Reducing ACL Injury Risk

In my first summary of a pair of excellent review studies addressing non-contact ACL injuries, I listed the modifiable factors. Review that article here. This article summarizes how and why training can reduce the risk of injury.

You can also access the full review studies here and here. (References in parentheses refer to these articles)

KEY POINT

"The implementation of interventions that incorporate core stability training, including proprioceptive exercise, perturbation, and correction of body sway, has the potential to reduce knee, ligament, and non-contact ACL injury risk in both female and male athletes [79]." By contrast, "continued performance in the presence of faulty technique increases the likelihood of the athlete’s sustaining a training induced knee injury." [56]

It seems that non-contact ACL injuries can be brought down to posture at landing or during movements. Most of the athletes (if not all) who would fall into the the at-risk category are not aware of it. A good start for reducing risk would be education- teach them what it is that is putting them at risk so that they are aware. Consequently, athletes will work with trainers and coaches in their efforts towards "technique modification, proprioception training, and plyometric training [as this] is essential to evoke changes in kinematics and kinetics of joints during sports tasks."

The conundrum of training is that some of the components that will be mentioned here as measures of training effectively, and thereby reducing the risk of injury, are age specific. Young players learn to kick, throw and jump as is appropriate for their level. As they mature physically, training/coaching generally only "matures" in that the intensity is increased- kick further, jump higher, run faster. This is not surprising at all as there is a natural shift from the fun in learning the fundamentals to more results driven competition. Feedback in practice is concerned with tactics, rather than technique; positional play, when it is incumbent upon coaches and trainers to realize that there are fundamentals at every level. But, thus driven, the window at which training technique would be most effective both for performance and injury risk reduction may be passed by. A recent article was published suggesting that "ACL injury risk can be reduced by 72% in players who begin an injury prevention program during their early and mid-teenage years."

STRENGTH TRAINING FOR MUSCLE WEAKNESS

The injury prevention/performance enhancing benefits of strength training do not lie in the strength of the muscle. Instead, they lie in the muscle's ability to activate and react in time and in sequence with the other muscles involved in a movement. Consider the hamstrings.

Hamstrings have an important role in protecting the ACL by preventing or decreasing the anterior, varus-valgus, and rotatory displacement of the tibia on the femur. Hamstring recruitment reduces the load imposed on the ACL from the more powerful quadriceps [155, 185], and by resisting anterior and lateral tibial translation and transverse tibial rotations, may help to provide dynamic knee stability [104].

Strength training is the obvious solution to cases of muscle weakness. Overall, training aimed to improve hamstring-to-quadriceps, hip, and trunk muscular strength is considered adequate to reduce the risk of non-contact ACL injuries. Eccentric loading of the hamstrings was shown, in various studies, to be more effective in increasing hamstring-to-quadriceps strength ratios than traditional concentric exercises in professional and semi-professional male soccer players.

STRENGTH TRAINING FOR ALIGNMENT AND JOINT STABILITY

Dynamic joint stability is provided by muscles as well as the elastic components of the musculotendinous unit, the sensorial and neural system. Single-component preventive programs have limited impact on biomechanical risk factors, as they may be too focused on the muscle component thereby minimizing potential improvements in other important components of the dynamic joint stability function.

Because muscle actions must be coordinated and co-activated in order to protect the knee joint [185], antagonist–agonist relationships are crucial for joint stability. Hence, preseason and continued in-season conditioning focused on hamstring strengthening is indicated. Research also shows that "coactivation of gluteus maximus and medius and hip joint position are essential elements to provide a safe biomechanical profile." Further, "relative weakness of the abductors, extensors, and external rotators, compared to the flexors and adductors, coupled with increased hip flexion , may severely limit the ability of the gluteal muscles to stabilize the hip and maintain a neutral alignment of the hip and knee [69]."

Both joint stability and muscle co-activation were adequately addressed following plyometric training. "The plyometric component of preventive programs trains the muscles, connective tissue, and nervous system to effectively carry out the stretch-shortening cycle and focuses on proper technique and body mechanics [29]. Following training, a significant increase in firing of adductor muscles during the preparatory phase was noted. A significant increase in preparatory adductor-to-abductor muscle co-activation was identified, as well as a trend toward reactive quadriceps-to-hamstring muscle co-activation."

LANDING

Landing presents a load of several time the athlete's body weight on the knee. Typically its the "extended hip and knee joint posture upon landing" that poses a risk. Coaches need to teach their athletes to land softly, "with initial contact at the forefoot with hip and knee flexion with knees over the toes." Also, it was shown that increased trunk flexion during landing avoided excessive anterior translation. This is a more desirable landing technique in order to reduce the risk of non-contact ACL injuries [32, 65]- land on the balls of their feet and sink into the ground, bending knees and hip, as in a squat.

EXPECT THE UNEXPECTED

Cutting maneuvers performed in response to changes in the direction of play, or a dribble constitute a particularly high risk position in soccer. This is mainly because players have so little time to respond if they want to keep up. The results are unplanned movements, unaccompanied by the small steps and other changes in posture which ordinarily decrease the external varus/valgus and internal/ external rotation moments applied to the knee. Warned, players can prepare better and it is therefore advisable to add components of visual cue interpretation to increase the time available for pre-planned movement.

In support, Nyland et al. support programs that focused on coordinated lower extremity closed kinetic chain tasks-

• mini-squats, 
• single-leg vertical and horizontal hopping,
• lateral shuffles in a mini-squat position,
• back pedaling, and 
• quick multidirectional movement responses to cues [56]. They recommended that these tasks should be performed progressively and with an emphasis on movement quality.

Besier et al. further recommended that training should involve drills that familiarize players with making unanticipated directional changes. Applying all reason and precautions for safety, practice should mimic as much as possible the game situation, particularly in speed and tempo. Drills are often slow so that even if players are familiar with certain movements, it is the added speed and perturbations that predispose them to injury during matches.

Applying these will go a long way in decreasing the risk of injury and improving performance as well.