For years, strength and conditioning coaches have leaned on eccentric training as the gold standard for adding sarcomeres in series and lengthening fascicles.
But this new review and meta-analysis suggest something different.
The evidence points toward tension at long muscle lengths, not contraction mode, as the key driver of sarcomere addition.
That distinction changes how we should program for fascicle length and resilience.
Do we need eccentric contractions to add sarcomeres, or does any form of high tension at long muscle lengths drive adaptation?
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What the Researchers Did
- Conducted a historical review of the theories linking contraction type, muscle length, and sarcomere number.
- Performed a systematic review and meta-analysis of animal studies using eccentric exercise models (like downhill running or NMES-evoked eccentric contractions).
- Compared downhill (eccentric-biased) vs uphill (concentric-biased) running protocols.
- Reviewed the small but growing number of human studies using ultrasound and microendoscopy during Nordic hamstring training.
Key Findings
1. The real trigger is long-length tension.
- Both active and passive forces applied at longer muscle lengths are what consistently drive sarcomere addition.
- Simply moving a muscle through large excursions without high force isn’t enough.
2. Eccentrics aren’t automatic.
- Downhill running produced small increases in sarcomere number (~5%) in some muscles.
- Other muscles, like the vastus lateralis, showed no change—even when fascicles were confirmed to lengthen eccentrically.
- Overall, the eccentric meta-analysis showed no strong effect.
3. Long-length isometrics are powerful.
- In animal studies, isometrics performed at long muscle lengths triggered up to 30%+ increases in sarcomere number in just 8 weeks.
- This adaptation was larger and more consistent than with classic eccentrics.
4. Human data confirm, but timing matters.
- 3 weeks of Nordic hamstrings: fascicle length increased, but sarcomere number did not.
- 9 weeks: both fascicle length and sarcomere number increased, with region-specific adaptations.
- Translation: sarcomere addition in humans is slower and varies by region.
What This Means for Coaches
- Focus on the where, not the how ➔ The critical factor is loading muscles at longer lengths, not whether the contraction is eccentric, concentric, or isometric.
- Leverage long-length isometrics ➔ These can be a high-yield, joint-friendly way to drive longitudinal growth.
- Expect regional variation ➔ Don’t assume every fiber in a muscle adapts equally. Periodize joint angles and positions to distribute strain.
Limitations
- Most direct sarcomere number data comes from animal studies, where remodeling rates are faster.
- Measuring sarcomere number in humans is invasive and rare, so the dataset is still small.
Coach’s Takeaways
- Load long, not just heavy ➔ Prioritize joint angles that stress the muscle in lengthened positions.
- Long-length isometrics are underutilized ➔ They’re simple, effective, and adaptable.
- Spread the strain ➔ Rotate angles and positions to manage tissue tolerance and target regional adaptations.
I hope this helps,
Rasmey
Reference
Blazevich AJ, Herzog W, Nunes JP (2025). Triggering sarcomerogenesis: Examining key stimuli and the role attributed to eccentric training—Historical, systematic, and meta-analytic review. Journal of Sport and Health Science.
