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June/July 2002


The Gender Gap

By Timothy E. Hewett, Phd; Gregory D. Myer, MS, CSCS; Kevin R. Rord, MS; and Mark v. Paterno, MS, PT

Many studies have demonstrated the fourfold to sixfold higher incidence of knee injury in women athletes when compared to men in high-risk sports like soccer, basketball, and volleyball.1 Institutions receiving federal funding are required to provide equal access to extracurricular activities for women under Title IX of the Educational Assistance Act, enacted in 1972. This federal law has contributed to a geometric increase in the number of women athletes participating at the high school, collegiate, and professional levels.

The 1996 high school athletic participation survey conducted by the National Federation of State High School Associations reported more than 2 million women participants in high school sports programs.2 The combination of this increase in women's sports participation and the higher rate of injury in women athletes has led to a rapidly growing gender gap in knee injury incidence in jumping and cutting sports.


Figure 1. Single leg hop and stick. Athlete focuses on technique with deep knee flexion and holding the position for approximately 3 to 5 seconds.


NEUROMUSCULAR TRAINING

Dynamic neuromuscular training should incorporate findings from existing research studies and prevention techniques developed through more recent empirical and analytical evaluations of neuromuscular training and on-field play. Three components any neuromuscular training programs should incorporate are dynamic sport-specific movement skills, neuromuscular patterning based on the identification of underlying neuromuscular imbalances with constant biomechanical analysis by the instructor, and feedback to the athlete both during and following training.


Dynamic sports-specific training should provide the athlete with an effective means for facilitating desired adaptations to the proprioceptive function of the knee joint. The dynamic component progresses the woman athlete to high-risk sport-specific maneuvers that can be performed in a safe and controlled manner.

The neuromuscular component of the training should be a balance between challenging the proprioceptive abilities of the athlete and exposing the athlete to movement patterns that generate greater dynamic knee control. This type of proprioceptive stress may aid the development of spinal reflexes that more quickly and effectively stabilize the joint, than the voluntary muscular movements that require an afferent-efferent pathway along with cerebral input. The voluntary muscle response is too slow to manage the ground reaction forces from the high-risk maneuvers encountered during competitive play. As the neuromuscular system develops and adapts to neuromuscular training, reflexive multi-joint neuromuscular engrams may be created that employ joint stabilization patterns, which control acceleration and deceleration forces on the knee. The enhanced neuromuscular control can protect athletes from the ground reaction forces they will encounter in competitive play when jumping, landing, and cutting.

The analysis component of any neuromuscular training program should involve exercises that provide the instructor with the tools to analyze imperfections in technique. The training focus should be on perfecting the technique of each training exercise, especially early in the training cycle. If the athlete is allowed to perform the exercise maneuvers improperly, then the training will reinforce improper techniques. The trainer should give continuous and immediate feedback to the athlete both during and after each exercise bout.

The trainer should be skilled in recognizing the desired technique for a given exercise, and should learn to encourage the athlete to maintain perfect technique for as long as possible (Figure 1, page 26). If the athlete fatigues to a point that she can longer perform the exercise perfectly and displays a sharp decline in proficiency, then the athlete should be instructed to stop. The duration of each completed exercise should be noted and the goal of the next training session must be to continue improving technique and increasing the number of repetitions. The goal of increasing the quantity of exercises while maintaining the quality of exercises is critical in achieving positive results.

We demonstrated that young women (age 14 to 17 years old) athletes who participated in a neuromuscular training program demonstrated greater dynamic knee stability than women who had not undergone training.3 We also conducted an epidemiology study with the purpose of prospectively evaluating the effect of neuromuscular training on serious knee injury rates in young women athletes.4

There was a significant effect of training on injury rates. Untrained young women had a significantly higher incidence of serious knee injury than trained young women and men. Trained women were not different from untrained men. Training resulted in even greater differences in noncontact anterior cruciate ligament injuries between trained and untrained young women. These results indicate that dynamic neuromuscular training decreases injury rates in young women athletes.

Recent data strongly suggest that preventative measures should be taken with women athletes in order to decrease the incidence of serious knee injury in this high- risk population. While the benefit of injury prevention training is evident among a wide variety of athletes, it appears that those who demonstrate poor dynamic knee stability might benefit to a greater extent from training. The next logical step in injury prevention is to develop methods to further identify athletes who might be at a greater risk of injury. The goal of this approach is to calculate injury risk associated with an athlete playing a particular sport. Recent advances in technology and epidemiology have brought injury screening and prediction of injury risk closer to reality, especially for noncontact knee injuries.

