Muscle hypertrophy, strength, and volume: how does the relationship evolve?

Volume and frequency are often presented as simple levers for progress. However, they may not produce the same effects depending on whether the goal is strength or hypertrophy, and depending on whether all sets are direct or indirect…

Strength training is based on an idea that seems simple: exposing the muscle to a load repeated often enough to trigger an adaptation. In practice, this load is influenced by several variables, two of which play a central role in programming: weekly volume, usually expressed as the number of sets, and frequency, meaning the number of training sessions per week. These two parameters are used daily to design programs for hypertrophy, strength, or maintenance, but their actual effect depends heavily on how they are defined and measured.

The difficulty stems from the fact that not all sets likely produce the same stimulus. A set of bicep curls directly targets the biceps, whereas a row set may also involve them, but indirectly. Similarly, for strength, a set of squats directly trains a specific movement pattern, whereas a leg press set can strengthen the muscles involved in the squat without exactly replicating the same task. However, many studies analyzing training volume in strength training have grouped these exposures in a more general way, which may have obscured the relationship between training dose and muscle adaptation.

But in this case, what is the dose-response relationship between weekly volume or training frequency and gains in hypertrophy or strength, when we distinguish between sets that are directly specific and those that are only indirectly specific?

The Study

American researchers conducted a systematic review with meta-regressions to model how muscle adaptations evolve in response to a continuous dose. The authors included 67 studies, representing 2,058 participants. The population was predominantly male, with 79.1% men and 20.9% women, and relatively young, with a mean age of approximately 25 years. The interventions lasted an average of 10.4 weeks and included both trained and untrained participants.

The included studies had to compare at least two training conditions differing in volume and/or frequency, while controlling for important variables such as relative load and proximity to failure. The adaptations studied were either hypertrophy, measured by direct, muscle-site-specific methods such as ultrasound, MRI, CT scan, or biopsy, or maximum strength, measured by dynamic, isometric, or isokinetic tests.

The study’s major methodological contribution lies in how sets were quantified. The authors classified each set as direct or indirect. For hypertrophy, a direct set corresponded to an exercise where the measured muscle was likely the primary mover. For strength, a direct set corresponded to the exact exercise used in the test. Indirect sets were then counted using three methods: as a full set, as a zero set, or as a half-set. It was this last method, known as “fractional,” where an indirect set counted as a half-set, that best predicted muscle and strength gains across all models.

Results & Analyses

The main finding concerns weekly volume. For hypertrophy, as the number of weekly sets increased, gains in muscle size tended to increase. The best-fitting model was nonlinear, meaning that each additional set yielded a progressively smaller benefit. Around the average volume reported in the studies, the estimated slope was approximately +0.24% muscle size per additional fractional weekly set.

Of course, this result should not be interpreted as a universal formula where each set mechanically adds 0.24% muscle. The observed relationship is curved. The first few sets appear relatively effective, but additional gains become progressively more costly in terms of volume. The authors estimate that approximately 4 fractional weekly sets are sufficient to exceed the smallest detectable effect for hypertrophy, set here at 2.05%. After that, more and more additional sets are needed to achieve a further detectable gain: about 6 more sets in the 5–10 set range, and even more as one approaches high volumes.

Importantly, the study does not identify a clear plateau for hypertrophy. The data suggest diminishing returns, but not a complete lack of benefit beyond a fixed threshold such as 10, 15, or 20 sets. However, uncertainty increases significantly at high volumes, particularly beyond approximately 25 to 30 weekly fractional sets. The authors consider that volumes starting at 43 weekly sets are too poorly documented to draw firm conclusions.

For strength, weekly volume also shows a positive relationship with gains. However, the shape of this relationship is different. The best-fitting model indicates much more pronounced diminishing returns, with a functional plateau. Around the average volume, the estimated slope was approximately +0.21% of maximum strength per additional fractional weekly set.

For hypertrophy, adding volume appears to continue producing a useful signal, even if marginal efficiency decreases. For strength, a large portion of the detectable benefit appears to occur at relatively low volumes. The study estimates that a single fractional weekly set may be sufficient to exceed the smallest detectable effect for strength, set here at 3.96%, and that additional gains become much less pronounced beyond 3 to 4 sets. This does not mean that strength athletes should train with just one set per week. Rather, it means that, in the studies analyzed, the initial exposure to the tested exercise produces a very strong effect, likely amplified by motor learning, test familiarity, and specificity.

