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An Interview about ACL Injury Prevention

As I sorted through some old files, I found this interview that I conducted two years ago for Women’s Basketball. WB published someone else’s article on ACL injuries, so I put it into my files and forgot about it.

The interview is with Dr. Timothy Hewett, Director and Associate Professor with the Human Performance Laboratory at the Cincinnati Children’s Hospital Medical Center.

BM: Why do girls tear their ACL’s more often than boys?

Dr. Hewett: The increased predisposition of female athletes is a multi-factorial problem. Most ACL injuries occur by non-contact mechanisms, often during landing from a jump or making a lateral pivot while running. Biomechanical, neuromuscular, physiologic, hormonal and anatomic mechanisms may underlie these differences in the incidence of ACL injuries in males and females. However, we think that females use different neuromuscular control mechanisms than males, and this is the primary reason for the increased incidence of knee injury in female athletes. Basically, females have measurable neuromuscular imbalances that cause them to control their knees like ball-and-socket joints attached to loose springs, while males’ knees behave more like hinge joints attached to stiff springs.

BM: Are ACL injuries inevitable for some girls?

Dr. Hewett: ACL injuries often occur during landing from a jump, decelerating or pivoting on one foot while running. When females land, decelerate and pivot, increased knee instability, due to four neuromuscular imbalances commonly observed in female athletes, contribute to the ACL injury mechanism. These neuromuscular imbalances include:

1) Ligament dominance: decreased dynamic neuromuscular control of the joint, which is related to the knee abduction component of the injury mechanism.
2) Quadriceps dominance: increased quadriceps recruitment and decreased hamstring strength and recruitment, which is related to the extended knee position component of the injury mechanism.
3) Leg dominance: side-to-side differences in strength, flexibility and coordination, which is related to the asymmetrical foot weighting component of the injury mechanism
4) Core instability: increased trunk motion, which is related to the foot displaces away from the body center of mass component of the injury mechanism

We have much data that demonstrates that these four neuromuscular imbalances are the primary underlying mechanisms for the increased incidence of knee injury in female athletes.

BM: Is there anything coaches and athletes can do to reduce the likelihood of an ACL injury?

Dr. Hewett: In order to correct ligament dominance in female athletes, a neuromuscular training program must be designed to teach the athlete to control dynamic knee motion in the coronal (abduction and valgus) plane. The first concept that the athlete and coach are taught is the knee is a single-plane hinge, not a ball-and-socket joint. Reeducation of the female neuromuscular system away from multi-planar motion of the knee to dynamic control of knee motion in the sagittal plane only is achieved through a progression of single, then multi-planar exercises.

Quadriceps dominance can be corrected through the use of exercises that emphasize co-contraction of the knee flexor (hamstrings) and extensor (quadriceps) muscles. It is a challenging task to develop more appropriate firing patterns for the knee flexors, while utilizing exercises that also strongly activate the knee extensors. At angles greater than 45 degrees, the quadriceps is an antagonist to the ACL. Therefore, it is important to use deep knee flexion angles to put the quadriceps into an ACL agonist position. By training the athlete with deep knee flexion jumps, she learns to increase the amount of knee flexion and decrease the amount of time in the more dangerous straight-legged position. At the same time the athlete can reprogram peak flexor/extensor firing patterns, increasing co-firing and quadriceps firing in deep flexion for greater protection of the ACL.

Dominant leg imbalance can be addressed in the female athlete via exercises that force the correction of dynamic contralateral imbalances. These imbalances are addressed throughout the entire training protocol. Equal leg-to-leg strength, balance and foot placement are stressed through all the neuromuscular training exercises. In order to correct for leg dominance, the neuromuscular training must progressively emphasizes double then single movements through progressive training phases.

Finally, core instability, as evidenced by increased trunk motion, must be corrected in those female athletes that demonstrate instability with the latest core stability training techniques. These include progressive neuromuscular training techniques that target the balanced and synchronized turn-on of the dynamic stabilizing musculature of the trunk, pelvis and hip. Unstable surfaces, single-leg balancing and perturbation training should be employed.

BM: If you could offer one piece of advice to high school basketball coaches and players, in regards to ACL injuries, what would it be?

Dr. Hewett: This is an involved process, but in simplest terms, the neuromuscular imbalances that each female athlete demonstrates must be corrected. I would offer two pieces of advice, screen for imbalances in these young athletes and then train to correct them (as outlined above).

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