Mechanisms of non-contact ACL injuries

In a review of data spanning over a decade, a number of risk factors were identified as far as non-contact ACL injuries are concerned. Female athletes, it appears are at 6 times greater risk than their male counterparts in the same sport and when matched by age, level of play, etc. The review studies are very technical, with the authors having used biomechanical terms for large sections of it. It would negate their work to try to present their work any differently so I have highlighted the key things in an attempt to make it more "user friendly." For those with the prowess for biomechanics, anatomy and physiology, I have posted links to the actual studies at the bottom of the page.

Risk Factor Summary

• The "Position of No Return"
• Weak hamstrings (I discuss risk factors for hamstring injury here, and here.)
• Inefficient gluteal muscles
• Poor or lack of Pelvic Stability
• Poor or lack of Knee Stability
• Core Stability
• Decreased Muscle Stiffness
• Decreased Proprioception
• Fatigue

The greater majority are related to valgus collapse- the knees coming together when you drop down into a squat. This is not a risk factor in itself, but it increased the potential for an ACL injury in the presence of load. However, there is a position the authors dubbed "the position of no return" - hip low forward flexion, hip adduction, hip internal rotation, knee valgus, knee extension, and knee external rotation may place the ACL to a high risk of rupture.

Boden et al. reported a lower extremity alignment associated with non- contact ACL injury in which the tibia was externally rotated, the knee was close to full extension, the foot was planted during deceleration with valgus collapse at the knee [17]. Teitz reported very similar deceleration positions and indicated that most often the center of mass of the body was behind and away from the base of support (area of foot to ground contact) [177].

Anterior pelvic tilt places the hip into an internally rotated, anteverted, and flexed position, which lengthens and WEAKENS THE HAMSTRINGS and changes moment arms of the GLUTEAL MUSCLES [37]. Hamstring muscles are important to prevent static and dynamic genu recurvatum and to prevent anterior tibial displacement. Gluteal muscles are important to assist hip flexion (gluteus maximus) and to prevent a dynamic valgus collapse (gluteus medius). Anterior pelvic tilt also increases knee valgus and subtalar pronation. It is debated whether the risk is caused by the altered pelvic position itself, or by the functional malalignment it creates [167]. What is important in any case, is that PELVIC STABILITY IS KEY.

Femoral torsion is defined as the angle between the axis of the femoral neck and a transverse line through the posterior aspect of femoral condyles [122]. Femoral anteversion, an increase in the mentioned angle, may cause GLUTEUS MEDIUS INEFFICIENCY. A WEAK GLUTEUS MEDIUS may influence dynamic valgus collapse because of the muscles’ inability to keep the hip abducted, especially during weight-bearing activities such as landing, cutting, or changing direction.

Landing, cutting, and pivoting maneuvers in some females have been shown to differ from males [51, 52, 115]. Essentially, female soccer players perform playing actions with increased adduction and internal rotation of the femur, reduced hip and knee flexion angles, increased dynamic knee valgus, increased quadriceps activity (with a concomitant decrease in hamstring activity), and DECREASED MUSCLE STIFFNESS around the knee joint [69].

Studies show that ISOLATED QUADRICEPS CONTRACTION near extension strained the ACL more than exercises with co-contraction of both quadriceps and hamstrings [48]. Chappell et al. [28] found that female soccer, basketball, and volleyball players prepared for landing with increased quadriceps activation and decreased hamstring activation, which may result in increased ACL loading during the landing of the stop-jump task and the risk for non-contact ACL injury.

In contrast, WEAK HAMSTRINGS contribute to a greater ground reaction forces that place the ACL at a higher risk of rupture [71]. On the other hand, peak landing flexion (reflecting net quadriceps muscle activity) and extension moments (reflecting net hamstrings muscle activity) at the knee did not change after training and were not significant predictors of peak landing force.Hamstring muscles are important to decrease anterior shear forces and greatly reduce load on the primary restraint to anterior tibial motion, the ACL [7, 126]. Through knee joint compression, hamstrings limit anterior tibial translation by allowing the concave medial tibial plateau to limit anterior drawer [82] and by allowing more of the valgus load to be carried by articular contact forces, protecting the ligaments [71]. Moreover, hamstring compression could protect against torsional loading, which has been found to be greater for females compared to males [104, 189]. Women demonstrate decreased hamstrings-to-quadriceps peak torque ratios and increased knee abduction (valgus) moments compared to males [71]. Hamstring muscles are activated by ACL receptors when the ligament is placed under stress, which evinces the hamstrings' support to the ACL as an antagonist. This ACL receptor- dependent muscle activation suggests that DECREASED PROPRIOCEPTION could have an impact on KNEE STABILITY.

Muscles crossing a joint provide stability to that joint. In other words, muscle stiffness, or the resistance to dynamic stretch may protect ligaments from rupture when a load is applied. The quadriceps and hamstring muscles provide anterior–posterior joint stiffness. Others suggest that sagittal plane knee joint stiffness is also relevant for ACL injury prevention. Studies demonstrate that female athletes show less muscular stiffness than their male counterparts [58, 59, 67, 79, 88, 161, 186, 189]. Males activate their lower extremity muscles significantly earlier [67], and have longer activation duration in muscles that initiated and maintained knee (gastrocnemius) and lower extremity stiffness (gluteus) than women [88]. DECREASED MUSCULAR FITNESS in females was shown for both anterior tibial translation [58, 59, 81, 88, 186] and rotational forces [58, 59, 161, 189].

Since muscles contribute to joint stability, muscular fatigue might be a risk factor for ligament injuries. FATIGUED MUSCLES are able to absorb less energy before reaching the degree of stretch that causes injuries [108]. Gastrocnemius muscles act as a synergistic and compensatory dynamic knee stabilizer in a closed kinetic chain situations as the quadriceps femoris muscles fatigue [140]. McLean et al. concluded that fatigue-induced modifications in lower- limb control, such as this, may increase the risk of non-contact ACL injury during landings.

Comparative studies have demonstrated that female subjects prepared for landing with a decreased hip and knee flexion angle which may result in increased ACL loading during the landing of the stop-jump task and the risk for non- contact ACL injury [28]. It was postulated that a decreased hip and knee flexion angles at landing places the ACL at a greater risk of injury, because a GREATER PEAK LANDING FORCE is transmitted to the knee [74]. Burkhart et al. [25] reported in a prospective research study that an athlete who landed with an increased heel to flat-foot loading mechanism was more likely to sustain to a non-contact ACL injury during competitive play.

Trunk displacement in any plane was greater in athletes with knee, ligament, and ACL injuries than in uninjured athletes. Lateral displacement was the strongest predictor of ligament injury. Trunk displacements, proprioception, and history of low back pain predicted knee ligament injury with 91% sensitivity and 68% specificity. This model predicted knee, ligament, and ACL injury risk in female athletes with 84, 89, and 91% accuracy, but only history of low back pain was a significant predictor of knee ligament injury risk in male athletes [199]. Therefore, CORE STABILITY may be an important component of ACL injury prevention programs.

Here is the link to Part 1 of the original review article.

In this article, I discuss ways of reducing the risk of injury, now that we know what the risk factors are.