ACL Injury Literature Review

In this blog post we will examine recent scientific studies on ACL injury. But why should we care?

ACL injury is costly. Replication of injuries is not feasible and measurement is tough. We establish interesting observations and recurring patterns of non-contact ACL injury. In-vivo studies and regressions show that some populations are at higher risk of non-contact ACL injury. Finally, we review literature that shows that we can screen for problematic movement patterns and institute injury prevention programs. Effective injury prevention programs have been found to decrease non-contact lower injury incidence by 52% in females and 82% in males [p].

We will attempt to provide light and understanding towards:

  1. The Cost of an ACL Injury: Why Should We Care?
  2. Measurement of non-contact ACL injury
  3. How non-contact ACL injury happens
  4. At-risk populations for ACL injury
  5. Can we screen for problematic movement patterns and mitigate ACL injury risk?

I. The Cost of an ACL Injury: Why Should We Care?

The cost of an ACL injury is high. As many as 250’000 individuals are affected and the cost to the American healthcare system exceeds $2B on annual basis, per the CDC [j].

The injured athlete’s quality of life is impacted. The surgery cost of ACL injury is on the order of $10’000 [u][v]. This cost does not quantify the impact to functional health nor quality of life, nor does it include tens of physical therapy sessions.

For athletes with previous ACL injury, rate of return to sport is 81%-82%, previous level of competition is ~64%, and competitive sport is 44%-55% [x][y][z].

Functional health is decreased and return to full health and sport form may be hindered by physical and/or psychological factors (e.g. fear of reinjury). Fear of reinjury is not irrational, as the data show that the probability of a follow-on ACL injury is higher for athletes that have experienced ACL injury than those that have not experienced ACL injury. Return to full lower extremity function can take years, and risk of reinjury during the first year can be higher than risk of reinjury after the first year [g].

Now that we’ve established the costliness of ACL injury, let’s examine the biomechanics in more depth.

II. Measurement of Non-Contact ACL Injury and Biomechanics

Up to 80% of ACL injury is non-contact, therefore further biomechanics and movement pattern investigation is warranted [c]. While the literature review on the topic if ACL injury may be without a clear causal relation, we will focus on and point to recurring themes and other interesting observations.

Biomechanical loading can be performed by in vitro tests of knee and lower extremity load testing. ACL injury analysis in-vivo, however, can only be examined post-injury. Video footage is the main mechanism through which ACL injury mechanisms can be examined.

Measurement Error

In order to characterize ACL injury, we should be able to quantify certain biomechanical metrics such as knee flexion, hip flexion, tibia rotation, valgus angle etc. ACL injury literature tends review ACL injury post-factum utilizing video review technology. Krosshaug et. al. found a 20° - 30° error in ACL analyst researcher knee flexion observed value from true angle [d]

16.7% of undergraduate physical education majors accurately quantified sagittal plane elbow flexion [q].

Although the studies are limited in scope, physical education and movement science experts in the above two studies demonstrate that despite expert domain-level knowledge and experience, experts should be cautious of assuming accurate quantification of body limb angles such as knee and elbow flexion.

Knudson performed a low sample size test in getting college students, kinesiology professors and assistant college basketball coaches to perform visual observation and range of motion scoring in the sagittal plane. 6 out of 10 college students consistently and accurately rated RoM, while only 1 out of 5 kinesiology professors and none (out of 6) basketball coaches were able to do so [r].

Despite the limited sample size and non-randomized control trial, it can be hypothesized that college students are potentially recently trained and exercised in observing and recording ROM values. Conversely, kinesiology professors and assistant collegiate basketball coaches potentially do not actively take part in and/or have never recorded range of motion values in basketball players.

In non-contact ACL injury videos, it is possible to identify large expressions of valgus knee, an overextended leg or an eccentric center of gravity. However, one should be wary in the accuracy of lower extremity movement values extracted by relying on unaided visual observation.

III. ACL injury: how does it happen?

In-vitro tests (i.e. cadaver lower extremity tests)

In in-vitro experiment, Markolf et. al. found that the combination of internal tibial torque and anterior tibial force produced the greatest resultant loading to the knee ligaments [h]. The maximum knee ligament combined loading is most prominent at full extension or hyperextension. Kiapour et. al. found that a combined loading sequence of knee abduction and internal tibial rotation resulted in higher knee strain than uniplanar loads alone [i]. However, Markolf’s and Kiapour’s studies utilized cadaver knee samples in a quasi-static environment, which does not necessarily replicate in-vivo conditions.

DeMorat et. al. found that sudden large force loading of the quadriceps muscle at a low flexion angle (20 degrees) induces knee valgus (2.3 degrees), internal hip rotation (5.5 degrees) and anterior tibial displacement (19.5 mm)[l].

