Hypertrophy
Meta-Analysis
2016
Effects of Resistance Training Frequency on Measures of Muscle Hypertrophy: A Systematic Review and Meta-Analysis
By Brad J. Schoenfeld, Dan Ogborn and James W. Krieger
Sports Medicine, 46(11), pp. 1689-1697
<h2>Abstract</h2>
<p><a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="Training frequency">Training frequency</a>—the number of times a given muscle group is trained per week—represents a fundamental yet incompletely understood variable in the design of <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a>-oriented resistance training programs. While anecdotal bodybuilding tradition has long favored high-frequency, high-volume "bro split" approaches in which individual muscle groups are targeted once per week with high volumes, emerging scientific evidence has begun to challenge this paradigm. The present <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> sought to quantitatively synthesize the available evidence on the <a href="/terms/dose-response-relationship/" class="term-link" data-slug="dose-response-relationship" title="dose-response relationship">dose-response relationship</a> between training frequency and measures of skeletal muscle hypertrophy.</p>
<p>Following a comprehensive systematic search of electronic databases, studies were included <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they directly compared different training frequencies for the same muscle group while equating or accounting for total <a href="/terms/training-volume/" class="term-link" data-slug="training-volume" title="weekly volume">weekly volume</a>. Eligible studies used ultrasonography, magnetic resonance imaging, or computed tomography to assess hypertrophic outcomes including muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a>, muscle thickness, or muscle volume.</p>
<p>The meta-analysis revealed that training muscle groups twice per week produced significantly greater hypertrophy compared with once-per-week training, with a small-to-moderate pooled <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="effect size">effect size</a>. Evidence for an additional benefit of training three or more times per week compared with twice per week was limited and did not reach statistical significance. These findings suggest that training frequency per se influences hypertrophic adaptations, independent of total volume, though the mechanisms underlying this frequency advantage remain to be fully elucidated. The results support structured training programs in which each major muscle group is targeted at least twice weekly, a recommendation consistent with current evidence-based guidelines for resistance training program design [1].</p>
<h2>Introduction</h2>
<p>The optimization of resistance training program variables for maximal skeletal <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="muscle hypertrophy">muscle hypertrophy</a> represents an enduring area of scientific inquiry with broad relevance to recreational fitness, competitive athletics, and clinical applications including sarcopenia management and functional rehabilitation. Among the many variables that practitioners manipulate—including load, volume, exercise selection, rest periods, and tempo—<a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a> has received comparatively less systematic scientific investigation despite its fundamental importance in program design [1].</p>
<p>Traditional bodybuilding practice, which has heavily influenced popular training culture, has historically emphasized high-volume, low-frequency approaches in which individual muscle groups are trained once per week with large numbers of sets (the so-called "bro split"). The purported rationale for this approach is that muscles require extended recovery periods between training sessions, and that a single high-volume session per week provides sufficient stimulus while allowing adequate repair and <a href="/terms/supercompensation/" class="term-link" data-slug="supercompensation" title="supercompensation">supercompensation</a>. However, this practice-based logic has been increasingly questioned as the scientific understanding of skeletal <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> (MPS) kinetics has advanced.</p>
<p>Research on MPS dynamics following a resistance exercise bout demonstrates that elevated rates of MPS return to baseline within approximately 24–72 hours in trained individuals, though the precise time course is influenced by training status, volume, and individual variability [2]. <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="If">If</a> the <a href="/terms/anabolic-window/" class="term-link" data-slug="anabolic-window" title="<a href="/terms/protein-timing/" class="term-link" data-slug="protein-timing" title="anabolic window">anabolic window</a>">anabolic window</a> following each training session is finite, then distributing a given weekly <a href="/terms/training-volume/" class="term-link" data-slug="training-volume" title="training volume">training volume</a> across multiple sessions per week—such that each session is followed by a discrete MPS elevation—might theoretically produce a greater cumulative anabolic stimulus compared with a single weekly mega-session. This theoretical framework suggests that frequency may serve as an independent anabolic variable, not merely a vehicle for volume delivery.</p>