The second major finding concerns frequency. For hypertrophy, the frequency-independent effect, once volume is accounted for, is small and difficult to identify. The average slope was positive, around +0.32% muscle size per additional weekly session, but the credibility interval included negligible effects. Additional analyses limited to direct comparisons confirmed this caution. In short, increasing frequency does not appear to produce a clearly identifiable gain in hypertrophy if total weekly volume is already accounted for.

This observation qualifies a commonly held belief: that stimulating a muscle more often is necessarily superior, particularly because post-exercise muscle protein synthesis declines after 24 to 48 hours. The study does not deny the practical value of a higher frequency. Rather, it shows that, based on the available data, frequency appears primarily to be a means of organizing volume, distributing fatigue, and improving the quality of sessions, rather than a systematic independent lever for hypertrophy.

For strength, the conclusion is different. Training frequency shows a clear positive relationship with strength gains. The estimated average rate of increase was approximately +3.27% of maximum strength per additional weekly session, with diminishing returns. Increasing training frequency from 1 to 2 sessions per week corresponded to a notable increase in the estimated gain, after which the benefits slowed beyond that point.

The most plausible explanation is specificity. Maximum strength is not merely a property of the muscle; it is also a performance in a given task. Repeating the tested movement—or similar movements—more often can improve coordination, confidence, effort strategy, and the ability to express available strength. This logic explains why frequency may have a more visible effect on strength than on hypertrophy, especially when strength tests are closely related to the exercises performed.

The choice of the fractional method is also a finding in its own right. Counting all partial sets as full sets likely overestimates the actual stimulus. Ignoring them entirely underestimates it. Counting them as half-sets better explained the data, suggesting that a multi-joint or non-specific exercise can contribute to muscle adaptation or test performance, but with lower average effectiveness than a directly targeted exercise. This rule remains, however, an approximation, not a biological law.

Practical Applications

For hypertrophy, this study reinforces the idea that weekly volume is a major lever, but that its value must be considered in terms of return on investment. A low volume may be sufficient to produce a detectable adaptation, particularly in untrained individuals or in muscles that have not been previously stimulated. Increasing volume can then improve gains, but the cost rises: more time, more fatigue, greater recovery demands, and greater uncertainty at very high volumes.

For coaches and advanced athletes, the goal is therefore not to automatically aim for the highest possible number of sets. A more robust approach involves identifying the volume that produces measurable progress, then gradually increasing it if recovery, in-session performance, and adaptation metrics warrant it. High volumes may be appropriate for certain muscles, individuals, or phases, but superior results are not guaranteed in all contexts.

For strength, frequency appears to have a clearer practical importance. Spreading the training of a movement across several weekly sessions can be useful, especially when the goal is to improve a specific test or performance. This does not mean increasing the number of sessions indefinitely. Diminishing returns suggest that there is likely a point where additional repetitions offer little benefit and may even compromise recovery or execution quality.

Distinguishing between direct and indirect sets can also improve programming. A bench press should not be counted as a direct triceps set in the same way as a triceps extension, but it should not be ignored either. A leg press can contribute to squat strength or quadriceps development without being equivalent to a squat or a leg extension if the measured objective is very specific. This nuance helps better estimate the actual stimulus received by a muscle or movement.

Finally, these results apply primarily to young, healthy adults in relatively short training programs. They therefore do not provide relevant or definitive information for individuals over 70 years of age, highly advanced athletes, long training periods, or contexts where fatigue, injuries, and adherence become critical factors. Here, the best conclusion is that there is no universal ideal volume, but that the training dose must be interpreted with specificity, progression, and caution.

Reference

Pelland JC, Remmert JF, Robinson ZP, Hinson SR & Zourdos MC. The Resistance Training Dose Response: Meta-Regressions Exploring the Effects of Weekly Volume and Frequency on Muscle Hypertrophy and Strength Gains. Sports Med 56, 481–505, 2026.