| | ACL Injury Prevention: Injury Mechanisms

Author / John

Injury prevention in strength and conditioning is an interesting beast.  If an S&C coach does their job correctly, non-contact injuries are an afterthought in the athlete or team’s success.  If done incorrectly, there is only one person to blame and that S&C coach better take a good look at their program and pinpoint where they failed to prepare their athletes.

Unlike strength or speed, injury prevention practices are difficult to measure, and a coach rarely sees a deficiency until non-contact injuries occur.  From an athlete’s perspective, the potential loss of a season, long road to recovery, and long term disability can be costly and life altering.

Torn-ACL-Prevention-Training-Power-AthleteThe anterior cruciate ligament (ACL) tear is one of the most common serious non-contact injuries to the knee joint.  An astonishing 70% of ACL injuries occur in a non-contact moment, typically during deceleration, lateral pivoting, or landing tasks associated with high loads on the knee joint (1).  These actions are unavoidable in the sporting arena, often occurring instantly without an athlete thinking about their knees.  It only takes a single moment of high dynamic loading for something to go awry.

A strength coach must be aware of the greater risk female athletes face for non-contact ACL injuries, currently reported to be 4-6 times more likely than male athletes in comparable high-risk sports (2).  Many factors contribute to this underlying gender disparity, including related extrinsic (physical and visual perturbations, bracing, and shoe-surface interaction) and intrinsic (anatomic, hormonal, neuromuscular, and biomechanical differences between genders) variables (3).


Photo credit: Johanna Zabal, Head S&C Marymount Univeristy (@CoachJo_CSCS)

Mechanisms for Injury

There are many factors that play a role in a non-contact ACL injury, including the vital aspects of the sport task situation, the athlete’s action and interaction with an opponent, the body’s biomechanic synergy, and the action of the injured joint/tissue biomechanics.

This article will present four ACL injury mechanisms that are modifiable neuromuscular control deficits, which can be effectively attacked in training through understanding, proper integration, and execution.  Far too many coaches improperly apply preventive techniques or corrective exercises.  The work is being done without prudency.  Corrective exercises must correct, and potential risk factors and mechanisms of injury must be understood before application.

Ligament Dominance

Ligament dominance is most familiar to the coach’s eye.  This is mostly present in taller, leaner athletes such as volleyball and basketball players, and especially in teenage athletes whose muscles have not caught up to their growth spurt.  Ligament dominance occurs when muscles do not sufficiently absorb the ground reaction forces, forcing the joint and ligaments to absorb force over a short time period, with total volume depending on the sport.  A high amount of force sustained over a short period of time leads to higher impulse forces, which likely results in ligament rupture (3).

ACL-Injury-Prevention-Valgus-Knee-Power-AthleteThe large, powerful muscles of the posterior chain are designed to absorb and dissipate force through flexion and extension in the sagittal plane.  The glutes, hamstrings, and calf complex must be properly recruited to absorb the substantial reaction forces imparted by the ground. Otherwise, the forces travel to the knee joint and ligaments.  Without the posterior chain activating, a collapse inward occurs, evidenced internal rotation at the hip upon landing and valgus knee towards the midline.  Muscles which allow frontal plane movement at the knee are not as effective at dissipating forces, which then end up at the joint and ligament.

This control imbalance between the neuromuscular and ligament control of dynamic knee joint stability is demonstrated by an athlete’s inability to control lower extremity frontal plane motion during high-risk sports and in the weight room.  The issue is exacerbated when training programs have athletes move through the frontal plane, instead of using the primary movers through the sagittal plane.  Muscles can only protect if they are contracted appropriately to control the imposed load (4), but, thanks to their training, their neuromuscular control only knows to load through the frontal plane, versus into posterior chain through the sagittal plane.

Quad Dominance

Quadriceps dominance occurs when an athlete uses their quadriceps instead of the posterior chain muscles to control, compress and stabilize the knee.  Over time, an imbalance between knee extensor and flexor strength, recruitment and coordination is developed.  This relates very closely to ligament dominance, and is especially prevalent in female athletes who typically first activate their quadriceps during deceleration (3).

