Since the passage of Title IX in 1972, girls' participation in high school sports has increased dramatically, by one estimate by more than 900%. The spectacular performance of the U.S. women in the London Olympics, along with the increase in the popularity of women's professional sports, have had a profound influence on a new generation of aspiring female athletes. "Be like Mia" has been replaced with "Be like Abby" and "Be like Hope."
The speed, power, and intensity displayed by female athletes have dramatically increased over the past decade. Such more aggressive style of play has led, predictably, to an increase in musculoskeletal injuries. One of the more common is a sprain or rupture of the anterior cruciate ligament (ACL).
Knee injuries are the most common cause of permanent disability in female high school basketball players, accounting for up to 91% of season-ending injuries and 94% of injuries requiring surgery. In the United States, 20,000 to 80,000 high school female athletes experience ACL injuries each year, with most in soccer and basketball.
ACL: vital role in many sports
Straight-ahead sports like jogging, swimming, and biking place little stress on the ACL. Sports such as soccer, basketball and volleyball that involve cutting, planting and changing direction, in which the ACL plays a vital role, put athletes, particularly females, at greatest risk of ACL injury.
Less than a third of all reported ACL injuries involve contact from an outside force such as an opposing player, goalpost or another object on the field/court. Over two-thirds are non-contact ACL injuries resulting from
- One-step/stop deceleration
- Cutting movements
- Sudden change in direction
- Landing from a jump with inadequate knee and hip flexion (at or near full extension)
- Lapse of concentration (resulting from unanticipated change in the direction of the play)
Women suffer more ACL injuries
Recent studies reveal that young female athletes are four to six times more likely than boys to suffer a serious non-contact ACL injury.
- According to the Centers for Disease Control and Prevention more than 46,000 female athletes age 19 and younger experienced a sprain or strain of the ACL in 2006.
- Nearly 30,000 of the injuries required reconstructive surgery.
- Overall, girls are 8 times more likely to suffer an ACL injury than boys.
- As many as 70% of ACL injuries involve little or no contact with the other player.
- At the age of 14 years, girls have 5 times higher rates of ACL tears than boys.
No easy explanation
Women may be more prone to non-contact ACL injuries because they run and cut sharply in a more erect posture than men, and bend their knees less when landing from a jump.
Why women and girls are more prone to ACL injuries than men and boys defies easy explanation is also likely due to a number of anatomical and hormonal differences between men and women:
- Narrower intercondylar notch and smaller ACL: Not only does the intercondylar notch (the groove in the femur through which the ACL travels) tend to be smaller in women, but so is the ACL itself, leading it more prone to injury.
- Wider pelvis: Women typically have a wider pelvis, which makes the thigh bones angle downward more sharply than in men. The greater the so-called "Q" angle, the more pressure is applied to the inside (medial aspect) of the knee, which can cause the ACL to tear. "One thought is that women have more of a knock-knee alignment," says Dr. Barry Boden of the Orthopedic Center in Rockville, Md., who is conducting research on the injury with funding from the National Institutes of Health. "The alignment of women's knees tends to bend inward when women land." Some researchers believe that this inward bend may predispose women to ACL injuries.
- More lax ligaments: Women's ligaments tend to have more "give" (laxity) than men's (what one docter characterized as "ligament dominant" rather than "muscle dominant.") Research has also shown that women's muscle tissue is more elastic than males. Excessive joint motion combined with increased flexibility may be a significant contributing factor in the higher rate of torn ACLs among women.
- Slower reflex time: Research shows that the muscles stabilizing the knee may take a millisecond longer to respond in women than in men. Scientists suspect that this small difference in contraction time also leads to a higher rate of injury.
- Greater Quadriceps/Hamstring Strength Ratio: It is well-established that female athletes typically have poor hamstring strength, which is considered one possible risk factor for ACL ruptures. If the hamstring cannot balance the power of the quadriceps (front thigh muscle), the imbalance can cause significant stress to the ACL, leading to injury.
- Changes in estrogen levels: One recent study suggests that changes in estrogen levels during a woman's menstrual cycle may affect ACL strength, predisposing women to the higher injury rate. A 2007 article in The British Journal of Sports Medicine, however, found "no conclusive evidence directly linking an increase in ACL injury to a predictable time in the menstrual cycle."
- Flat-footed landing: Dr. Boden believes that one of the major reasons any athlete suffers ACL damage is because they land in a flat-footed position, as opposed to landing on the balls of their feet. "If the calf muscles are not absorbing the force, and if the knee is not in the proper position, the knee buckles and tears the ACL," explains Dr. Boden.
Strength training, conditioning, coaching reduces risk
Several recent studies demonstrate that the rate of ACL injury among women can be significantly reduced by following a proper neuromuscular training and conditioning program.
