PhD Pinboard: Measuring Hamstring Muscle-Tendon Unit Geometry
- Jo Clubb
- 39 minutes ago
- 9 min read
This PhD Pinboard article by Steph Lazarczuk explores the impact of training on hamstring muscle-tendon unit geometry.
I’m Dr Steph Lazarczuk. As a Sport Rehabilitator with experience across football (soccer), rugby, cricket amongst other sports, I’ve seen my fair share of hamstring strain injuries (HSIs) in the medical room. Successfully returning athletes to sport following these injuries has always been a challenge.
We know that hamstring strains are either the most common injury or near the top of the leaderboard for sports involving running, changes of direction, kicking and other potentially injurious mechanisms (1,2). We also know that a large proportion of these injuries happen where muscle and tendon interlink and where most force is being transferred from one tissue to another. This makes the musculotendinous junction and the aponeurosis itself (the part of the tendon which muscle fibres attach to) a region of particular interest (Figure 1).
Note: Add = adductors, BFlh = biceps femoris long head, BFsh = biceps femoris short head, SM = semimembranosus, ST = semitendinosus.
In the second half of the 2010s and into the 2020s, we saw an influx of research describing hamstring muscle adaptations to training and injury. However, given the importance of tendon tissue, I wanted to know how these structures might be altered. I had the questions, but nobody had the direct answers, and so my PhD was born out of a desire to find out what happens to hamstring tendons in the presence of either injury or training activity.
Key Findings
To set the baseline, I wanted to establish what training stimuli are necessary to induce adaptation in lower limb tendons. We chose to focus on three key themes:
change in geometry (i.e., cross sectional area [CSA] of tendon structures)
mechanical properties (i.e., stiffness)
material properties (i.e., elastic modulus).
We found that resistance training led to small increases in tendon CSA, moderate changes in tendon stiffness, and large changes in modulus (3) (Figure 2). High strain protocols delivered through resistance training led to larger increases in tendon stiffness and modulus than low strain protocols.
Strain is the percentage of change in length or width of a tissue relative to its starting length/width, so high strain protocols had greater tendon lengthening during exercises than low strain protocols. For tendons, strain influences adaptation by stressing the embedded tenocytes (the main cell type in tendons) and this leads to various chemical markers being released which then alter the tissue.
Note: apon = aponeurosis; CSA = cross-sectional area; ecc = eccentric; HE = 45° hip extension exercise; inj = previously injured limbs; musc:apon = the ratio between muscle and aponeurosis tissues, i.e., muscle volume to aponeurosis interface; NHE = Nordic hamstring exercise; vol = volume
Earlier work estimated that a narrower biceps femoris long head (BFlh) aponeurosis accompanied by a wider muscle, led to greater amounts of strain being experienced by the muscle tissue at or near to the proximal musculotendinous junction (4,5). This led to a focus on the geometry of these structures for my remaining PhD studies, identifying whether the size of them and the ratio between them, was altered by prior injury or training activity.
There are several methods used to assess tendon geometry. For my PhD, the volumes of tendons and the surface areas of aponeuroses were important and so we favoured MRI to visualise these structures simultaneously.
We found that in limbs with a history of HSI, there were larger proximal BFlh aponeuroses and correspondingly smaller muscle-to-aponeurosis volume ratios in comparison to healthy, control participants who had no history of hamstring injury (6). At the moment, it’s unclear whether these differences in tissue size pre-date injury or whether it is a direct result of it.
[I]n limbs with a history of HSI, there were larger proximal BFlh aponeuroses and correspondingly smaller muscle-to-aponeurosis volume ratios in comparison to healthy, control participants who had no history of hamstring injury
We also investigated the use of the Nordic hamstring exercise or the hip extension exercise over ten weeks on hamstring muscle-tendon unit geometry. We knew from earlier studies that these exercises led to muscle hypertrophy (7), but were able to establish there was a limited effect on hamstring aponeuroses and free tendons.
Interestingly, we found that the muscle hypertrophy wasn’t uniform along the length of the muscle (Figure 3), or between muscles (8). Notably, the hip extension exercise created the largest increase in BFlh muscle hypertrophy. As the muscle size increased more than tendon size, this also increased the muscle-to-aponeurosis ratio, which could have implications for the stress and strain experienced by the muscle fibres.