Amanda-Ruller-Power-Athlete-ACL-Injury-PreventionThis deficit is not a great strategy for an athlete because they’re using a single muscle group with a single tendinous insertion for stability and control, not only putting that tendon at risk, but also increasing stress on the ACL (3).  The primary action of the quadriceps is knee extension and force production, with only the rectus femoris assisting with hip extension action.  While the quad can assist in deceleration, solely relying on this muscle group for force reduction can overload the muscles, patellar tendon, or ACL.

Athletes that demonstrate quadriceps dominance may increase their risk of ACL injury when they cut and land.  Evidence of a quad dominant athlete can be seen during jumping, landing, squats, and even deadlifts.  An athlete who lands with small knee flexion angles, avoids eccentric loading of the posterior chain, and catches in flat foot position demonstrates characteristics of a quadriceps dominant athlete. They will likely demonstrate excessive landing contact noise (1), unlike a ninja.  These same faults will be witnessed on the squat, via avoidance of eccentric load during the descent.

Quad dominance during the deadlift is an execution error that can still result in a successful lift.  As demonstrated in the video below, the hamstring is no longer the primary mover when the shoulders creep behind the bar and the knees shoot in front during ascension of the barbell.  While an athlete can mark it in their training log, the rep was not prudent for ACL injury prevention once the posterior chain gave way to the quadriceps.


Primary activation of the hamstrings versus the quads and having a posterior chain dominant athlete is necessary when training athletes, recreational or serious.  The hamstrings pull the tibia posteriorly and take stress off the ACL while the quadriceps pull forward adding to the anterior stress on the ACL (3).  While strengthening the hamstring is an obvious solution most coaches apply, leg curls alone cannot attack this limiting factor.  It is not just about building strength in the hamstring, it is about educating it how to load and reduce force, which will be covered in ACL Injury Prevention: Neuromuscular Reeducation.


The third modifiable neuromuscular control deficit is lower limb asymmetry, as most athletes have preferred legs for planting and kicking/driving.  To identify imbalances and subsequently, injury risks, use the single leg assessments per Plyometric Training: Assessment.

Power-Athlete-Team-Series-ACL-Injury-PreventionA few things to consider:

  1. The difference between limbs in muscle recruitment patterns, strength, and flexibility, tends to be greater in women than men (3).
  2. When muscle strength and size deficiency is expected for asymmetry sport athletes (soccer, tennis, baseball), identify alignment problems and stability through full range of motion.

Trunk Dominance

The final modifiable neuromuscular control deficit is trunk dominance. This poses the greatest risk.  This deficit is the inability to precisely control the trunk three dimensionally, an imbalance between the trunk’s inertial demands and the control and coordination to resist it (1).  Athletes, typically females, who do not adequately sense trunk position, or allow greater movement following landing or change of direction, have greater risk of future knee, ligament, and ACL injury (3).  Factors contributing to trunk dominance include growth and maturation factors, frontal and transverse plane neglect in training, and limited spatial awareness.

ACL-Prevention-Training-Power-AthleteWhen the trunk is out of control, the center of mass risks moving outside the foot base.  Sometimes proper foot planting isn’t enough to compensate for trunk dominance, so you can imagine how a misaligned step combined with loss of trunk control can be disastrous.  Very large ground reaction forces exist outside of knee joints and ligaments, especially moving at high speeds on the field or court.  These high forces and torques are loading the joints and ligaments, and when the center of gravity is not controlled, these forces increase and begin to travel in multiple directions.  As discussed in Plyometric Training: Landing, control of the trunk is essential for not only plyometrics, but all high force, multidirectional activities.

Empowering Performance

These four neuromuscular imbalances comprise the formula for a potential non-contact torn ACL. Contributing factors are affected by trunk position and control.  Power Athlete constantly harps on posture and position and hopefully the message is clearer now more than ever.