<p>The available evidence from individual studies has been mixed, with some investigations demonstrating clear frequency advantages and others finding no differences when volume is equated. Methodological heterogeneity, small sample sizes, and varying definitions of frequency have complicated the interpretation of this literature. The current <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> was designed to aggregate the available data and provide a more definitive quantitative estimate of the frequency-hypertrophy relationship, with the goal of informing evidence-based training recommendations.</p>
<h2>Methods</h2>
<h3>Search Strategy</h3>
<p>A systematic literature search was conducted in PubMed/MEDLINE, EMBASE, CINAHL, and SPORTDiscus databases from inception through the search date. The following Medical Subject Headings (MeSH) and keyword terms were used in combination: "resistance training," "strength training," "weight training," "<a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a>," "<a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a>," "muscle size," "<a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a>," "muscle thickness," "muscle mass," and "muscle volume." Boolean operators (AND, OR) were applied, and the search was restricted to peer-reviewed English-language publications. Reference lists of relevant articles and systematic reviews were additionally hand-searched.</p>
<h3>Inclusion and Exclusion Criteria</h3>
<p>Studies were deemed eligible for inclusion <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they: (a) were original randomized controlled trials or controlled trials with human participants; (b) included at least two conditions differing in training frequency for a given muscle group; (c) held total weekly <a href="/terms/training-volume/" class="term-link" data-slug="training-volume" title="training volume">training volume</a> constant or statistically controlled for volume differences between conditions; (d) used validated imaging modalities (ultrasound, MRI, or CT) to assess hypertrophy; and (e) had a minimum duration of four weeks. Studies were excluded if they were acute exercise studies, review articles, case reports, or if they did not provide sufficient data for <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="effect size">effect size</a> computation.</p>
<h3>Data Extraction</h3>
<p>Two reviewers independently extracted data using a standardized extraction form. Extracted variables included: publication year, study design, sample characteristics (n, age, sex, training background), frequency conditions compared, training protocol specifics (exercises, sets, repetitions, load, duration), muscle group examined, measurement method, and hypertrophic outcomes (means ± SD pre- and post-intervention). Where multiple time points were reported, post-intervention values were used. Discrepancies between reviewers were resolved by discussion until consensus was reached.</p>
<h3>Statistical Analysis</h3>
<p>Effect sizes were calculated as Hedges' g corrected for small sample bias, with 95% confidence intervals. <a href="/terms/eccentric-contraction/" class="term-link" data-slug="eccentric-contraction" title="Negative">Negative</a> effect sizes indicated superior outcomes in the lower-frequency group, and <a href="/terms/concentric-contraction/" class="term-link" data-slug="concentric-contraction" title="positive">positive</a> effect sizes indicated superiority of higher-frequency training. Effects were pooled using a random-effects model. Heterogeneity was quantified using the I² index (low: 25%; moderate: 25–75%; high: 75%). Moderator analyses were conducted using meta-regression for training volume, training status, duration, and muscle group. Significance was set at p 0.05 [3].</p>
<h2>Results</h2>
<h3>Literature Search Outcomes</h3>
<p>The systematic search identified 3,312 records after deduplication. Screening of titles and abstracts excluded 3,198 records, leaving 114 full-text articles for eligibility assessment. Of these, 10 studies satisfied all inclusion criteria and were retained for the <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a>. The included studies enrolled a combined total of 268 participants (age range 18–35 years; 74% male). Training durations ranged from 8 to 20 weeks across studies [1].</p>
<h3>Frequency Comparisons</h3>
<p><strong>Once versus twice per week</strong>: The primary analysis compared training each muscle group once versus twice per week at equated weekly volumes. Pooled results across seven studies revealed a statistically significant advantage for twice-weekly training (Hedges' g = 0.35, 95% CI: 0.11–0.59, p = 0.004). Heterogeneity was low (I² = 22%), suggesting reasonably consistent effects across studies. In relative terms, muscles trained twice weekly showed approximately 6.8% greater <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> than once-weekly training after 8–16 weeks.</p>
