Strength and conditioning is no longer solely concerned with making an athlete stronger, more explosive or ensuring their energy system demands are up to par for their sport. Now more than ever, parents and coaches want to ensure that their athletes are training to prevent injury. After all, it’s hard to optimize your performance on the field or court if, well…you aren’t on the field or court. Unfortunately, that’s a difficult caveat in itself. Predicting injury is next to impossible. The body is extremely resilient and adaptable, and what may cause issues in one individual may never affect another individual; that is why we see marathon runners with moderate knee valgus (knees cave inwards,) who never experience injury and bodybuilders with excessive thoracic flexion (rounded upper backs) who can overhead press pain-free. They’ve been able to build up gradual tissue load tolerance, conditioning the tissue to handle whatever tasks you are asking of it, the body has adapted to the perceived ‘faulty’ pattern, and as such they don’t experience any negative effects to those movements (or probably never will if they’ve made it this far). In elite athletes, altering their biomechanics to what you feel is ‘more ideal’ or to get them out of a position that you feel is a ‘faulty’ movement is a slippery slope – they have made it to the elite level they are at for a reason (unless pain is present, be careful messing with a good thing).
When it comes to training however, research has provided some rather glaring evidence pertaining to dynamic injury mechanisms (which are a completely different beast than the functional, overuse and ‘postural’ related injuries mentioned above). One of the most common issues that coaches attempt to address in training is knee valgus (knee caves inward), and educating the athlete on controlling and absorbing forces at the knee properly. Excessive knee valgus that exceeds the load that the ACL can handle has been correlated with ACL injury in numerous studies. However, it’s NOT the only contributing factor that needs to be addressed in an evidence-based ACL injury prevention training program ACL…
It’s well documented now that females are more prone to non-contact ACL injuries compared to their male counterparts (up to 2-10x greater risk), and that females generally have larger amounts of knee valgus that takes place at the time of injury (Hewett et al, 2009). As such, this is one of the primary focuses that coaches emphasize when training the female athlete. But wait, how do we explain other studies that show ACL tears occurring in males despite significantly decreased knee valgus angles compared to their female counterparts at the time of injury (Krosshaug et al, 2009)? Males are less tolerant to the load? Perhaps, but not likely. Clearly, knee valgus isn’t the only major factor we need to control as strength coaches.
The Other Factors – things you should also consider in your ACL injury prevention training programs.
- Knee flexion angle
The knee is specifically designed to act as a shock absorber for the body. Issues begin to arise when we perform movements that don’t maximize the beneficial capabilities of the knee. Case in point, improperly performing dynamic tasks under load such as cutting, pivoting and change of direction. Studies have shown, rather convincingly, that the vast majority of non-contact ACL injuries occur when there is less than 30 degrees of knee flexion (ie. during a hop step or layup) at both initial contact and 40 milliseconds after contact (which is approximately the moment when the ACL actually tears) (Koga et al, 2010). Knee valgus alone isn’t enough to promote ACL injury; knee valgus + an extended knee + an external load/force = INJURY. Training tip: during jumping, landing, lateral acceleration and deceleration, or any other instance where change of direction is rapid and load is high, teach the athletes to land and absorb the forces at a MINIMUM of 30 degrees knee flexion. Land softly to reduce vertical ground reaction forces.
- Center of Gravity/Increased Heel Loading
Foot position plays an important role in promoting stability, allowing for an explosive push-off in linear and lateral tasks, and is a key component to the universal ‘athletic stance’ seen in virtually every sport that involves foot-to-ground contact. It turns out that the way we load the foot plays a role in ACL injury as well. Upon video analyses of 54 ACL injuries occurring in basketball and soccer, it was noted that a vast majority of the injuries occurred when the center of gravity was located behind the knee. Furthermore, ‘flat-foot’ contact was noted in two-thirds of the injured females, and all of the injured males (Griffin et al, 2000). Placing an emphasis on landing with the toes makes it virtually impossible for the center of mass to be behind the knee (Griffin et al, 2000). Training tip: when accelerating and decelerating in a linear direction, teach the athletes to remain at a minimum of 30 degrees of knee flexion, and never attempt to decelerate using the heel while the leg is in a straightened position.
- Trunk Control
Our bodies are designed to be as efficient, and natural as possible. We learned here that inefficient athletes are essentially weeded out via natural selection at higher levels of play, and we learned from the same blog that natural strategies of trunk control should always be a main priority in training. The same principles apply for ACL injury prevention. Female athletes generally have a tendency towards knee valgus combined with lateral trunk movement away from the direction of collapse (Donnelly et al, 2012) (ie. knee caves in one way, body keeps travelling in the opposite direction). Training tip: placing an emphasis on maintaining trunk control and redirecting the whole-body center of mass towards the desired direction of travel can drastically help reduce incidence of ACL injuries (Donnelly et al, 2012).
The Wrap Up – what you need to know.
- Knee flexion angle is one of the biggest contributors to ACL injury – when landing, cutting or changing directions, always ensure you remain in an athletic stance with the knees bent to a minimum of 30 degrees.
- Make sure your center of mass is never behind your body – when landing from a jump, decelerating from a sprint, or changing directions, never attempt to decelerate using the heel or a flat-foot approach while the leg is in an extended position (it’s next to impossible for your center of mass to be behind your body if you ‘stay up on your toes’).
- Get your center of mass moving in the direction of travel you wish to go – never try to stop and turn back against the direction you are travelling. Focus on staying low, utilizing the balls of your feet to decelerate as fast as you can, and accelerate in the opposing direction by getting your center of mass moving in that direction.
By: Ian Schnarr, CSCS
Certified Strength and Conditioning Specialist
Director of Performance @ Redline Conditioning
Build to Perform.
Elevate your knowledge, training and performance.
Video analysis of trunk and knee motion during non-contact anterior cruciate ligament injury in female athletes: lateral trunk and knee abduction motion are combined components of the injury mechanism
T Hewett-J Torg-B Boden – British Journal of Sports Medicine – 2009
Mechanisms of Anterior Cruciate Ligament Injury in Basketball: Video Analysis of 39 Cases
T. Krosshaug-A. Nakamae-B. Boden-L. Engebretsen-G. Smith-J. Slauterbeck-T. Hewett-R. Bahr – The American Journal of Sports Medicine – 2006
Mechanisms for Noncontact Anterior Cruciate Ligament Injuries: Knee Joint Kinematics in 10 Injury Situations From Female Team Handball and Basketball
H. Koga-A. Nakamae-Y. Shima-J. Iwasa-G. Myklebust-L. Engebretsen-R. Bahr-T. Krosshaug – The American Journal of Sports Medicine – 2010
Noncontact Anterior Cruciate Ligament Injuries: Risk Factors and Prevention Strategies .
L. Griffin et al. – Journal of the American Academy of Orthopaedic Surgeons – 2000.
Optimizing whole-body kinematics to minimize valgus knee loading during sidestepping: implications for ACL injury risk. Donnelly et al – Journal of Biomechanics – 2011.