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.

The Wisdom of Coaching II

The Charlie Francis Training System is one of the most powerful books I have ever read. I highly recommend it to anybody who is interested in learning how to train (or become!) a successful athlete. In a previous post, I shared some of the quotes from it. Here I share a few more (all unless otherwise referenced), as well as quotes from other sources:

"The basic principle underlying all training is to bring only a fully regenerated (rested/recovered from a previous performance) athlete to each training element. The element is then performed with the highest quality possible."

"The great majority of athletes are [still] training and performing with hard-spasmed muscle (both organ and skeletal muscle) with the result that a full complement of tissue is not being presented to the training stimulus, let alone involved in performance. This fact becomes doubly significant when one considers that the body must use protective inhibition in the presence of such spasmed tissue in order to survive training and performance without injury."- Paul Patterson

"If you try to communicate with an athlete when he or she is upset, very little of what you say will penetrate."

"I never write programs on paper for an athlete because....this [manner] will prevent the athlete from ever taking responsibility for their own training. Athletes must interpret training programs through their bodies; programs written on paper redirect the athlete's focus and encourage athletes to interpret training programs through their heads."

"Some coaches dump too much information on the their athletes which is either an attempt to prove to the athletes that they are smart or it is a reflection of the coaches' poor communication skills."

"Whenever you are in a really pushy type of program and are always trying to get athletes to do stuff, you can't be listening to them because they are not listening to themselves; they are simply busy trying to do what you asked them to do."

"You must keep the lines of communication with your athletes open, otherwise you are both in real trouble."

"Plan your work, work your plan."

Some thoughts from other authors:

Direction-Specific Leg Power: The traditional test for power has been the vertical jump test, highly relevant considering that we jump quite often for headers- depending on the position, of course. However, to suggest heading is the only place power is utilized in the game is to grossly underestimate its demands. Kicking, running, cutting maneuvers all require power but they are not vertical in direction. As such Jennifer K. Hewit, PhD, CSCS discusses Direction-Specific Leg Power. This seems obvious but we rare quantify power in these 'other-direction' movements, giving them their proper value in the testing process. To quote, "... testing athletes's power capabilities in only one direction limits the amount of information gained from testing that is used to develop and implement appropriate training programs.." "...therefore, assessing athletes' unilateral leg power capabilities across multiple directions is recommended when compiling a complete athlete profile for programming, performance and progression purposes." More info here.

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.