The Problem
For both males and females, approximately 70% of anterior cruciate ligament (ACL) injuries occur during non-contact situations such as landing from a jump or during turning, twisting activities when a sudden knee imbalance occurs. Serious knee injuries occur 2-10 times more frequently in female athletes than in male athletes; the difference in injury rate increases with increasingly difficult sports activities. One in every 10 collegiate female athletes and one in every 100 high school female athletes will sustain a serious knee injury every year. In fact, ACL injuries in female athletes are so widespread that coaches of high school and college teams expect at least one player to be sidelined by an ACL injury every season.
The Cincinnati Sportsmedicine Research and Education Foundation is nationally recognized for performing important research for over three decades on preventing and treating knee injuries. One of our most significant research efforts has been the analysis of knee ligament injuries: how they occur and the best treatment for them. Fifteen years ago, we noticed an increase in anterior cruciate ligament injuries in female athletes. In a rigorous study of the incidence of injury in soccer, Dr. Thomas Lindenfeld and Dr. Frank Noyes at the Cincinnati Sportsmedicine Orthopaedic Center reported that the incidence of serious knee injury was approximately six-fold higher in female than male players when normalized per player-hour. . We dedicated research staff to studying this problem and have identified several factors which place the female athlete at greater risk than male athletes for knee injuries.
The Cause
Theories abound for the discrepancy between male and female knee injury rates. The majority of the research has focused on the following areas:
Structural/Anatomic Theories
- Wider pelvis; Q-angle
- Joint laxity
- Narrow intercondylar notch
Hormonal – Estrogen Theories
- Collagen strength deficit
- Joint laxity
Training Differences
- Strength
- Technique
- Coaching
While there are many theories, it is most likely that the increased knee injury incidence in female athletes over male athletes has multi-factorial causes. 
Anatomic differences do exist, and there is little to be done to correct these differences, but we must be aware of them. Estrogen both directly and indirectly affects the female neuromuscular system; hence, estrogen may have effects on neuromuscular patterning in female athletes. Finally, training differences between males and females may account for some percentage of the difference in knee injury rates with documented differences in the biomechanics of landing and lower extremity strength variations between females and males.
Males demonstrate relatively higher use of the hamstring and gastrocnemius musculature in landing from a jump, which protects the knee ligaments. Conversely, Female athletes most often demonstrate contraction of their quadriceps in response to an anterior tibial translation. Quadriceps contraction at knee flexion angles between zero and forty degrees (where injury most often occurs) significantly increases strain on the ACL. The hamstrings are an ACL agonist; ie, they resist strain on the ACL at these angles. In addition, female athletes demonstrate more anterior knee laxity and significantly less strength than their male counterparts.
Knee Injuries Female Athletes
The Treatment
Regardless of the reasons for the increases in incidence, the fact remains that when faced with an injury situation in which the athlete is slightly off-balance or must make a sudden stop or twist, female athletes may not have the coordination and muscle strength to react immediately and avoid injury. This problem with muscle coordination is not as prevalent in male athletes. When an athlete has insufficient muscle strength, coordination, and balance to react immediately there is an increased risk for a serious knee injury.
In 1996, our research at Cincinnati received a major national award for our published study which showed that hamstrings strength and power, jumping ability and knee control in female athletes were abnormally low in a high percentage of female athletes compared to male athletes. We believed that female athletes did not perform the right training and conditioning programs to achieve knee balance and thus prevent ACL injuries.
After participating in a six-week progressive, three-phase conditioning program incorporating stretches, jump/plyometric training, and strength and coordination exercises, later known as SportsmetricsTM, we proved that female athletes had better leg strength and power and attained limb symmetry equivalent to that of male athletes the same age. Importantly, female athletes decreased knee landing forces; jumping and landing mechanics changed from stiff-legged, “wiggle-wobble” landings to more controlled knee joint actions which provided stability; and the athletes demonstrated improved jump height. In short, SportsmetricsTM training can protect the knee while increasing jump power and performance.
Other researchers have also demonstrated that neuromuscular training increased hamstrings strength in females significantly and corrected imbalances in hamstrings to quadriceps strength ratios and side-to-side (dominant versus non-dominant leg) hamstrings strength. Females demonstrate mean hamstrings strength patterns which are significantly lower than that of males prior to training, but equal to males after training.
More recently, we focused our efforts on an epidemiological study to evaluate the effects of training in a “real world” setting. Pre-season screening questionnaires were administered to 829 female and 434 male high school soccer and volleyball athletes. A subgroup of 366 female athletes participated in SportsmetricsTM training six weeks prior to their season.
Throughout the school year, certified athletic trainers submitted weekly team and individual injury reports during the three-month sports season including the number of practice and competition exposures for all athletes. An injury risk exposure was defined as one athlete participating in one practice or match. Individual injury reports included type of injury, mechanism of injury, and treatment. A serious knee injury was defined as a knee ligament sprain or rupture causing an individual to seek medical care by a trainer or physician, causing a practice or match to be discontinued and resulting in at least five consecutive days of lost time from practice or match competition. All injuries were confirmed by a physician, entered into a database, and followed up until the athlete returned to full activity or quit the team.
The non-trained female athlete group sustained ten serious knee injuries. For the 
subgroup of females trained with SportsmetricsTM, only two serious knee injuries were documented, and both of these ACL tears were contact injuries. The control group of non-trained males also reported only two serious knee injuries (both non-contact injuries). Our research showed that the female athletes who completed the SportsmetricsTM program decreased their risk for knee injury by 3-4 fold compared to those who did not complete the program, and the injury rate for these trained female athletes was statistically similar to the male athletes.
The entire Sports Medicine community agrees that training programs for female athletes are an integral part of preventing these ACL injuries. As published in 2000 in their Journal, the American Academy of Orthopaedic Surgeons, with joint sponsorship of the American Orthopaedic Society for Sports Medicine, the Orthopaedic Research and Education Foundation, the National Athletic Trainers Association Research and Education Foundation and the National Collegiate Athletic Association, stated that while there is not presently a “clear understanding of the cause of non-contact ACL injuries…prevention programs designed to increase neuromuscular control, improve balance and teach avoidance strategies for at-risk situations appear to be effective in decreasing injury rates.”