
Yes. In a Finnish dataset with participants aged 19 to 78, age had no influence on muscle growth or strength gain after 20 to 24 weeks of training. The differences between individuals were large though, at every age.
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Every number in this article was checked against the original publication.
This article is information, not medical advice. If you have symptoms, are pregnant or take medication, talk to your doctor before changing anything.
In the largest analysis, 178 studies with 5097 participants, heavy programmes with several sets, at 80 per cent of your maximum or more, scored best for strength. Lighter training makes you stronger too. Train the movement you want to get stronger at and gradually add load or reps. Training to failure proved unnecessary for strength.
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Twice a week is enough for most people. With the same number of sets, spreading them over one, two or three sessions makes little difference to muscle mass. For strength, the most recent research points to a possible advantage for training more often. The practical minimum is four sets per muscle group per week. For more muscle mass, the newest position stand recommends ten sets or more.
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