How prevalent are hamstring strain injuries (HSI) in sport?
Hamstring strain injuries have progressively increased in soccer, to a level which has and still does leave them as the most common soft-tissue injury in the sport (1). Generally, HSI occurs during sprint- or stretch-induced activities (2), and although have a relatively low absence time, have high reinjury rates (3). Shockingly, hamstring injuries alone cost Europe’s top 5 leagues €104,000,000 in the 2023/2024 season and have been found to negatively impact expected final league ranking (4). Considering that each final league ranking in the English Premier League was worth £2,200,000 last season, bettering our understanding of HSI management is well-worth considering.
Why Hip extension and not knee flexion?
Traditional hamstring exercise and assessment is characterised by knee flexion dominant movements. This is largely due to the fact that knee flexion largely isolates the hamstring muscles from the remainder of the lower-limb muscles. However, substantial evidence exists to place greater importance on the hip extensor muscles vs. knee flexors for sprinting and maximal acceleration, and for HSI management. Therefore, traditional methods have been challenged (5), and a refocus towards hip extension as a driver for performance enhancement and injury management has been made (6).
Targeting the culprit, the biceps femoris long head (BFlh)
The biceps femoris long head (BFlh) is often documented as the most common HSI, at 83% of total HSI’s (7), with mechanisms generally involving high-speed running and sprinting movements. Studies have found that the BFlh is up to 45% more prone to fatigue in soccer protocols vs. the other hamstrings (8), which is partly explained by a reduction in hip flexion during running and subsequent reduced hip extension power. Traditional hamstring exercises like prone leg curls and Nordics primarily target the medial hamstrings (semitendinosus and semimembranosus) at shorter muscle lengths (9, 10). However, studies suggest that training the BFlh as a hip extensor may be more effective for injury prevention than during knee flexion. Here’s how:
- Selective BFlh activation: Hip extension exercises, such as glute bridges and hip thrusts, and Romanian deadlifts selectively target the BFlh due to its increased moment arm at the hip vs. at the knee.
- Fascicle length adaptation: Training at different muscle lengths, particularly through eccentric (muscle lengthening) contractions with hip flexion, can increase fascicle length properties, an important factor for injury prevention.
- Improved strength endurance: Hip-dominant training enhances hamstring strength endurance to a greater extent than knee-dominant training, which is crucial for preventing fatigue-related hamstring strains that often occur towards the end of competitions.
Target the synergist, the gluteus maximus (Gmax)
During sprinting and maximal acceleration, the Gmax plays a crucial role as the prime mover, showcasing high amounts of muscle excitation and generating significant torque that contributes to horizontal force and forward propulsion (11, 12, 13). In contrast, the hamstrings act more as force transducers (14) and are therefore somewhat secondary to the Gmax.
Studies have shown that poor concentric hip extension torque (15) and reduced Gmax function during the front swing phase of sprinting (16) increases HSI risk. In addition, fatiguing the Gmax prior to exercise elicits substantial increases in BFlh muscle excitation, potentially increasing risk of injury (17). Furthermore, whilst the hamstrings act first to resist lengthening forces under controlled conditions (18), neuromuscular fatigue during repeated sprinting may lead to increased reliance on the Gmax to counteract these forces and protect the hamstrings from excessive strain.
Therefore, enhancing Gmax force production and muscle excitation through targeted hip extension training may be an effective strategy for managing HSI risk (19). The barbell hip thrust is one exercise proven to substantially activate the Gmax (20, 21) where a 42% increase in hip extension moment has been found when compared to a barbell back squat (22) such efforts to increase Gmax muscle function are likely to improve the coordinative components of combined hamstring and Gmax force coupling and reduce the load imposed on the hamstrings during exercise.
Harnessing the base; hip extension for trunk control
Lumbo-pelvic control plays a crucial role in hamstring injury (HSI) prevention. An anterior pelvic tilt can irritate the hamstring origins, potentially leading to nerve inhibition and increased hamstring length (23). Athletes with an anterior pelvic tilt during running exhibit an increased HSI risk due to several factors:
- Increased hamstring length: The anterior tilt positions the hamstring origins further from their insertion points, increasing muscle length during activity.
- Altered muscle coordination: Deficient synergistic coupling between the Gmax and hamstrings and subsequent muscle excitation and force generation.
- Reduced trunk muscle activity: Decreased activity of trunk muscles contributes to instability, reduced force production and increases the risk of injury (24, 25).
In relation to the above, reduced Gmax function is a contributing factor to anterior pelvic tilt, which can lead to increased erector spinae activity (26) and hip flexor tightness (27, 28), further compromising lumbo-pelvic control. Posteriorly tilting the pelvis during exercise therefore may be beneficial due to increased Gmax activation (29) and strengthening the hip extensors may reduce overall pelvic motion during running (30).
Finally, inefficient trunk control can also contribute to “dynamic anterior pelvic tilt”, characterized by an anterior-to-posterior tilting or “jolting” motion during the swing phase of running. This rapid change in pelvic position, combined with knee extension, may subject the hamstrings to significant stretch and potential strain due to rapid changes in muscle length and tension in short timeframes (31).
