Effects of the Nordic Hamstring Exercise on Muscle Architecture
What does it do?
Researchers at Stanford University and collaborating institutions have published new evidence showing how the Nordic hamstring exercise (NHE) reshapes hamstring muscle structure at both gross and microscopic levels. The findings, published in the Journal of Sport and Health Science, tracked 11 recreationally active participants over 9 weeks of supervised NHE training, followed by 3 weeks of detraining, using advanced MRI to measure real-time changes in muscle architecture and microstructure.
The result: NHE delivered substantial muscle growth across all four hamstring heads, but the adaptations were far from uniform. The semitendinosus showed the largest gains at 26% growth, followed by the biceps femoris short head at 22%. The biceps femoris long head (the most commonly studied hamstring) grew only 9%, while the semimembranosus barely budged at 6%. Beyond raw size, the muscles got longer and thicker at the fiber level, changes that researchers argue directly explain why NHE cuts hamstring injury rates nearly in half. But here’s the critical finding: after just 3 weeks without training, fiber lengthening disappeared almost entirely, though the muscle size gains and structural toughness largely stuck around. Let’s break it down.
Aim
To determine how the four individual hamstring muscles adapt in structure and composition to eccentric training (specifically NHE), and to clarify what happens to these adaptations during brief detraining. Previous work focused heavily on the biceps femoris long head because it’s the most commonly injured muscle, but researchers needed a complete picture of all four hamstrings to understand which muscles respond best to this popular injury-prevention exercise.
Methods
Eleven recreationally active participants (5 males and 6 females, average age around 27-28 years) with no history of lower limb injury performed a supervised 9-week NHE training program. Each session involved 3 training days per week with 4 to 5 sets of 6 to 8 repetitions, totalling 72 to 120 reps weekly. The exercise was done under controlled conditions: participants knelt on a padded board with secured ankles, leaned forward with their knees extending at roughly 18 degrees per second until they could no longer control the movement, then used their upper body to return to the start position. A metronome maintained the pace.
The researchers used advanced MRI imaging at three time points: before training, immediately after the 9-week program, and again after 3 weeks of detraining (when participants stopped NHE but continued other exercise). MRI measured architectural parameters (muscle volume, fiber tract length, and fiber tract angle) and microstructural parameters (water diffusion metrics that reflect tissue composition at the microscopic level).
Results
Muscle Growth
After 9 weeks, the semitendinosus grew 26%, the biceps femoris short head grew 22%, the biceps femoris long head grew 9%, and the semimembranosus showed minimal change at 6%. This non-uniform growth pattern directly correlated with muscle fiber composition and mechanical advantage: muscles with higher proportions of fast-twitch (type-II) fibers and better mechanical leverage about the knee grew larger.
Fiber Architecture
All four hamstrings showed significant increases in fiber tract length: semitendinosus (18%), biceps femoris short head (11%), biceps femoris long head (7%), and semimembranosus (10%). Fiber tract angle (the pennation angle of muscle fibers) did not change significantly. These length increases indicate the addition of new sarcomeres in series along each fiber.
Microstructure (Water Diffusion)
Axial diffusivity (5%), mean diffusivity (4%), and radial diffusivity (5%) all increased significantly, reflecting changes in water movement within muscle tissue. These increases suggest larger fiber cross-sections and concurrent microtrauma from the intense eccentric loading. Fractional anisotropy, which measures the directionality of water diffusion, showed no significant change.
After 3 Weeks Detraining
The semitendinosus lost 8% of the volume it gained but remained 13% larger than baseline. The other muscles held most of their size gains. However, fiber tract length declines were dramatic: all four hamstrings returned to baseline lengths, essentially losing all the length adaptations in just 21 days. Remarkably, the diffusion metrics (indicators of fiber cross-section and tissue composition) remained elevated compared to pre-training levels across all muscles.
Practical Takeaways
The Nordic hamstring exercise works. It produces substantial, measurable muscle growth across the entire hamstring group, with particular emphasis on the muscles with the best mechanical advantage for protecting the knee. The semitendinosus and biceps femoris short head respond most aggressively, growing 22-26%, which matters because these muscles activate most heavily during the eccentric phase of NHE.
The growth happens through two distinct mechanisms: muscles get thicker (more sarcomeres in parallel, boosting force production) and fibers get longer (more sarcomeres in series, reducing strain per sarcomere during overstretching). Both adaptations contribute to injury prevention.
Consistency is non-negotiable. The dramatic loss of fiber length gains after just 3 weeks of detraining is sobering. Stop NHE for even a short period during competition season or injury recovery, and you’ll lose the length adaptations that help protect against overstretch injuries. The muscle size and fiber thickness persist longer, which is why strength remains relatively stable, but the protective benefit of longer fibers fades fast. For maximum injury prevention, maintain consistent NHE participation, even at reduced volume.
The biceps femoris long head doesn’t respond like the others. Despite being the focus of most previous research due to its high injury rate, it shows only a modest 9% growth compared to the semitendinosus at 26%. This suggests that while NHE is effective for injury prevention across the board, other hamstring muscles may benefit more from the stimulus. This doesn’t diminish NHE’s value for the long head, but it highlights why a comprehensive training approach matters.
Practical application: Perform NHE 2 to 3 times per week without missing multiple weeks in a row. You’ll build protective muscle architecture. Don’t expect the exercise to hit all four hamstring heads equally; focus on multiple exercises to target the entire hamstring muscle group for resilience and more complete development.
Reference
Pai S, A., Andrews, M. H., Gurchiek, R. D., Pincheira, P. A., Barbieri, M., Friedrich, T., Kogan, F., Gold, G. E., Mazzoli, V., Lichtwark, G. A., Delp, S. L., & Chaudhari, A. S. (2025). Hamstring muscle architecture and microstructure changes following Nordic hamstring exercise training and detraining. Journal of Sport and Health Science, 14, 101070. https://doi.org/10.1016/j.jshs.2025.101070
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