Lastly, in a cross-sectional study we compared elite sprint and jump athletes who had a long-term history of training with recreationally active individuals, examining the muscle-tendon unit geometry, muscle fibre types, and athletic performance (sprint and eccentric knee flexor strength).
Somewhat unsurprisingly, the elite group had larger hamstring muscles and tendons (including larger aponeuroses), had a greater proportion of Type II (‘fast twitch’) muscle fibres, were faster, and were stronger than the recreationals (9). The ratio between muscle and tendon, however, was not different between the groups suggesting that over time, and with continuous loading, the tendons were able to catch up with muscular hypertrophy.
We also looked at the correlation between muscle-tendon geometry and muscle typology with maximal sprint velocity and eccentric knee flexor force. Typology with medial hamstrings muscle and tendon volumes were associated with maximal sprint velocity, while BFlh and semimembranosus muscle plus semitendinosus tendon volumes were associated with knee flexor force (9). We know from earlier work that muscle volumes are associated with sprint ability, but we have now identified that tendon volumes are also associated with performance.
Practical Relevance of Hamstring Muscle-Tendon Unit Geometry
Sometimes to get to the application, we need to backtrack and describe, understand, or clarify the foundational science, and so this PhD was designed to be a balance between the two perspectives.
The early review reinforces the notion that strain as a stimulus is important for adaptation, and that there are several avenues through which you can apply this. While muscle contraction type was not directly related to tendon adaptations, eccentrics and isometrics may reasonably apply greater strain than concentric actions. Meaning, we can make use of these contraction types in our resistance training programmes to stimulate tendon changes.
We already had a fair idea about what happens to the hamstring muscles when you load them, but little idea about how the tendons fit into the picture. My PhD work shows that 10 weeks of training with the Nordic or hip extension exercises wasn’t long enough to change the geometry of hamstring tendon tissue. However, other recent work reports that change after 12 weeks (10), which is mirrored in other lower limb tendons.
Given the final cross-sectional study of my PhD, which showed that the ratio between hamstring muscle and tendon tissue was similar between trained and relatively untrained populations, it’s plausible that tendon tissue can ‘catch up’ with hamstring adaptations given sufficient time. So as practitioners, we need to consider the consistency of our loading over long durations if we are to create tendon change.
While we know that stiffer Achilles and patellar tendons are associated with better sprint and jump performance, we don’t know what the optimal stiffness for the hamstring tendons might be.
Conclusion and Future Directions
Ultimately, my PhD highlights the need to consider the WHOLE muscle-tendon unit when reporting adaptation and that both structures are important for athletic performance. Muscles and tendons work together for function so ideally we need to be examining both.
The primary focus of my PhD was on the geometry of these muscle-tendon unit structures, and additional work is needed to explore the effect of other exercise-based strategies on hamstring tendon adaptation. This would help to clarify the optimal training variables needed for inducing the adaptations we’d like to see.
Currently our understanding of how the mechanical and material properties in the hamstrings affect performance and injury risk is limited. While some exploration using shearwave elastography and myotonometers has begun, these struggle to provide information about the hamstring aponeuroses as these are thin and deep structures. Elastography sets a ‘region of interest’ which is usually larger than the aponeurosis is thick and myotonometers cannot reach through several layers of skin/fascia/muscle etc., so other structures may contribute to findings attributed to the aponeurosis.
Consequently, we need to find reliable methods to assess the aponeurosis itself. We know that other tendons experience changes in mechanical and material properties quicker than for geometric properties, and presumably the same is true for the hamstrings, but to what extent is unclear.
Finally, additional investigation into the functional implications of changing hamstring tendon properties (either on performance or injury risk or both) will be useful for practitioners moving forward. While we know that stiffer Achilles and patellar tendons are associated with better sprint and jump performance, we don’t know what the optimal stiffness for the hamstring tendons might be.
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Frequently Asked Questions (FAQs)
What is strain in tendon biomechanics?
Strain is simply the percentage of deformation (either change in length or width) relative to its starting length/width. So, if a tendon was 10 cm in length, and then it was stretched by 1 cm, that tissue has experienced 10% strain. You can have positive strain, where the tissue gets longer/wider, or negative strain, where the tissue gets shorter/narrower. Given that our bodily structures must maintain their volume when we start stretching them, if we lengthen a tendon, then we simultaneously narrow it. Strain seems to be important as a stimulus for adaptation and as a mechanism for injury.
