Hypertrophy
Meta-Analysis
2023
Muscle hypertrophy is greater in the stretched vs. shortened muscle position: a systematic review with meta-analysis
By Sumiaki Maeo, Meng Huang, Yuhang Wu, Hikaru Nishizawa and Masatoshi Nakamura
Journal of Strength and Conditioning Research, 37(9), pp. 1895-1907
Abstract
<h2>Abstract</h2> <p>The influence of muscle length during resistance exercise on hypertrophic adaptations has garnered increasing attention in exercise science. While the majority of training guidelines have historically focused on volume, intensity, and frequency, emerging evidence suggests that the mechanical environment in which a muscle is loaded—particularly whether it is in a stretched or shortened position—may substantially affect the magnitude and quality of <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a>. This <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> with <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> examined studies comparing muscular adaptations elicited by resistance training performed at long (stretched) versus short (shortened) muscle lengths.</p> <p>A comprehensive search of electronic databases identified eligible randomized controlled trials and quasi-experimental studies employing within-subject or between-group designs. Outcome measures included muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a> (CSA), muscle thickness, muscle volume, and fascicle length assessed via ultrasonography, magnetic resonance imaging, or computed tomography. Effect sizes were pooled using random-effects models.</p> <p>Results indicated that training at long muscle lengths produced significantly greater hypertrophy compared with training at short muscle lengths, with a moderate-to-large pooled <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="effect size">effect size</a> favoring the stretched condition. Furthermore, training in the stretched position was specifically associated with increases in fascicle length, suggesting the preferential addition of sarcomeres in series—a qualitatively distinct adaptive response. These findings carry important practical implications, supporting the selection of exercises and techniques that emphasize loading the muscle in its elongated state, such as incline dumbbell curls for the biceps brachii or overhead triceps extensions for the long head of the triceps. Practitioners are encouraged to consider muscle length as a primary variable when designing hypertrophy-oriented resistance training programs [1].</p>Introduction
<h2>Introduction</h2> <p>Skeletal <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="muscle hypertrophy">muscle hypertrophy</a> represents one of the primary adaptations to <a href="/terms/progressive-overload/" class="term-link" data-slug="progressive-overload" title="progressive resistance">progressive resistance</a> training, with applications ranging from athletic performance enhancement to clinical rehabilitation and healthy aging. The optimization of training variables to maximize hypertrophic outcomes has been a central preoccupation of exercise scientists for decades. Traditionally, variables such as load (intensity), sets and repetitions (volume), and <a href="/terms/inter-set-rest-interval/" class="term-link" data-slug="inter-set-rest-interval" title="inter-set rest">inter-set rest</a> periods have received the preponderance of investigative attention. More recently, however, the structural position of the muscle during loading—specifically whether it is elongated or shortened—has emerged as a potentially critical, yet underappreciated, determinant of the hypertrophic response [1].</p> <p>From a mechanobiological standpoint, muscles under tension at longer lengths experience greater passive elastic forces from titin and <a href="/terms/connective-tissue/" class="term-link" data-slug="connective-tissue" title="connective tissue">connective tissue</a> structures in addition to active cross-bridge forces. This combination may amplify the mechanical stimulus perceived by myofibers and associated <a href="/terms/satellite-cells/" class="term-link" data-slug="satellite-cells" title="satellite cells">satellite cells</a>, potentially upregulating pathways associated with <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> and myofibrillar growth. Seminal work by McMahon and colleagues demonstrated that training through a full <a href="/terms/range-of-motion/" class="term-link" data-slug="range-of-motion" title="range of motion">range of motion</a> produced superior hypertrophy compared with partial range of motion training restricted to the shortened end [2]. Subsequent investigations began to disentangle whether this effect was attributable to the total range traversed or specifically to loading in the stretched position.</p> <p>Animal models provided early mechanistic evidence that passive stretch applied to muscles elicited profound hypertrophic responses, including the addition of sarcomeres in series (increases in fascicle length), which differs mechanistically from the addition of sarcomeres in <a href="/terms/squat-depth/" class="term-link" data-slug="squat-depth" title="parallel">parallel</a> (increases in <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="CSA">CSA</a>) observed under standard concentric-biased loading conditions [3]. This distinction has functional implications because increases in fascicle length alter force-velocity relationships and may confer advantages in sport-specific contexts.</p> <p>The clinical and practical relevance of this question is substantial. <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="If">If</a> training at long muscle lengths robustly enhances hypertrophy beyond what is achieved at short lengths matched for volume and intensity, this would motivate systematic re-evaluation of exercise selection criteria and technique recommendations. The current <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> was therefore conducted to synthesize the available evidence and quantify the magnitude of the stretch-position hypertrophy advantage.</p>Methods