Hamstring strength testing
Considering the key role the hip extensors play in the BFlh injury discussion, it comes as a surprise that many hamstring strength tests remain to be knee flexor dominant (32, 33, 34). By nature, these tests largely selectively target the medial hamstring muscles (SM & ST) rather than the most commonly injured BFlh, and do so in positions of limited hip flexion, leaving the hamstrings at a relatively short length for assessment.
Interestingly, isometric knee flexion assessments tend to elicit peak forces of between 400 to 600 N, whereas
Traditional hamstring assessments are also largely compromised by setup orientation, limited force production, open chain movements, and a non-specific direction of force application. Each of these factors, and more, pose a threat to the quality of data that Sports Science and Medicine practitioners are fed from sports technology, and may lead to misinformed results and subsequent training prescription. Further reading on this topic can be found in our Gold Standard Research and Development for Hip Extension Assessments article.
The future of hamstring training and assessment
At Metrics, we have designed and implemented an alternative option for “hamstring assessment” across the past 8 years in elite sport. The Hip Extension Bench (HEB) is a patented hip thrust style isometric exercise, holding the capacity to selectively target the BFlh or the Gmax depending on the hip flexion angle you choose to test at. The system also elicits forces of up to and above 4-fold that of knee flexion counterpart assessments. The HEB leans heavily on stringent and detailed research and design considerations, backed by a PhD thesis. This enables practitioners to have total confidence in the quality, transparency and relatedness of the data that it extrapolates. Our system has 6 Key Features that we believe set it aside from competitors, providing great strides in the right direction to better understand hamstring injury risk and subsequent reduction in the coming years.
Reading list:
1. Ekstrand et al. (2022) - https://pubmed.ncbi.nlm.nih.gov/36588400/
2. Tyler et al. (2017) - https://pubmed.ncbi.nlm.nih.gov/27632842/
3. Ekstrand et al. (2020) - https://pubmed.ncbi.nlm.nih.gov/31182429/
4. Turnbull et al. (2024) - https://pubmed.ncbi.nlm.nih.gov/38514294/
5. Guex et al. (2013) - https://pubmed.ncbi.nlm.nih.gov/24062275/
6. Morin (2014) - https://journal.aspetar.com/en/archive/volume-2-issue-3
7. Ekstrand et al. (2012) - https://pubmed.ncbi.nlm.nih.gov/22144005/
8. Wilmes et al. (2021) - https://pmc.ncbi.nlm.nih.gov/articles/PMC8594518/
9. Bourne et al. (2017) - https://pubmed.ncbi.nlm.nih.gov/27660368/
10. Hegyi et al. (2019) - https://pubmed.ncbi.nlm.nih.gov/30230042/
11. Edouard et al. (2018) - https://pubmed.ncbi.nlm.nih.gov/30555346/
12. Bartlett et al. (2013) - https://pubmed.ncbi.nlm.nih.gov/24218079/
13. Hoskins & Pollard (2005) - https://pubmed.ncbi.nlm.nih.gov/15922230/
14. Morin et al. (2015) - https://pubmed.ncbi.nlm.nih.gov/26733889/
15. Hoskins & Pollard (2005) - https://pubmed.ncbi.nlm.nih.gov/15922230/
16. Sigiura et al. (2008) - https://pubmed.ncbi.nlm.nih.gov/18678956/
17. Schuermans et al. (2017a) - https://pubmed.ncbi.nlm.nih.gov/28263670/
18. Iguchi et al. (2023) - https://pubmed.ncbi.nlm.nih.gov/37616534/
19. Motomura et al. (2019) - https://pubmed.ncbi.nlm.nih.gov/30430278/
20. Contreras et al. (2015) - https://pubmed.ncbi.nlm.nih.gov/26214739/
21. Williams et al. (2021) - https://pubmed.ncbi.nlm.nih.gov/33332802/
22. Otsuka et al. (2021) - https://pubmed.ncbi.nlm.nih.gov/34197487/
23. Panayi (2009) - https://pubmed.ncbi.nlm.nih.gov/20538228/
24. Schache & Murphy (2000) - https://pubmed.ncbi.nlm.nih.gov/10953901/
25. Schuermans et al. (2017b) - https://pubmed.ncbi.nlm.nih.gov/28704884/
26. Tateuchi et al. (2012) - https://pubmed.ncbi.nlm.nih.gov/22464201/
27. Van Gelder et al. (2015) - https://pubmed.ncbi.nlm.nih.gov/26618061/
28. Mills et al. (2015) - https://pubmed.ncbi.nlm.nih.gov/26673683/
29. Kim & Seo (2015) - https://pubmed.ncbi.nlm.nih.gov/25995555/
30. Ford et al. (2013) - https://pubmed.ncbi.nlm.nih.gov/23274608/
31. Fousekis et al. (2011) - https://pubmed.ncbi.nlm.nih.gov/21119022/
32. Opar et al. (2013). - https://pubmed.ncbi.nlm.nih.gov/23886674/
33. Hickey et al. (2017) - https://pubmed.ncbi.nlm.nih.gov/29073840/
34. Wollin et al. (2016) - https://pubmed.ncbi.nlm.nih.gov/25683733/