In-vivo video analysis

Hewett et. al. investigated a local USA high school sixth through twelfth grade basketball, soccer and volleyball players for nearly a decade [q]. The research team identifies four characteristics of non-contact ACL injury:

  1. Knee buckling (valgus knee) upon landing.
  2. A relatively straight landing leg (overextended knee or low knee flexion).
  3. Majority or all of the weight is placed on the landing leg (rapid and high quadriceps loading).
  4. A laterally tilted trunk (this condition offsets the center of gravity, and potentially causes knee moment).
[Be aware of valgus knee. Created by EuMotus]
[Be aware of valgus knee. Created by EuMotus]

Hewett defines four characteristics that can be used to describe the differences in risk factors when comparing female to male athletes [q].

  1. Ligament dominance. The ligaments absorb most of the force or impulse from situations such as athletes landing from a jump. The quadriceps muscles are weaker and absorb less of the force, thereby increasing ligament loading.
  2. Quadriceps dominance. Females tend to be more prone to quadriceps dominance. In this case, the quadriceps muscle primarily absorb force. Hewett states that the stiffening of the leg caused by quadriceps dominance is associated with lower knee flexion. Quadriceps dominance is also linked with anterior tibial translation. A counteracting mechanism in reducing anterior tibial translation and thereby ligament stress is the posterior kinematic chain in hamstring activation. IOC states similar findings [c].
  3. Leg dominance. Leg dominance is the athlete’s preference of using a certain foot for activities - e.g. jumping, landing or kicking a ball with one of the feet. Hewett finds association in leg asymmetry to greater risk of lower extremity injury [t]. Leg dominance is especially evident in females.
  4. Trunk dominance. Trunk or core dysfunction dominance refers to the athlete’s inability to control their body in space. Hewett makes reference to accelerated growth years, where male’s muscles grow in relation to their bodies, while women’s muscle to body growth remains similar.

Koga et. al. examined 11 ACL injury episodes utilizing video review techniques [a]. Despite anecdotal evidence (as limited by video footage of ACL injury episodes), and a small sample size, the research team found several recurring themes in ACL injury episodes:

  1. “immediate valgus” (the author noted valgus and the other two conditions are apparent within 40ms of ground contact),
  2. “internal rotation motion”, and
  3. forward moving (“anterior tibial translation”).

Based on these observations, Koga et. al. propose an ACL injury mechanism consisting of lateral knee compression due to:

  1. Valgus loading
  2. Anterior loading “caused by quadriceps muscle contraction“ (e.g. muscle contraction caused by single foot landing)
  3. Tibia anterior translation, combined with internal rotation.

Accordingly, the authors make the recommendation that ACL injury prevention programs should contain an element of eliminating problematic movement patterns during knee flexion characterized by valgus knee and internal rotation of the tibia. The author suggests that a preventative mechanism could potentially be found in utilizing knee and hip flexion to distribute loading forces generated by landing on the ground, as well as avoiding internal rotation. Interestingly, while the author mentions knee and hip flexion as central to knee loading, the author does not mention ankle flexion as a potential aid in reducing knee loading.

In 10 ACL injury video studies of female handball players, Koga et. al. found a neutral mean valgus angle at initial contact and an increase of 12 degrees of mean valgus angle 40 ms later [b]. In addition, the research found that at initial contact, knees had a mean external rotation of 5 degrees, and had rotated internally by a mean value of 8 degrees during the first 40 ms, followed by an external rotation of 17 degrees.

In a review of decades of literature, in vitro in vivo and computer simulation studies, Shimokochi and Shultz found that ACL injuries and high ACL loading tend to happen during the following conditions [k]:

  1. with the leg hyperextended,
  2. during deceleration or acceleration,
  3. during quadriceps muscle contraction,
  4. dith a hip internal rotation,
  5. while lacking hamstring co-contraction,
  6. during combined valgus loading and hip rotation.

Manchester United’s Zlatan Ibrahimovic single leg hyperextended knee landing resulted in ACL and PCL tears.

In their ACL injury literature review, Shimokochi and Shultz also found that knee stress significantly declined at greater knee flexion angles - generally 40/45 to 60 degrees and at 90 degrees, compared to lesser knee flexion angles - e.g. 0 to 40/45. The authors also found that excessive lower extremity hyperextension increases the ACL load level, and therefore increases the chance of ACL injury. Furthermore, Shimokohchi and Shultz found that an internal knee moment combined with a quadriceps force produced greater ACL strain that an external knee moment combined with a quadriceps force.

Kim et. al. hypothesized that tibiofemoral bone bruises were present as a result of non-contact ACL injury [m]. Considering a limited sample size of eight subjects (5 male & 3 female), the researchers utilized magnetic resonance imaging to confirm kinematics hypothesis of key predictors of injury. The kinematics model was constructed by envisioning the potential contact between femur and tibial bones after ACL rupture. The team found three statistically significant characteristics of extended knee ACL injury:

  1. Valgus knee angle of 5 degrees,
  2. Internal tibial rotation of 15 degrees, and
  3. Anterior tibial translation of 22mm.

Kim et. al.’s study was in the range of DeMorat et. al.s’ in vitro study findings on amplitude of lower extremity characteristics of non-contact ACL injury [l][m]. An interesting perspective that Kim brings to light through the review of the literature is that the valgus knee “could be a result of buckling of the knee after ACL rupture.” Kim reinforces that despite observed patterns in vivo, in vitro and during simulation, causality is still not yet clear.