According to Dr. Thomas Haverbush, a Michigan orthopedic surgeon, a training program developed at the University of Vermont Medical School designed to prevent ACL injuries in skiers led to a 69% decrease in the number of knee injuries among ski patrol personnel and instructors who received the training compared with those who did not.
In the same article, Dr. Haverbush reported that a six week training program in Cincinnati in which athletes were trained to rely more on hamstring muscles than quadriceps in order to protect the knee could reduce the ratio of knee ligament injuries in female athletes as compared to men from five times higher to only one or two times higher.
More recently, a study reported in the American Journal of Sports Medicine suggests that the ACL injury rate for female athletes, particularly non-contact ACL injuries, can be significantly reduced if the athlete follows a specific exercise program called the Prevent Injury and Enhance Performance (PEP) program before practices and games.
Designed in 1999 by a team of experts at the non-profit Santa Monica Orthopedic and Sports Medicine Research Foundation, the PEP program consists of a series of 19 warm-up, stretching, strengthening, plyometric, and sport-specific agility exercises that can be completed in less than 30 minutes without any specialized equipment.
Two prevention programs reduce risk, increase performance
A 2011 study in the journal Sports Health: A Multidisciplinary Approach,5 found that two prevention programs, Sportsmetrics and PEP not only significantly reduced ACL injury rates but improved athletic performance.
In general, experts say there are four ways to reduce the risk of ACL injury:
- Proper leg muscle strength training and core training;
- Proper neuromuscular (balance and speed) training;
- Proper coaching on jumping and landing and avoiding any straight knee landing;
- Proper footwear and orthotics if necessary (the amount of traction or "grippiness" of athletic shoes needs to fall within an optimal range that minimizes rotational friction to avoid injury yet optimizes transitional friction to allow peak performance in activities such as cutting and stopping).
A recent meta-analysis of published ACL injury prevention programs determined that plyometric and strengthening components were more important than balance training and that the favorable effects of training were most pronounced in female soccer players younger than 18 years.6
Another meta-analysis7 found that various types of neuromuscular and educational interventions appear to reduce the incidence rate of ACL injuries by approximately 50%, but that the estimated effect varied appreciably among the studies, the reasons for which were unexplained.
Here's a video from SafeKids Worldwide featuring U.S. Women's Soccer star, Ali Krieger, which shows 7 exercises every young athlete can do to help prevent knee injuries.
For a good overview of anterior cruciate ligament injuries in young athletes by a physician at Lurie Children's Hospital of Chicago, click here.
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3. Barber-Westin SD, Noyes FR, Galloway M. Jump-land characteristics and muscle-strength development in young athletes: a gender comparison of 1140 athletes 9 to 17 years of age. Am J Sprts Med. 2006;34(3):375-384.
4. Alentorn-Geli E, Myer GD, Silvers HJ, et al. Prevention of non-contact anterior cruciate ligament injuries in soccer players: part 1. Mechanisms of injury and underlying risk factors. Knee Surg Sports Traumatol Arthorsc. 2009;17(7):705-729.
5. Noyes F, Barber Westin S. Anterior Cruciate Ligament Pevention Training in Female Athletes: A Systematic Review of Injury Reduction and Results of Athletic Performance Tests at n. 1, 12, 17. Sports Health: A Multidisciplinary Approach. Published online December 13, 2011 as DOI: 10.1177/194173811430203 (accessed December 14, 2011)
6. Yoo JH, Lim BO, Ha M, et. al. A meta-analysis of the effect of neurmuscular training on the prevention of the anterior cruciate ligament injury in female athletes. Knee Surg Sports Traumatol Arthrosc. 2010;18(6):824-830.
7. Gagnier J, Morgenstern H, Chess L. Interventions Designed to Prevent Anterior Cruciate Ligament Injuries in Adolescents and Adults. A Systematic Review and Meta-Analysis. Am J Sports Med. 2012; 20(10). DOI: 10.1177/0363546512458227 (published online ahead of print on September 12, 2012).
8. National Institutes of Health Medicine Plus. An Athlete's Nightmare: Tearing the ACL. Accessed August 7, 2013
9. Wild CV, Steele JR, Munro BJ. Why do girls sustain more anterior cruciate ligament injuries than boys?: a review of the changes in estrogen and musculoskeletal structure and function during puberty. Sports Med 2012;42:733-49.
10. Shea KG, et al. Anterior cruciate ligament injury in pediatric and adolescent soccer players: An analysis of insurance data. J Pediatr Orthop 2004;24(6):623-628.
11. Lopiano DA. Modern history of women in sports. Twenty-five years of Title IX. Clin Sports Med 2000;19(2):163-173,vii.
Updated August 7, 2014