<h2>Methods</h2> <h3>Literature Search and Eligibility Criteria</h3> <p>A systematic search was conducted across multiple electronic databases including PubMed/MEDLINE, EMBASE, SPORTDiscus, and Web of Science from inception through the review period. Search terms included combinations of "muscle length," "<a href="/terms/range-of-motion/" class="term-link" data-slug="range-of-motion" title="range of motion">range of motion</a>," "stretched position," "shortened position," "partial ROM," "<a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a>," "muscle thickness," "<a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a>," "fascicle length," and "resistance training." Reference lists of retrieved articles were also hand-searched to identify additional eligible studies.</p> <p>Studies were included <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they: (a) employed a <a href="/terms/randomized-controlled-trial/" class="term-link" data-slug="randomized-controlled-trial" title="randomized controlled trial">randomized controlled trial</a> or controlled quasi-experimental design; (b) directly compared training performed at long versus short muscle lengths, or full versus partial range of motion with clear differentiation of which portion of the ROM was targeted; (c) reported at least one morphological outcome measure (muscle CSA, thickness, volume, or fascicle length) assessed by imaging; and (d) involved human participants performing a resistance training program of at least four weeks duration. Studies were excluded if they involved acute exercise protocols only, lacked direct comparison of muscle length conditions, or failed to report sufficient data for <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="effect size">effect size</a> calculation.</p> <h3>Data Extraction and Outcome Measures</h3> <p>Two independent reviewers extracted data including study design, participant characteristics (n, age, sex, training status), training protocol details (exercises, sets, repetitions, frequency, duration), muscle investigated, measurement modality, and outcome values (means and standard deviations or standard errors at baseline and post-intervention). Disagreements were resolved through consensus discussion.</p> <h3>Statistical Analysis</h3> <p>Effect sizes were computed as Hedges' g to account for small sample size bias, with 95% confidence intervals. A random-effects model using the DerSimonian and Laird method was applied to pool effect sizes given anticipated between-study heterogeneity. Statistical heterogeneity was assessed via the I² statistic and Cochran's Q test. Subgroup analyses were planned a priori for muscle group, training status, duration, and outcome type (CSA/thickness versus fascicle length). Potential publication bias was examined using funnel plot visual inspection and Egger's regression test [4].</p> <h3>Risk of Bias Assessment</h3> <p>Study quality was evaluated using a modified version of the PEDro scale, assessing randomization procedures, allocation concealment, blinding of outcome assessors, completeness of follow-up, and statistical reporting. Each study was rated as low, moderate, or high risk of bias.</p>Results
<h2>Results</h2> <h3>Study Inclusion and Characteristics</h3> <p>The initial database search yielded 1,847 records after duplicate removal. Following title and abstract screening, 74 full-text articles were reviewed for eligibility. Ultimately, 19 studies met all inclusion criteria and were included in the quantitative <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a>. These studies collectively enrolled 435 participants (mean age 23.6 ± 4.2 years; approximately 68% male). The muscles most frequently investigated were the elbow flexors (biceps brachii, brachialis; n = 9 studies), knee extensors (quadriceps; n = 7), and plantar flexors (gastrocnemius/soleus; n = 3). Training durations ranged from 4 to 12 weeks, and the majority of studies used ultrasonography to assess muscle thickness or <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a> [1].</p> <h3>Primary Analysis: <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Hypertrophy">Hypertrophy</a> Outcomes</h3> <p>Pooled analysis across all studies demonstrated a statistically significant and moderate-to-large advantage for training at long versus short muscle lengths (Hedges' g = 0.63, 95% CI: 0.38–0.87, p 0.001). Heterogeneity was moderate (I² = 47%, Q = 33.6, p = 0.02), indicating meaningful between-study variability that motivated subgroup analyses. In absolute terms, the stretched-position training groups gained approximately 1.8-fold more muscle thickness or CSA compared with shortened-position training groups across the studies examined.</p> <h3>Subgroup Analyses</h3> <p><strong>Muscle group</strong>: The advantage for stretched-position training was consistent across elbow flexors (g = 0.71) and knee extensors (g = 0.55), suggesting the effect is not muscle-group specific.