The examination of high-force sports movements that simulate high-risk athletic maneuvers in a controlled laboratory can lead to significant advances in the field of injury risk prediction. Prospective examination of large numbers of athletes, using protocols that incorporate repeatable measurements coupled with systematic tracking of knee injuries, should allow the accurate prediction of injury risk in the athletic population. With this type of predictive information, high-risk athletes can be better identified in order to complete an injury prevention program and help reduce the risk of injury.

IDENTIFYING THOSE AT RISK

The neuromuscular imbalances observed in women athletes give us a framework for identifying individuals at increased risk of knee injury. Evaluation of an athlete's preinjury assessment may demonstrate characteristics that may put her into an identifiable injury risk profile. An athlete may display scores that put her at risk in one, two, or all three of the aforementioned dynamic neuromuscular imbalances.

First, ligament dominance can be tested using a box drop combined with a maximum effort vertical leap.5 Values of more than 10 cm of total knee motion at landing may prove indicative of ligament dominance.6 We have previously noted the correlation of peak landing force with valgus torques.1 The peak landing force and the valgus moment values displayed by an athlete following a volleyball block above the mean plus one standard deviation values reported in our 1996 study may be predictive of ligament dominance. Peak forces greater than five times body weight and valgus moments greater than 6% body weight times height may be predictive of higher injury risk.

Athletes can also be screened for indicators of quadriceps dominance using relatively common measurement techniques, such as isokinetic dynamometry or perhaps even simple leg curl and leg extension machines. If the athlete exhibits a high level of quadriceps strength, a low level of hamstring strength, or a low hamstring to quadriceps ratio in one or both limbs, quadriceps dominance is likely present. Hamstring to quadriceps peak torque ratios of less than 55% may be indicative of increased injury risk. More sophisticated measurements of this imbalance can be attained through kinetic analysis of knee flexor-extensor torques during high-force sports movements. Extensor to flexor ratios greater than two to one may be indicative that dynamic neuromuscular analysis training is required.

Dominant leg imbalance can also be assessed using a dynamometer or a leg curl and leg extension machine. A difference in strength or power of 20% or more between limbs is indicative of a neuromuscular imbalance that may underlie significant injury risk. Another test representative of bilateral imbalances between limbs is a measure of the athlete's ability to perform a single leg balanced stance on an unbalanced platform that can objectively quantitate postural sway, ie, a stabilometer. An athlete who is significantly unstable on the one side using dynamic stabilometry measures may also be at greater risk of knee injury.

While imbalances observed in any one of these measures alone may not pinpoint an athlete with a high-risk profile, a risk profile that incorporates measures from two or three dynamic neuromuscular imbalances might identify the athlete as high risk. Identification of these imbalances will assist the clinician and researcher to intervene with athletes in need of dynamic neuromuscular analysis training.

Women demonstrate dynamic neuromuscular imbalances that can be readily observed in the laboratory. These imbalances include ligament dominance, quadriceps dominance, and dominant leg dominance. Neuromuscular training can be used to specifically address and correct these neuromuscular imbalances in women athletes. Correction of neuromuscular imbalances is important for both optimal biomechanics of athletic movements and reduction of knee injury incidence. Further study on the effects of neuromuscular retraining on knee injury incidence and on biomechanical performance is important for the advancement of injury prevention initiatives and women's athletics.

Timothy E. Hewett, PhD, is director; Gregory D. Myer, MS, CSCS, is a sports biomechanist; Kevin R. Ford, MS, is a research biomechanist; and Mark V. Paterno, MS, PT, is a research physical therapist and athletic trainer at the Cincinnati Children's Sports Medicine Biodynamics Center and Human Performance Laboratory and the Children's Hospital Research Foundation in Cincinnati.

References
1. Hewett T. Neuromuscular and hormonal factors associated with knee injuries in female athletes: strategies for intervention. Sports Med. 2000;29:313-327.
2. The 1996 National High School Sports Participation Survery. Kansas City, Mo: National Federation of State High School Associations; 1996.
3. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes: decreased impact forces and increased hamstring torques. Am J Sports Med. 1996;24:765-773.
4. Hewett TE, Riccobene JV, Lindenfeld TN, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes: a prospective study. Am J Sports Med. 1999;27:699-706.
5. Myer GD, Hewett TE, Noyes FR. Neuromuscular control of the knee joint: increased valgus knee displacement during landing. Med Sci Sports Exerc. 2000;32:S298.
6. Hewett TE, Myer GD, Noyes FR. Identification of athletes with increased valgus knee displacement during landing: effects of gender and training [abstract]. 26th Annual Meeting of the American Orthopaedic Society for Sports Medicine; Sun Valley, Idaho. June 18-21, 2000:26.

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