<p><strong>Twice versus three or more times per week</strong>: Three studies compared training frequencies of two versus three or more sessions per week at equated volumes. The pooled effect was small and non-significant (Hedges' g = 0.19, 95% CI: −0.06 to 0.44, p = 0.13), indicating insufficient evidence for an additional hypertrophic benefit of very high training frequencies beyond twice per week [2].</p>
<h3>Moderator Analyses</h3>
<p>Meta-regression analyses examined potential moderators of the frequency effect. <a href="/terms/training-volume/" class="term-link" data-slug="training-volume" title="Training volume">Training volume</a> (total weekly sets) was not a significant moderator after controlling for frequency condition (β = 0.02, p = 0.71), providing evidence that the frequency advantage observed was not simply an artifact of volume differences. Training status (untrained versus trained) approached but did not reach significance as a moderator (p = 0.09), with a trend toward larger frequency effects in trained individuals. Training duration and muscle group were not significant moderators.</p>
<h3>Study Quality</h3>
<p>Quality assessment revealed that six studies were rated as low-to-moderate risk of bias. Four studies had moderate-to-high risk due to absence of allocation concealment or blinded outcome assessment. Sensitivity analyses excluding high-risk studies produced a similar overall effect estimate (g = 0.31, 95% CI: 0.06–0.56), indicating robustness of the primary finding.</p>
<h2>Discussion</h2>
<h3>Twice-Weekly Training Produces Superior <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Hypertrophy">Hypertrophy</a></h3>
<p>The principal finding of this <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> is that training each muscle group twice per week elicits significantly greater hypertrophy compared with once-per-week training at equivalent weekly volumes. This frequency advantage, while modest in absolute terms (Hedges' g = 0.35), represents a meaningful practical difference that compounds over extended training periods. Several plausible mechanisms may account for this observation.</p>
<p>As discussed in the introduction, the temporal dynamics of <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> following a resistance exercise bout are finite. In trained individuals, MPS rates typically peak within hours and return toward baseline within 24–48 hours, meaning that a muscle group stimulated only once per week experiences a prolonged period of minimal anabolic activity between sessions [2]. By contrast, twice-weekly training generates two discrete MPS peaks within the same seven-day window, potentially creating a greater cumulative anabolic stimulus. Additionally, there may be advantages to the session-level dose being halved when volume is redistributed across two sessions: each individual session may be performed with higher quality (less fatigue-induced technique degradation, better <a href="/terms/motor-unit/" class="term-link" data-slug="motor-unit" title="motor unit">motor unit</a> recruitment) when sets are not concentrated into a single mega-session [3].</p>
<h3>Frequency Beyond Twice Per Week</h3>
<p>The absence of a statistically significant advantage for training three or more times per week compared with twice per week requires careful interpretation. The nonsignificant result does not prove equivalence; rather, it reflects that the available evidence is insufficient to conclude superiority at very high frequencies. Three factors complicate this analysis: the number of eligible studies comparing high-frequency conditions was small (n = 3), the statistical power to detect small additional effects was limited, and practical adherence challenges at very high training frequencies may introduce confounding in real-world settings. It remains conceivable that specific populations (advanced athletes with high recovery capacities) or specific circumstances (body-part specialization phases) might benefit from training frequencies exceeding twice per week [1].</p>
<h3>Practical Recommendations for Program Design</h3>
<p>Based on the current evidence, each major muscle group should be trained at least twice per week to optimize hypertrophic adaptations. This can be achieved through various training splits: upper/lower body splits (training upper and lower body on alternating days), push/pull/legs splits with each category trained twice weekly, or full-body routines performed two to four times per week. The specific split is less important than ensuring each muscle group receives adequate stimulation at least twice within each seven-day period.</p>
<p>When redistributing volume from a once-weekly to a twice-<a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="weekly frequency">weekly frequency</a>, practitioners may initially reduce per-session set volume proportionally (e.g., from 20 sets in one session to 10 sets per session twice weekly) and then progressively increase as adaptation and recovery capacity improve. The evidence suggests this redistribution, even without increasing total weekly sets, will yield superior hypertrophic outcomes over time [1,3].</p>