Eccentric Training May Not Build Longer Muscles: Meta-Analysis Reveals Force at Long Lengths as Key Driver
Historical and meta-analytic review challenges conventional wisdom, demonstrating that training position matters more than contraction type for fiber length adaptation
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Muscle growth happens at the cellular level through the addition of sarcomeres in series—the fundamental contractile units that determine fiber length and function. Despite widespread promotion of eccentric training for building longer muscle fibers and reducing injury risk, this comprehensive review challenges that assumption with rigorous historical analysis and meta-analytic evidence from animal studies. The findings suggest that the critical stimulus is not the eccentric contraction itself, but rather the application of high force at long muscle lengths.
Key Points
Aim
This review examined whether eccentric exercise is truly the optimal stimulus for increasing serial sarcomere number (SSN) in muscle fibers, which influences muscle function and injury protection. The researchers conducted a detailed historical overview of mechanical factors regulating muscle fiber length and performed a systematic meta-analysis of animal studies to determine if eccentric training effectively increases SSN.
Methods
The researchers conducted an extensive literature search with snowballing to build the historical review. For the meta-analysis, they systematically reviewed animal studies comparing fiber-lengthening exercises to control conditions using random-effect meta-analyses. Studies included voluntary exercise (downhill/uphill treadmill running in rats and mice) and involuntary exercise (neuromuscular electrical stimulation) protocols. Risk of bias was assessed using the SYRCLE tool, and effect sizes were calculated as Hedges’ g with 95% confidence intervals.
Results
Historical Evidence
The historical review demonstrated that applying forces at long fiber lengths—not greater excursions—plays the critical role in increasing SSN. Animal studies showed SSN changes exceeding 20% over several weeks with various activities, particularly with isometric contractions at long muscle lengths (32% increase), long-length immobilisation (13-18% increase), and intermittent passive stretching (10% increase).
Meta-Analysis Findings
The meta-analysis revealed a lack of effect of eccentric resistance training in animal models, with only a 1% increase in SSN (Cohen’s d = 0.19, 95% CI: -0.29 to 0.67, p = 0.449). When examining downhill running studies specifically, a moderate 5% increase was observed (ES = 0.78, p < 0.001), while uphill running showed no changes (-2%, ES = -0.26, p = 0.214).
Importantly, no significant SSN changes were detected when analysing only muscles confirmed to work eccentrically (vastus lateralis during downhill running and electrically stimulated muscles) (1% change, ES = 0.27, p = 0.240). The neuromuscular electrical stimulation protocols also showed no significant effect (1% change, ES = 0.19, p = 0.449).
Human Studies
Only two human studies exist measuring SSN changes with exercise. Both used eccentric Nordic hamstring training. After 3 weeks, no significant SSN change was detected (-1.4%) despite a 21% increase in fascicle length. After 9 weeks, SSN increased by 25% centrally and 49% distally in the biceps femoris long head, suggesting longer durations may be necessary in humans compared to animal models.
