When most coaches think about building stronger hamstrings, they think about muscle.
But behind every strong hamstring is an equally important piece of tissue that rarely gets mentioned: the tendon and aponeurosis.
These are the connective “ropes and sheets” that transmit muscle force to bone.
Two recent papers from Lazarczuk and colleagues (2024 & 2025) help paint a clearer picture of how the hamstring muscle–tendon unit actually adapts.
One looked at short-term resistance training. The other compared elite sprinters and jumpers to recreational athletes. Together, they show the difference between 10 weeks of lifting and 10 years of sprinting.
Lets break them down...
Short-Term Muscle Gains
Lazarczuk et al. published a study in 2024 investigating the short term effects of training on hamstring muscle-tendon geometric adaptations to resistance training.
The researçhers asked, can 10 weeks of training change the size or shape of the tendons and aponeuroses that transfer that force?
Methods
Thirty recreationally active men trained twice per week for 10 weeks in one of three groups:
- Nordic Hamstring Exercise (NHE): eccentric only, supramaximal load
- 45° Hip Extension: progressive concentric–eccentric loading
- Control: no intervention
MRI scans measured:
- Muscle volume
- Free tendon and aponeurosis volumes
- Biceps femoris long head (BFlh) muscle–aponeurosis interface area
- Regional cross-sectional area (CSA) along the length of each muscle
Results
- Muscle grew, tendons didn’t ⮕ Both training groups increased hamstring muscle volume (especially mid-belly of the semitendinosus and BFlh), but free tendon and aponeurosis volumes showed almost no change.
- No change in interface area ⮕ The BFlh aponeurosis area stayed the same in all groups.
- Muscle-to-interface ratio ⮕ Increased in the hip-extension group; meaning the muscle grew faster than its connective tissue.
- Practical Interpretation ⮕ short-term training builds the “engine” faster than the “rope,” potentially increasing localized strain at the point where the two meet (the same region where hamstring strains occur most often).
Takeaway
Ten weeks is long enough to grow muscle and add strength, but too short to remodel tendon or aponeurosis tissue. Tendons adapt slowly, taking months or years of high-strain exposure to truly change their structure.
This has training and injury implications, as the authors stated:
"Given the lack of adaptation in tendon geometry, practitioners may need to consider how exercise-programming choices affect the ratios between the size of the force-generating muscle and force-transmitting tendon/aponeurosis."
Long-Term Tendon Adaptations
Following the 2024 study, Lazarczuk et al. published a study in 2025, examing the differences in hamstring muscle-tendon unit geometry and function between elite sprint and jump athletes and recreational controls.
The researchers asked, what does the hamstring unit look like in elite athletes who’ve spent years sprinting, jumping, and lifting?
Methods
Researchers compared 15 elite sprinters/jumpers to 15 recreationally active men.
They used MRI to measure all hamstring muscles, tendons, and aponeuroses, and magnetic resonance spectroscopy (MRS) to estimate fiber type through carnosine content (a proxy for fast-twitch muscle).
Results
- Muscles and tendons both grew ⮕ Elites had larger muscles (ST, BFlh, BFsh) and both free tendons and aponeuroses were significantly larger.
- Interface area scaled proportionally ⮕ The BFlh muscle–aponeurosis interface was much larger in elites, but the ratio between muscle size and interface area stayed balanced; a sign of coordinated adaptation over time.
- Fiber type shifted faster ⮕ Elites showed about 1.5× higher carnosine levels, reflecting a greater proportion of Type II fibers.
- Performance links ⮕ Sprint velocity correlated with SM + ST geometry and fast-twitch profile (R² ≈ 0.65); Nordic force correlated with BFlh + SM muscle volume and ST tendon volume (R² ≈ 0.59).
Takeaway
Years of high-intensity loading lead to parallel growth of muscle and tendon/aponeurosis structures. This proportional scaling likely protects the musculotendinous junction and allows elites to handle extreme sprint forces efficiently.
Or, as the researchers put it, "...elite sprinters/jumpers possessed larger hamstrings and were both stronger and faster than recreationally active individuals. Their correspondingly larger aponeuroses and free tendons likely reflect adaptations to withstand the forces associated with high-level performance."
10 Weeks vs 10 Years
When you line these two studies up side by side, the picture becomes clear: muscle adapts fast, tendon adapts slow.
What happens over 10 weeks in the lab looks very different from what develops over 10 years on the track.
Elites look the way they do because they’ve spent years accumulating high-strain mechanical load that gradually thickened and strengthened their connective tissues.
Now of course, tendon changes do not take 10 years, but generally speaking, muscles adapt in weeks while tendons can take months before appreciable improvements are observed.
Coaching Takeaways
- Think beyond muscle ⮕ Strength and hypertrophy come first, but tendon and aponeurosis adaptation is what makes those gains durable.
- Play the long game ⮕ Tendons remodel over months and years. Plan progressive phases that include heavy eccentrics, long-range isometrics, and sprint exposure.
- Balance loading patterns ⮕ Don’t let hip-dominant hypertrophy outpace knee-dominant eccentric work. Keep connective tissues keeping up with the muscle.
- Respect the lag ⮕ After off-seasons, injuries, or rapid hypertrophy phases, connective tissue may be behind. Gradual re-loading prevents the “strong-muscle / weak-rope” scenario that raises strain risk.
In short, ten weeks builds muscle, while ten years builds a balanced muscle–tendon system.
And that balance is what separates a fast hamstring from a fragile one.
I hope this helps,
Ramsey
References
Lazarczuk, S. L., Collings, T. J., Hams, A. H., Timmins, R. G., Shield, A. J., Barrett, R. S., & Bourne, M. N. (2024). Hamstring muscle–tendon geometric adaptations to resistance training using the hip extension and Nordic hamstring exercises. Scandinavian Journal of Medicine & Science in Sports, 34(9), e14728.
Lazarczuk, S. L., Hams, A. H., Bellinger, P. M., Timmins, R. G., Lievens, E., Kennedy, B., Opar, D., Barrett, R. S., & Bourne, M. N. (2025). Differences in hamstring muscle–tendon unit geometry and function between elite sprint and jump athletes and recreationally active controls. Scandinavian Journal of Medicine & Science in Sports, 35, e70151.