Rep Range: Do You Need Heavy Sets to Build Muscle?

Load in strength training is most often expressed as a percentage of the maximum you can lift once (1RM), or as the number of reps you can do with a given weight to failure (RM). The two measures are related, though not strictly — they depend on the exercise, the person and the day. Studies use conventional divisions that allow papers to be compared.

The mechanism explaining why light loads can work is this: during a set taken to failure, as fatigue builds the body recruits additional motor units, until in the end — despite the light weight — even the largest, high-threshold ones are working. This is a mechanistic hypothesis consistent with the size principle of motor units, not a measurement from muscle-mass trials; we treat it as an explanation, not proof.

This yields a key condition: light sets work if they end very close to failure. A light set stopped at 15 reps with 10 in reserve does not recruit high-threshold units and does not give a comparable stimulus. All the studies discussed below that found similar muscle growth relied on sets taken to failure — worth remembering, because without that condition the conclusions do not carry over.

This is the foundational paper cited by most modern recommendations. The result has two layers. First: muscle mass — similar regardless of load. Second: dynamic strength — clearly better with high loads, consistent with the principle of specificity (you train what you want to improve). The lack of difference in isometric strength suggests part of the advantage of heavy sets concerns skill at performing a specific movement with a heavy weight, not the muscle's raw capacity to generate force.

Note that this meta-analysis required all sets to be performed to failure — a restriction with practical importance. In real training every set is rarely taken to failure, and light sets not to failure probably give a weaker stimulus than heavy sets not to failure, because the latter recruit high-threshold motor units from the first repetition. That is a logical inference, not something directly tested in the paper described.

This paper confirms the conclusions of the previous one and adds a valuable detail: the difference in strength appears mainly between low loads and the rest, while between moderate (9–15 RM) and high loads the difference is small and not statistically significant. Practically, this means building strength does not require going down to single reps — the moderate range works too, and heavy sets give an extra benefit of limited certainty.

A second practical takeaway: the only groups in which the authors found significantly greater hypertrophy were untrained people (compared with trained) and people doing more sessions. This is a reminder that rep range is a secondary variable compared with consistent, regular training.

This study has two important features. First, it concerns people with training experience, so it answers the objection that “light sets only work for beginners”. Second, it used muscle biopsies and DEXA scanning, direct measures of muscle mass and fibres, not just circumference measurements. At the same time the sample is moderate (49 people), the group is young men, and the duration is 12 weeks; it is a good indication, not a final ruling.

The second part of the conclusion is also especially interesting: no correlation between the acute post-training hormone surge and later muscle gain. This matches what we write in the article on rest between sets — transient hormone swings are not a reliable indicator of how effective training is.

This study sets a practical lower boundary: a load around 20% 1RM (very light) was clearly weaker than 40% and above despite equated volume. Loads of 40–80% 1RM gave a similar increase in muscle cross-section (about 20% in the quadriceps). Conclusion: light sets are effective, but “light” does not mean “anything goes” — at around 20% 1RM effectiveness was clearly lower (the study did not test values between 20% and 40%, so we do not know the exact boundary).

A meta-analysis comparing load ranges with equated volume (Carvalho et al., 2022) completes this picture: for hypertrophy no differences were found between loads, while for dynamic 1RM strength higher loads gave a greater gain even when volume load was the same. The authors also note little difference between low and very low loads for strength.

If light sets give similar muscle growth, why not always do them? Because they demand two things that carry a price. The first is very high effort: for a set at 30–40% 1RM to approach failure you must do 25–40 reps, and the last ones are metabolically exhausting (burning, breathlessness). For many people this is subjectively harder than a heavy set of 8–10.

The second is time and volume. A set of 30 reps takes longer, and the rest afterwards is often longer because fatigue is felt systemically (breathlessness, heart rate). In many compound exercises (squat, deadlift) the limiting factor becomes not the muscle but breathing or grip, so the set ends earlier than the muscle would allow. That is why in practice light sets work best in isolation exercises and on machines.

The third issue is judging failure. As we note in the article on RIR, people often misjudge how many reps they have left; with light sets this error is especially costly, because a set ended with 8–10 reps in reserve gives practically no stimulus. That is another argument for using light sets where failure is safe.

The ranges above are not the result of one study but a synthesis of meta-analytic evidence and common sense. A good template for most people is to mix ranges: main lifts at 5–10 reps (for strength and technique), and accessory and isolation work at 10–20 (for volume and joint comfort). This is not proven better than a single range, but it uses the strengths of both approaches.

The myth “heavy sets give only strength” is especially harmful for people who fear heavy training “won't build size”. In reality heavy sets of 5–8 reps also stimulate growth, while developing strength that later lets you lift more weight in the size range. The myth “light sets do nothing” discourages people with joint problems from an effective alternative.

If we had to reduce this topic to one thought, it would be: effort counts for muscle growth, not a magic rep range; load counts for strength. Most people do best combining main lifts at 5–10 reps with isolation work at 10–20, keeping proximity to failure and consistency. Light sets are a valuable tool, especially with joint pain, not an inferior one.

The greatest benefit of these studies is freedom of choice. If you cannot train heavy — because of joints, injury, lack of equipment or safety — you do not lose the chance to build muscle, provided you work truly close to failure. And if you like heavy sets, you need not worry that you will “run short” of muscle.