Address imbalances with intelligent, targeted plyometrics, footwork, weight lifting, and change of direction.  Our next article with introduce movements that target these four mechanisms, focusing on trunk control, knee position, and posterior chain dominance through all three planes of motion.

Many studies and several meta-analyses of literature show that prevention programs attacking the above four critical neuromuscular imbalances reduce risk of an ACL tear between 30 and 80 percent (3).  If a female athlete is 4-6 times more likely than a male to rupture her ACL, an intervention that can gradually decrease risk from 4-6 times to 1 reduces the risk by 80 percent.  Now that’s empowering performance.


1) Myer, G. D., Brent, J. L., Ford, K. R., & Hewett, T. E. (2011). Real-time assessment and neuromuscular training feedback techniques to prevent ACL injury in female athletes. Strength and Conditioning Journal, 33(3), 21–35.
2) Myer, G. D., Chu, D. A., Brent, J. E., & Hewett, T. E. (2008). Trunk and Hip Control Neuromuscular Training for the Prevention of Knee Joint Injury. Clinics in Sports Medicine, 27(3), 425–ix.
3) Hewett, T. E., Ford, K. R., Hoogenboom, B. J., & Myer, G. D. (2010). Understanding And Preventing ACL Injuries: Current Biomechanical And Eepidemiological Cconsiderations – Update 2010. North American Journal of Sports Physical Therapy : NAJSPT, 5(4), 234–251.
4) Verkhoshansky, Y., & Siff, M. (2009, pg. 472). Supertraing: 6th Edition. Rome: Ultimate Athlete Concepts.


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John Welbourn is CEO of Power Athlete and Fuse Move. He is also creator of the online training phenomena, Johnnie WOD. He is a 9 year veteran of the NFL. John was drafted with the 97th pick in 1999 NFL Draft and went on to be a starter for the Philadelphia Eagles from 1999-2003, appearing in 3 NFC Championship games, and for starter for the Kansas City Chiefs from 2004-2007. In 2008, he played with the New England Patriots until an injury ended his season early with him retiring in 2009. Over the course of his career, John has started over 100 games and has 10 play-off appearances. He was a four year lettermen while playing football at the University of California at Berkeley. He graduated with a bachelor's degree in Rhetoric in 1998. John has worked with the MLB, NFL, NHL, Olympic athletes and Military. He travels the world lecturing on performance and nutrition for Power Athlete. You can catch up with John as his personal blog on training, food and life, Talk To Me Johnnie and at Power Athlete.


  1. Ingo B on January 22, 2015 at 9:19 am

    Dominick Cruz is guilty of this. He destroyed his comeback momentum. Again.

  2. menacedolan on January 22, 2015 at 11:35 am

    Minus the factors of Hormone fluctuations that weaken the ligaments and differences in Q angles between athletes (both non modifiable), you nailed it.
    Great work Tex!!

  3. simonsglcfc on January 23, 2015 at 12:38 am

    Hi Tex,

    Do you have an email I can reach you on?


  4. Luke Summers on January 23, 2015 at 3:23 pm

    @mcquilkin, must I remind you that I’m a STANDOUT STATE CHAMP in a real sport where you use your body to conquer your opponent, not a little game where you play catch with a stick.

    Besides that SIGNIFICANT oversight, decently written article

  5. jhackett14 on January 29, 2015 at 6:46 am

    Enjoyed the article, and it kind of hits home with me a little bit. My cousin who is a high school senior basketball player that has already signed with Mercer in Georgia, tore her ACL Tuesday night coming to a jump stop. So I am anxiously awaiting the next article to help prevent this is girls as well as guys. Our football team this year had one ACL and meniscus injury. The girls basketball team at my school had also had 2 ACL injuries so far. So needless to say I can’t wait to read the next article.

  6. […] of the primary mechanisms for a torn ACL in contact sports is the inability to control the body at top end speed.  Learn […]

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