What are ACL injuries?
The ACL connects the femur to the tibia and helps maintain proper knee movement and stability. Injury gradings typically range from grade 1, a mild sprain causing stretch of the ligament but with maintained joint stability, to grade 3, a complete rupture, fully torn ligament and significantly reduced joint stability. The ligament is often torn centrally and will require surgical intervention or a carefully managed conservative approach for athletes looking to reengage in their sport.
What causes an ACL injury?
An ACL injury is typically caused by a few common mechanisms:
Sudden changes in direction (e.g. performing an offensive step / cut in Rugby, or pivoting in Basketball)

Abrupt stops (e.g. a feint in American Football or pressing an opponent in Soccer)

Jumping and landing (e.g. a jump shot in Basketball or hurdling an opponent in American Football)

Direct impact to the knee (e.g. a tackle in Soccer or a sack in American Football)

How prevalent are ACL’s?
Sports like Soccer, American Football, Basketball and Tennis maintain a particularly high prevalence of ACL injuries. In recent years, knee ligament injuries have also increased substantially in many sports (1, 2, 3) and are now found to be the most costly injury in the top 5 European Soccer Leagues at €141.22 M in the 2023/24 season (4). In Basketball, injury rates are found to be as high as 0.25 ACL injuries per 1000 exposures between 2000 and 2015 (5) and the average cost of recovery is as high as $2.9 M per player (6).

What role does hip extension have in ACL injuries?
Hip extension plays a significant role in the management of ACL injuries. If we think of the key mechanisms involved in ACL injuries (horizontal and vertical deceleration and change of direction), each of them require a triple flexion action, meaning bending of the hips, knees and ankles. The muscles required to decelerate hip flexion and cope with the high ground reaction forces exposed to the body in these movements are the hip extensor muscles. It’s also known that stability around the pelvis is essential to maintain suitable function and force coupling of the distal muscles surrounding the knee and ankle joints. Therefore, with suboptimal hip extensor function, it is clear that the lower extremities may be exposed to magnitudes of force and instability that are detrimental to the joints.
The four key ways in which enhanced hip extension capacity may support ACL injury risk reduction are:
- Decreased knee extensor reliance
- Increased force attenuation at the hip
- Improved landing mechanics
- Synergist to the hip abductors and external rotators
Simply put, a decreased reliance of the hip extensor muscles increases load on the knee extensors and the stress placed upon the ligaments surrounding the knee (7). Therefore, it’s likely that strategies to increase hip strength will provide a protective effect for the knee, by reducing the load tolerance requirement and subsequent shear load on the knee (8). In sport specific terms, individuals with weaker hip extensors are likely to land from a jump with an increased magnitude of load placed on the knee extensor muscles (8). The same is likely the case for change of direction tasks, whereby poorer hip strength will increase the hip to knee reliance ratio (9).
From a kinematics (visual movement) perspective, triple flexion is an aspect of dynamic movement that should provide balance throughout the lower-limbs. When the relative amount of flexion in one joint is reduced, this will likely cause greater reliance elsewhere. This has been evidenced with lower hip flexion angles (9) and increased knee valgus (8) during change of direction and jump tasks in individuals with weaker hips - thus increasing stress about the knee joint.
How can hip extension training reduce ACL injury risk?
From a training perspective, it has been widely shown that appropriate strength and conditioning programmes reduce ACL injury risk (10, 11, 12). Interestingly, participants who become more favourably hip dominant due to strength and conditioning interventions (13) are able to reduce their knee adductor moments in drop landing and change of direction tasks (7). Across an 8 year intervention study in Basketball, a hip strength programme supported the reduction of ACL injury rates from 0.25 / 1000 hrs to 0.10 / 1000 hrs (14).
According to research, it’s largely been the hip abductors and external rotators that are responsible for these key improvements. For instance, Hietamo et al. (2020) (15) found a direct association between lower hip abduction strength and knee ligament injuries, and Khayambashi et al. (2016) (16) evidenced that those who went on to sustain ACL injury had reduced hip external rotation and abduction strength. There is no specific research evidence observing the direct correlation between hip extension strength and ACL injury risk. However, considering the synergistic role of the powerful hip extensor muscles, it would be theorised that these provide a significant supporting role. Further to this, it is interesting to find limited research in this area, perhaps due to the perceived lack of suitable equipment to collect data with… (until now). Therefore, at Metrics we are motivated to continue to broaden our research interests and investigations to provide the field with further answers in the coming months and years. If you’d like to collaborate with us on bettering our understanding of research in any of the fields discussed above, please feel free to get in touch via the contact page here.
References:
1. Project Play (2023) - https://projectplay.org/news/2023/11/22/analysis-serious-knee-injury-among-teen-athletes-grows-26
2. Garcia-Mansilla et al. (2025) - https://isakos.com/2025/Abstract/20521
3. Hunter (2024) - https://www.skysports.com/football/news/11095/12926431/future-of-football-why-acl-injuries-have-been-on-rise-in-womens-game-and-the-technology-and-solutions-to-fix-it
4. Howdens (2024) - https://www.howdengroupholdings.com/reports/2023-24-mens-european-football-injury-index
5. Stojanovic et al. (2023) - https://onlinelibrary.wiley.com/doi/10.1111/sms.14328
6. Vaudreuil et al. (2021) - https://pubmed.ncbi.nlm.nih.gov/34604424/
7. Stearns & Pollard (2013) - https://pubmed.ncbi.nlm.nih.gov/23425687/
8. Pollard et al. (2017) - https://pmc.ncbi.nlm.nih.gov/articles/PMC5593213/
9. Davies (2016) - https://research.stmarys.ac.uk/id/eprint/1178/
10. Magana-Ramirez et al. (2024) - https://pubmed.ncbi.nlm.nih.gov/38395699/
11. Arundale et al. (2023) - https://pubmed.ncbi.nlm.nih.gov/36587265/
12. Grimm et al. (2014) - https://pubmed.ncbi.nlm.nih.gov/25451790/
13. Pollard (2017) - https://pmc.ncbi.nlm.nih.gov/articles/PMC5593213/
14. Omi et al. (2018) - https://journals.sagepub.com/doi/10.1177/0363546517749474
15. Hietamo et al. (2020) - https://onlinelibrary.wiley.com/doi/full/10.1002/tsm2.172
16. Khayambashi et al. (2016) - https://pubmed.ncbi.nlm.nih.gov/26646514/