Can You Actually Train Your Muscles to Get Longer?

There's a kind of muscle growth almost nobody talks about, and it isn't your fibers getting thicker. It's your fibers getting functionally longer, made up of more individual contractile units stacked end to end. It's called sarcomerogenesis, and yes, you can trigger it on purpose. There's also a genuine, current scientific argument going on about exactly how.

What a sarcomere even is

A muscle fiber isn't one continuous contracting unit, it's built from thousands of tiny repeating segments called sarcomeres, connected end to end like train cars. That chain of sarcomeres, wrapped up, is what makes a muscle fiber. Everything we usually talk about, fiber size, hypertrophy, is about those cars getting individually bigger. Sarcomerogenesis is different: it's the muscle adding more cars to the train. More sarcomeres in series means the fiber is functionally longer, can produce force across a wider range of motion, and by most accounts, holds up better against injury, especially in muscles like the hamstrings that get hurt often specifically at long, stretched lengths.

The direct proof it's trainable

For a long time this was mostly inferred from animal studies and indirect ultrasound measurements in humans. That changed with a 2024 study that measured it directly, inside living human muscle, using a technique called second harmonic generation microendoscopy [1]. Twelve people did 9 weeks of Nordic hamstring exercise, an eccentric-only hamstring movement, three sessions a week. The result: sarcomere number increased 25% in the central hamstring and a striking 49% in the distal portion, alongside real gains in fascicle length, muscle volume, and a 40% strength increase. Three weeks of detraining afterward brought some of that back down, but not all the way to where they started.

That's about as direct a "yes, this is real, and yes, you can trigger it through training" as this field has produced.

Where it gets genuinely contested

Here's the part I think is actually the most interesting, and it's happening right now, not settled decades ago.

A major 2025 review pooled the available evidence, mostly from animal studies, and reached a provocative conclusion: it argued that eccentric muscle action itself isn't what matters. What actually triggers sarcomere addition, the review claimed, is high force applied while the muscle is at a long, stretched length, whether that force comes from an active eccentric contraction or even just passive stretch. In their own meta-analysis, eccentric training by itself showed a statistically null effect on sarcomere number in animal models [2].

That got pushed back on hard, within months, in the same journal. A different group argued the original meta-analysis pooled in poorly designed studies (too short, badly timed, or run in old, dysfunctional animals) right alongside well-designed ones, watering down a real effect, and that it excluded muscles that showed bigger responses. When they reran the analysis using only the well-designed eccentric studies, the effect wasn't null at all, it was moderate to large [3].

Nobody's resolved this yet. What both sides agree on: something about training a muscle hard at a long, stretched length triggers this adaptation. What they're actively arguing about is whether the eccentric contraction itself is the specific ingredient, or just one common way of getting a muscle into that stretched, loaded state.

What about drugs?

Worth answering directly, since it's the obvious next question. At the molecular level, sarcomere addition is regulated by real, identifiable pathways, including IGF-1/mTOR signaling and myostatin, a protein that normally puts the brakes on muscle growth. Drugs that block myostatin do exist and are an active area of research [4]. But right now those are investigational treatments aimed at muscle-wasting diseases, not something available or sensible for a healthy lifter to use. As far as the current evidence shows, this is a mechanical adaptation you earn through training at long muscle lengths, not something you can shortcut with a compound.

Where that leaves you

If you train a muscle hard while it's in a long, stretched position, however exactly that works, you can genuinely add functional length to it over a period of weeks, not days. The debate is over the precise mechanism, not over whether it's real. That's not a bad place for a training question to land, actual disagreement among serious researchers, over a phenomenon that's directly, physically demonstrated to happen.

If you want to dig into the actual studies, including the back-and-forth between the two research groups, they're in the Research Library.

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References

1. Andrews, M. H., Pai, A. S., Gurchiek, R. D., et al. (2024). Multiscale Hamstring Muscle Adaptations Following 9 Weeks of Eccentric Training. Journal of Sport and Health Science, 14, 100996.
2. Blazevich, A. J., Herzog, W., & Nunes, J. P. (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, 14, 101073.
3. Power, G. A., Franchi, M. V., & Hinks, A. (2025). The Mechanical Loading Environment Associated with Eccentric Exercise Is One of the Key Stimuli to Trigger Sarcomerogenesis. Journal of Sport and Health Science, 15, 101089.
4. Hinks, A., Hawke, T. J., Franchi, M. V., & Power, G. A. (2023). The Importance of Serial Sarcomere Addition for Muscle Function and the Impact of Aging. Journal of Applied Physiology, 135(2), 375-393.

Full details and direct links for all four are in the Research Library.