IV. At Risk Populations for ACL Injury

The IOC’s report investigates the higher rate of female non-contact ACL injury head on. IOC states 2x higher rate of non-contact ACL injury for pre-college age (14-18 year old) females than in males [c].

In alpine skiing, 2.5x higher injury rate for females than males [e] (via [c]). Jordan et. al. state that risk factors that can be controlled by the athlete and their fitness and professional staff are centered around prevention of valgus loading, while suggesting hamstring and quad strength and coactivity as contributing factors [f].

Jordan et. al., in a review of 6 alpine skiing studies, found statistically significant different in knee injury female to male risk ratio of 3.1 and 2.3. The other four studies, however, found no difference in knee injury risk incidence [f].

Unfortunately, the literature consistently documents significantly higher risk of non-contact knee injury risk for female athletes, relative to male athletes. It is therefore of great importance for poor form to be screened using movement analysis tech, prior to the occurrence of symptoms such as pain, or even worse, injury. Athletes, coaches, team physios and trainers can and should ask the question: how can we reduce the incidence of these preventable non-contact events.

[Female players have higher incidence of non-contact ACL injury rates[w].]
[Female players have higher incidence of non-contact ACL injury rates[w].]

While there have been multiple studies investigating the friction interface between athlete footwear and playing surface, it is not clear which surfaces present a higher risk of knee injury. On aggregate, in our blog post on artificial turf vs. natural grass, we find no difference in injury incidence. However, different studies pointed to mixed directions.

Literature confirms that then properly designed and implemented ACL injury prevention programs can significantly reduce the incidence of non-contact ACL injury. ACL injury prevention programs are not a one-off event. Rather, systematic integration into the practice routines pre and post-season must be implemented to achieve injury risk reduction.

Sugimoto et. al. quantify the risk of non-compliance, identifying a discrete threshold of 66% compliance, which segments less than 66% compliant injury prevention programs as 3 to 5 times higher ACL injury incidence rate, than programs with more than 66% compliance [n]. Compared with a physical therapy non-compliance or non-adherence of 70%, this raises an important question - what can be done to improve the relationship and trust between patient and provider, athlete and trainer? Does the patient understand the consequence of not adhering to the treatment plan? If they do understand the potential consequence, why do they choose to not complete their treatment program? That’s a topic for another blog post.

Age, BMI, psychological factors (e.g. fear of reinjury) and gender were all related to return to sport rates. Regardless of all factors, the literature points in the direction of a significant number of athletes never achieving the same level of competition to never returning to sport altogether. While modern medicine enables the effective diagnosis and treatment of a significant number of athlete injuries, efforts should be focused on preventing preventable injuries in the first place.

V.Can we screen for signs of and mitigate the incidence of non-contact ACL injury?

Throughout the literature review, the causality of ACL injury is still unclear. While there are signs and characteristics of ACL injury, critical review of the knowledge to date and keeping abreast of the latest literature is recommended.

The above being said, the author recommends athletes, trainers and their PTs consider problematic movement pattern screening and prevention programs to start. Sadoghi et. al. found a 85% risk reduction in ACL injuries for males, and 52% risk reduction for females when participating in an ACL injury prevention program [p].

Considering adopting movement analysis software for problematic pattern screening? Get in touch below.

There are differing factors and faulty movement patterns that could lead to non-contact ACL injury in females than males. Accordingly, Hewett recommends that certain types of preventative exercises are better suited for one gender vs. the other. As an example, Hewett lists that women’s exercises should focus on dynamic trunk or core strength [q]. IOC lists quadriceps dominance and low knee flexion as potential risk factors for females [c].

Yoo et. al. found that combined pre-season and in-season ACL injury prevention programs are more effective than either solely pre-season or in-season only programs [o].

Jordan et. al. identified several screening factors throughout the elite alpine ski athlete literature including functional symmetry assessment, hamstring and quadriceps muscle strength, coactivity and force development (firing) ratios, as well as endurance / fatigue resistance [f].

Shimokochi and Schultz proposed that ACL injury training programs should be designed to prevent frontal and sagittal plane moments to the knee, prevent unopposed quadriceps loading, and should be designed to employ high degrees of knee flexion during acceleration and deceleration in athletes [k].

Kim et. al. reinforce that while causality is not clear, it is important to critically review the current ACL mechanism current understanding to date [m]. Kim suggests that landing on an poorly flexed or overextended knee carries a high degree of risk of non-contact ACL injury. In an injury prevention program, knee flexion mechanism (e.g. landing technique) training should be considered, and landing on a overextended lower extremity should be avoided.


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[h] Markolf, Keith L., et al. “Combined knee loading states that generate high anterior cruciate ligament forces.” Journal of Orthopaedic Research 13.6 (1995): 930-935.

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[u] Accessed October 2017.

[v] Accessed October 2017.

[w] KeithJJ via Pixabay. Accessed October 2017.

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[y] Ardern CL1, Taylor NF1, Feller JA2, Webster KE1. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. Br J Sports Med. 2014 Nov;48(21):1543-52. Epub 2014 Aug 25.

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