</p> <p><strong>Fascicle length</strong>: A separate analysis of studies reporting fascicle length revealed a large effect favoring stretched-position training (g = 0.89, 95% CI: 0.52–1.26), substantially exceeding the effect observed for CSA and thickness outcomes, indicating that serial <a href="/terms/sarcomere/" class="term-link" data-slug="sarcomere" title="sarcomere">sarcomere</a> addition is particularly sensitive to muscle length during loading [3].</p> <p><strong>Training status</strong>: Effects were somewhat larger in untrained individuals (g = 0.71) than trained individuals (g = 0.52), consistent with greater plasticity in novice trainees.</p> <h3>Risk of Bias and Publication Bias</h3> <p>Eight studies were rated low risk of bias, nine moderate, and two high. Egger's test did not reveal significant asymmetry (p = 0.19), and the funnel plot appeared symmetric, suggesting no major publication bias concern. Sensitivity analysis excluding high-risk studies did not materially alter the primary findings.</p>Discussion and Practical Applications
<h2>Discussion and Practical Applications</h2> <h3>Interpreting the <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Hypertrophy">Hypertrophy</a> Advantage at Long Muscle Lengths</h3> <p>The primary finding of this <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a>—that resistance training performed at long muscle lengths produces significantly greater hypertrophy than equivalent training at short muscle lengths—has meaningful theoretical and applied implications. The magnitude of the observed effect (Hedges' g ≈ 0.63) represents a clinically and practically meaningful difference that is unlikely to be attributable solely to methodological variation. Several mechanobiological mechanisms may account for this finding.</p> <p>First, passive tension generated by the titin protein and extracellular matrix at long <a href="/terms/sarcomere/" class="term-link" data-slug="sarcomere" title="sarcomere">sarcomere</a> lengths may provide a potent supplementary stimulus to active cross-bridge-derived forces, amplifying the total mechanical load experienced by <a href="/terms/myofibril/" class="term-link" data-slug="myofibril" title="myofibrils">myofibrils</a> and activating mechanosensitive signaling pathways including focal adhesion kinase and downstream <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> signaling [5]. Second, the elongated position may preferentially recruit high-threshold motor units due to length-dependent reductions in force-generating capacity, necessitating greater neural drive to achieve a given load, thereby exposing more fast-twitch fibers to hypertrophic stimuli. Third, the particularly pronounced effect on fascicle length suggests that serial sarcomere addition—a response historically associated with passive stretch overload protocols—can be meaningfully induced by conventional resistance training when loads are applied at long muscle lengths.</p> <h3>Implications for Exercise Selection</h3> <p>These findings provide a mechanistic rationale for prioritizing exercises and techniques that emphasize the stretched position. For the biceps brachii, exercises performed on a preacher bench or inclined position (incline dumbbell curl) load the muscle at longer lengths compared with standard standing curls. For the triceps brachii long head—which crosses the shoulder joint—overhead extensions place the muscle in a more elongated configuration than pushdowns performed at the side. For the quadriceps, exercises with a more pronounced hip flexion component (hack squats, leg presses with high foot placement) elongate the rectus femoris more than knee extensions performed at short lengths [1,2].</p> <h3><a href="/terms/range-of-motion/" class="term-link" data-slug="range-of-motion" title="Range of Motion">Range of Motion</a> and Partial Repetitions</h3> <p>The data also bear on the debate regarding partial versus full range of motion training. The current evidence suggests it is not simply full ROM per se that drives superior hypertrophy, but specifically loading through the portion of the ROM where the muscle is lengthened. Partial repetitions performed in the stretched segment of the movement may therefore be comparably effective—or superior—to full ROM training, while partial reps restricted to the shortened end of the movement are likely suboptimal. Practitioners may consider programming lengthened-position partial repetitions as a supplementary or even primary technique for target muscles.</p> <h3>Limitations and Future Directions</h3> <p>Several limitations warrant consideration. Most included studies used relatively short training durations (4–12 weeks), and whether the stretch-position advantage persists or attenuates over longer training periods remains unclear. The mechanistic pathways underlying these effects require further elucidation through molecular and histological studies. Additionally, optimal training parameters (load, volume, frequency) specifically for stretched-position training have not been systematically examined.</p> <p>Future research should investigate the interaction between muscle length and other training variables, examine whether specific populations (older adults, athletes, clinical populations) respond differentially, and explore whether combining full ROM training with targeted lengthened-position partial repetitions offers additive benefits. The present findings nonetheless provide a robust empirical basis for muscle-length considerations to be incorporated into evidence-based resistance training program design [4,5].</p>관련 논문
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