How do we measure hamstring tendon geometry?
There are several ways to measure the geometry of tendons. In my studies, we opted for MRI-based measures so that we could view the entire hamstrings muscle-tendon units. Other work has used ultrasound to look at cross-sectional area, thickness and volumes of free tendons and aponeuroses. While 2D ultrasound has been widely employed in research before, more exploration of extended field of view (which shows you a ‘sweep’ of the muscle-tendon unit) and 3D ultrasound have been implemented more recently.
Do we have a good method for assessing stiffness of hamstring tendons?
Not yet. Work with shearwave elastography has begun in the hamstrings but generally focuses on the muscle tissue because the hamstring aponeuroses are too thin to reliably capture. Other research groups have used myotonometers on more superficial tendons but for the hamstrings this would be limited to the most proximal portion of the biceps femoris or semimembranosus tendons or the distal tendons. Too many other structures lie over the top of the BFlh aponeuroses to get a reading from these directly.
References
1. Maniar N, Carmichael DS, Hickey JT, Timmins RG, San Jose AJ, Dickson J, et al. Incidence and prevalence of hamstring injuries in field-based team sports: a systematic review and meta-analysis of 5952 injuries from over 7 million exposure hours. British Journal of Sports Medicine. 2023;57:109–16.
2. Kerin F, Farrell G, Tierney P, McCarthy Persson U, De Vito G, Delahunt E. Its not all about sprinting: Mechanisms of acute hamstring strain injuries in professional male rugby union-a systematic visual video analysis. British Journal of Sports Medicine. 2022;56:608–15.
3. Lazarczuk SL, Maniar N, Opar DA, Duhig SJ, Shield A, Barrett RS, et al. Mechanical, Material and Morphological Adaptations of Healthy Lower Limb Tendons to Mechanical Loading: A Systematic Review and Meta-Analysis. Sports Medicine. 2022;52:2405–29.
4. Fiorentino NM, Epstein FH, Blemker SS. Activation and aponeurosis morphology affect in vivo muscle tissue strains near the myotendinous junction. Journal of Biomechanics. 2012;45(4):647–52.
5. Rehorn MR, Blemker SS. The effects of aponeurosis geometry on strain injury susceptibility explored with a 3D muscle model. Journal of biomechanics. 2010 Sept;43(13):2574–81.
6. Lazarczuk SL, Collings TJ, Hams AH, Timmins RG, Opar DA, Edwards S, et al. Biceps femoris long head muscle and aponeurosis geometry in males with and without a history of hamstring strain injury. Scandinavian Med Sci Sports. 2024 Apr;34(4):e14619.
7. Bourne MN, Duhig SJ, Timmins RG, Williams MD, Opar DA, Al Najjar A, et al. Impact of the Nordic hamstring and hip extension exercises on hamstring architecture and morphology: Implications for injury prevention. British Journal of Sports Medicine. 2017;51(5):469–77.
8. Lazarczuk SL, Collings TJ, Hams AH, Timmins RG, Shield AJ, Barrett RS, et al. Hamstring Muscle‐Tendon Geometric Adaptations to Resistance Training Using the Hip Extension and Nordic Hamstring Exercises. Scandinavian Med Sci Sports. 2024 Sept;34(9):e14728.
9. Lazarczuk SL, Hams AH, Bellinger PM, Timmins RG, Lievens E, Kennedy B, et al. Differences in Hamstring Muscle‐Tendon Unit Geometry and Function Between Elite Sprint and Jump Athletes and Recreationally Active Controls. Scandinavian Med Sci Sports. 2025 Nov;35(11):e70151.
10. Maeo S, Balshaw TG, Nin DZ, McDermott EJ, Osborne T, Cooper NB, et al. Hamstrings Hypertrophy is Specific to the Training Exercise: Nordic Hamstring versus Lengthened State Eccentric Training. Medicine & Science in Sports & Exercise [Internet]. 2024 June 6 [cited 2024 July 16]; Available from: https://journals.lww.com/10.1249/MSS.0000000000003490
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