Identifying ACL Injury Risk Factors in Athletes

The anterior cruciate ligament connects the femur to the tibia and is often a major injury site amongst athletes across multiple sports. Research by Hawkins and Fuller previously highlighted the incidence of anterior cruciate ligament injuries within elite senior and academy soccer players, with the most severe knee joint injuries (measured by time away from playing activity) involving the anterior cruciate ligament. McManus and Cross (2004) reported similar findings when investigating injury rates within elite senior and academy Australian rugby union players, with knee injuries (e.g. ACL and MCL sprains, etc.) being recorded as the most ‘costliest’ non-contact injuries based on severity of injury and time away from competitive play (e.g. games and training sessions missed). Knee injury occurrence during tackle-based contacts also accounted for a large number of ACL and MCL injuries within competitive games. Berson et al (1981) reported similar findings within squash players, with over one third of injuries being classified as strains or sprains and over half of the reported injuries involving the lower extremities, with the ankle and knee again being the most common injury sites. It is evident that ACL injury occurrence is unfortunately common across multiple sports, and should therefore be a key focus when preparing athletes who compete in such sports.  

Concentric Quadricep vs Eccentric Hamstring Strength

Knee ligament injuries are the result of excessive shear, compressive and torsion forces occurring within the knee ligament connective tissues, leading to ligament strain. Such injuries can be the result of a combination of factors, accumulating overtime and/or occurring within an instant. A lack of eccentric hamstring strength, in-relation to concentric quadricep strength, has previously been identified as a knee ligament injury risk factor (Myer et al, 2008). Upon landing the hamstrings contract in a paradoxical fashion, creating a posterior force upon the knee. This posterior force aids in reducing the shear forces experienced at the anterior knee connective tissues, including the ACL. Myer et al (2008) previously highlighted the importance of specifically developing hamstring strength to ensure the correct dynamic stabilisation of the knee upon impact (e.g. when landing and whilst in locomotion). Therefore, strength and conditioning coaches must ensure programs are structural balanced by including equal amounts of concentric quadricep (e.g. squats, split squats, etc.) and eccentric hamstring (e.g. nordics, glute ham raises, stiff legged deadlifts, etc.) within performance programs.   

Lumbopelvic Control

Zazulak et al (2006) recently highlighted the association between lumbopelvic control and an increased risk of ACL injuries in female athletes, suggesting a lack of torso positional control when performing change of direction or landing based tasks predisposes female athletes to an increased risk of ACL injury. Likewise, Lephart et al (2005) reported similar findings, suggesting the degree of knee and hip flexion upon landing is directly related to the ability of the soft tissue structures to absorb joint forces within the lower limbs. It is therefore apparent that knee ligament injury prevention measures should include lumbopelvic strength and stability-based training that develops the ability to resist torso rotational forces (e.g. anti-rotation based exercises).

Knee Valgus

Excessive knee valgus moments when performing changes of direction and landing has previously been identified as an ACL injury risk factor (Boden et al, 2010). McLean et al (2005) previously investigated peak knee valgus moments in male and female athletes whilst performing a sidestep manoeuvre and reported significantly greater knee valgus moments in the female group when compared to the recorded knee valgus moments within the male group. The authors also reported that the greater knee valgus moments were associated with greater initial hip flexion, internal rotation and knee valgus angles. These findings demonstrate the need for correct hip control when performing sidestep manoeuvres and the ability to resist excessive hip flexion upon impact, especially within female athletes. 

Tendon Stiffness & Proprioception  

Previously documented ACL injury prevention programs have highlighted the importance of tendon stiffness, with the most reduced ACL injury rates seen in those prevention programs that included plyometric based exercises (Hewett et al, 2006). Ingersoll et al (2008) also previously highlighted the importance of proprioceptive based training (e.g. BOSU single leg balance, etc.) within ACL prevention programs, as a means of increasing sensorimotor function within the lower extremities.

Collectively, these findings highlight the importance of reinforcing hip stability, torso control, reduced knee valgus, eccentric hamstring strength, tendon stiffness and proprioceptive feedback within programs aimed at reducing ACL injury rates in athletes.